Melt Synthesis, Structural, Characterization and Scaling of Swelling 2:1-Layer Silicate Materials

MeltSynthesis,Structural,Characterizationand ScalingofSwelling2:1LayerSilicateMaterials  Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat) an der Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth vorgelegt von Hussein Kalo aus Aleppo (Syria) Bayreuth 2012 MeltSynthesis,Structural,Characterizationand ScalingofSwelling2:1LayerSilicateMaterials Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat) an der Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth vorgelegt von Hussein Kalo aus Aleppo (Syria) Bayreuth 2012 Die vorliegende Arbeit wurde in der Zeit von März 2007 bis Mai 2012 in Bayreuth am Lehrstuhl Anorganische Chemie I unter Betreuung von Herrn Prof. Dr. Josef Breu angefertigt. Vollständiger Abdruck der von der Fakultät für Biologie, Chemie und Geowissenschaften der Universität Bayreuth genehmigten Dissertation zur Erlangung des Akademischen Grades eines Doktors der Naturwissenschaften (Dr. rar. nat.). Dissertation eingereicht am: 08.06.2012 Zulassung durch die Prüfungskommission: 09.08.2012 Wissenschaftliches Kolloquium: Amtierende Dekanin: Prof. Dr. Beate Lohnert Prüfungsausschuss: Prof. Dr. J. Breu (Erstgutachter) Prof. Dr. J. Senker (Zweitgutachter) Prof. Dr. H. Keppler Prof. Dr. G. Papastavrou This thesis is dedicated to my parents for their love and endless support My sincere acknowledgement to my supervisor Professor Dr. Josef Breu for his guidance, encouragement and enthusiastic support during the course of this research program. Acknowledgement This thesis is a cumulative result after years of research since joining Prof. Breu’s group in 2007. I have worked with a large number of people who contributed in various ways to my research. I would like to take the chance to express my gratitude to all of them in my unassuming acknowledgment. My grateful gratitude for the outstanding support and guidance goes to my supervisor Prof. Dr. Josef Breu. He was always a great source of motivation and guidance during my research. I would also like to deeply thank Dr. Wolfgang Milius for his advice, scientific discussions, and supervision in single crystal refinement. My thanks goes to my previous coworkers; Michael Möller for studying the hydration behavior in the humidity chamber and for SEM images, and to Daniel Kunz for AFM imaging. During my academic study period I was lucky to meet many people who helped me in different ways, in particular Dr. Micheal Schütz and Dr. Dunja Hirsemann. I would like to thank my colleagues in the laboratory; Matthias Stöter and Josef Hausner, and the entire ACI group. Many thanks goes to the technical staff; Mr. Bernhard Putz who taught me how to operate the frequency furnace technology and to all other helpful people who provided different measurements and help, especially to Beate Bojer, Dieter Will, Sonja Lutschinger, and Lena Geiling. My great appreciation goes to my professors back home at University of Aleppo; Nawzat Nabgaly. Abdalah Witte, Mohammad Abd AL-mattiy. I want to express my gratitude to my family for the support and encouragement. I am grateful for the unending love and support I receive from my parents, my wife, my sisters and my brothers. Finally, I would like to thank the ministry of higher education in Syria, University of Aleppo, for sponsoring my graduate studies. Content 1. Summary – Zusammenfassung . . . . . . . . . . . . . . . . . . . . . . 1 2. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1. Structure of clay minerals . . . . . . . . . . . . . . . . . . . . . . 6 2.2. Properties and characterization of swelling 2:1-layer silicate . . . . . . . . 8 2.3. Synthesis of swelling 2:1-layer silicate . . . . . . . . . . . . . . . . 9 2.4. Application of layer silicate . . . . . . . . . . . . . . . . . . . . . 11 3. Synopsis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2. Single crystal structure of hydrate sodium fluorohectorite . . . . . . . . . 13 3.3. Large scale melt-synthesis of sodium-fluorohectorite . . . . . . . . . . 16 3.4. Synthesis of lithium-fluorohectorite . . . . . . . . . . . . . . . . . . 19 3.5. Synthesis and structure of hydrate sodium brittle mica . . . . . . . . . . 22 4. Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 5. Individual contributions to joint publications . . . . . . . . . . . . . . . . . 28 5.1. Appendix 1. Crystal structure of hydrate synthetic sodium-fluorohectorite . . 30 5.2. Appendix 2. Large scale melt-synthesis of sodium-fluorohectorite. . . . . . 55 5.3. Appendix 3. Synthesis of lithium fluorohectorite . . . . . . . . . . . . 63 5.4. Appendix 4. Crystal structure of hydrate sodium brittle mica . . . . . . . 83 6. Curriculum vitae . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 7. List of publication . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 8. Declaration/Erklärung . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Summary - Zusammenfassung 1 Summary – Zusammenfassung Summary Melt synthesis, characterization, and refinement of single crystal structures of swelling 2:1layer silicates were the main fundamental topics of the presented thesis. In particular, large scale syntheses of both lithium and sodium fluorohectorite were successfully achieved. Furthermore, the crystal structure of one-, and two-layer hydrate of sodium fluorohectorite and the one-layer hydrate of sodium brittle mica were thoroughly investigated and characterized in detail. Swelling sodium fluorohectorite with good crystallinity in an ideal composition of Na0.85[Mg2.15Li0.85]Si4O10F2 was synthesized for investigating the hydrated structure. Melt synthesis was done in closed molybdenum crucibles using pure reagents (glass with composition Na2O-2SiO2, Li2SiO3 MgF2, MgO, SiO2). The crystal structures of oneand twolayer hydrate of sodium fluorohectorite were studied. The one-layer hydrate of sodium fluorohectorite (at relative humidity 45 %) showed two planes of interlayer sodium along [100]. The two-layer hydrate of sodium fluorohectorite showed sodium interlayer cations being located in the middle of the interlayer. In addition, sodium brittle mica with a target composition Na4[Mg6]Si4Al4O20F4 was successfully synthesized via melt synthesis in a gas tight molybdenum crucible and the refinement of the one-layer hydrate of sodium brittle mica was done. The synthetic sodium brittle mica swells only to the one-layer hydrate and could not be further hydrated to the twolayer hydrate. Generally, natural swelling layer silicates (smectites) usually contain impurities such as iron oxide (pigmentation material), quartz, and carbonate. However, these impurities hinder the employment of swelling layer silicates in industry for cutting edge and advanced applications. In addition, they suffer from small particle size under 5 µm limiting their aspect ratio. For industrial applications, pure synthetic swelling layer silicates with superior properties are highly desirable. Therefore, a large scale synthesis of sodium fluorohectorite Na0.6[Mg2.4Li0.6]Si4O10F2 was carried out in three steps. (i) Synthesis of glass, glass was used as precursor and low melting agent, the amorphous glass with composition Na2O-Li2O-6SiO2 was synthesized from sodium carbonate Na2CO3, lithium carbonate Li2CO3, and silicic acid SiO2·nH2O via melt synthesis in an open glassy carbon crucible at 1075 °C under flowing argon in a high frequency induction furnace, where the temperature was increased with a constant rate of 300°C/hr. (ii) Introduction 8 layer silicate type is close to zero and varies in 2:1-layer silicates type from 0.2 in montmorillonite and hectorite to 2.0 in brittle mica per formula unit [1,4]. In Table 1 the layer charges and the ideal formulae of some types of layer silicates are presented [1]. Table 1. Layer charge and the ideal formula of selected types of phyllosilicates [1]. group name charge (per formula unit). dioctahedral type trioctahedral type kaolinite – serpentine group ~ 0 kaolinite (Si2)IV(Al2)VIO5(OH)4 serpentine (Si2)IV(Mg3)VIO5(OH)4 talc-pyrophyllite group ~ 0 p yrophyllite (Si4)IV(Al2)VIO10(OH)2 talc (Si4)IV(Mg3)VIO10(OH)2 smectite group ~ 0.2-0.6 montmorillonite (Si4)IV(Al2yMgy)VIO10(OH)2,yM+·nH2O beidellite (Si4-xAlx)IV(Al2)VIO10(OH)2, xM+·nH2O hectroite (Si4)IV(Mg3-yLiy)VIO10(OH)2, yM+·nH2O saponite (Si4-xAlx)IV(Mg3)VIO10(OH)2, xM+·nH2O vermiculite group~ 0.6-0.9 vermiculite (Si4-xAlx)IV(Al2yMgy)VIO10(OH)2, (x+y)M+· vermiculite (Si4-xAlx)IV(Mg3-yM3+y)VIO10 (OH)2, (x-y)/2Mg2+· true mica group ~ 0.9-1.0 muscovite (Si3Al)IV(Al2)VIO10(OH)2, K+ p hlogopite (Si3Al)IV(Mg3)VIO10(OH)2 K+ 2.2 Swelling properties of 2:1-layer silicates Swelling layer silicates of the 2:1 family are rigid 2-dimensional polyanions with a rich intracrystalline reactivity resulting from solvation and exchange of the interlayer cations. Depending on the water vapour pressure, the layer charge and the type of interlayer cation the 2:1-layer silicate family forms different hydration states with one-, two-, and threeor even four pseudo-layer of water molecules between silicate layers [5-8]. Whereas, the stepwise increase of the interlayer space is a characteristic of the swelling 2:1-layer silicates with interlayer cations such as sodium or lithium as shown in Figure 3. The effect of the interlayer cation on the hydration behavior of smectites was studied by Cases et al. for alkaline-earth metals (e.g. Mg2+, Ca2+, Sr2+, and Ba2+) [9]. They observed a gradual variation of water adsorption and swelling depending on the hydration enthalpy of the cation [9]. Berend et al. have studied different alkaline metals (e.g. Li+, Na+, Rb+, and Cs+) Introduction 9 [10-12] as interlayer cation, where the rate of interlayer space filling increased in relation to vapour pressure. Eric Ferrage et al. investigated the influence of layer charge and charge location on the hydration properties of smectites and they found that as the layer charge was increased, the basal spacing shifted from the two-layer hydrate to the one-layer hydrate at the same relative humidity for both montmorillonite (charge location in octahedral sheet) and beidellite (charge location in tetrahedral sheet) [13,14]. Figure 3. Stepwise increase of the interlayer space of 2:1-layer silicates, d001~ 10 Å refers to zero-hydration, d001~12.3 Å to one-layer hydrate, d001~15.5 Å to two-layer hydrate, and d001~18.5 Å to three-layer hydrate. 2.3. Synthesis of swelling 2:1-layer silicates The motivation of clay synthesis is to produce highly pure material at the lowest possible temperature with remarkable properties such as homogeneity in layer charge density and high crystallinity. For preparation of synthetic clay two main paths can be used, either with a melt synthesis or by hydrothermal methods. 2.3.1. Melt synthesis The advantages of melt synthesis of layer silicates are:  Due to substitution of Fanions with OHin melt synthesis, the obtained fluorosilicates are more stable at high temperature than hydroxi-silicates. Introduction 10  Highly crystalline layer silicates can be achieved by using high temperature and slow cooling.  The method is easier to tune for the synthesis of different layer silicates such as Mg2+, Fe2+, Ni2+, and Co2+ rich octahedral sheets [15].  Homogeneity in layer charge density. Different methods are applied for the synthesis of layer silicates via the melt where the synthesis depends on the state and type of starting material such as minerals or rocks, glasses and gels. Fiore et al. has used minerals as precursor but impurities were observed in the final product. In addition, glass was used as a starting material as a source for metal and as a low flux agent [15-19]. The most used starting materials are gels, which can be prepared by one of these three methods: (i) using only organic salts like tetraethoxysilane (TEOS), tri-isopropyl aluminates, iron acetylacetonate, etc [20], (ii) using TEOS and nitrates of Mg2+, Al3+, or Fe3+ cations and heating the gels at 800 °C for a complete dehydration [21-23], and finally (iii) using sodium meta-silicate and chloride or as sulfate anions [24]. Nevertheless, gels take longer times to dry and the dried gel show heterogeneity of the element distribution which might affect the homogeneity of the layer charge of the synthetic layer silicate. 2.3.2. Hydrothermal Synthesis There are different drawbacks of using hydrothermal methods for the synthesis of layer silicates;  Due to the hydroxyl content in synthetic layer silicates the thermal stability of is low (not more than 350 °C).  Heterogeneity in the element distribution of the synthetic material.  Using low temperature generates small particles  Product suffering a high degree of stacking fault. Commonly, the hydrothermal process requires long periods and produces a product with a small particle size [25]. For example hectorite was synthesized by Carrado et al. [26] in hydrothermal treatment using silica sol, magnesium hydroxide sol, and lithium fluoride. The mixture was treated under reflux for 2 days and the product had a maximum particle size of 1-2 μm. Introduction 11 2.4. Application of layer silicates Layer silicate materials are extensively used in different industrial applications such as: polymer fillers [27], catalysis, ceramics, refractory bricks, paper, paint, in agriculture, and sensors [28]. Table 2 shows some applications of different layer silicates. Table. 2 Selected applications of layer silicates in industry Layer silicates type Industry Use mica electrical industry paint insulation UV-, heat-stable, and under-water paint vermiculite building industry packaging industry heat insulation, sound dissipation shock proof materials, thermal protection smectite agriculture building industry soil improvement antifriction agents for pipe jacking and shaft sinking additions to concrete and mortar kaolinite paper, plastics, rubber filler talc paper, plastics, rubber filler Synopsis 12 3. Synopsis 3.1 Motivation The hydrates of 2:1-layer silicates are widely employed and studied. When the structure of this type of materials was investigated typically natural clay minerals such as vermiculite were used. Studies on the one-layer hydrate demonstrated that the location of the interlayer cation is close to a one-side of the tetrahedral sheet. Other records claimed to show that the interlayer sodium cations are located in the middle of the interlayer in case of one-layer hydrate. Nevertheless, the crystal structure of one-, and two-layer of hydrate sodium fluorohectorite is still not well determined using complete single crystal X-ray diffraction data. Figuring out the hydrate structure can, however, help finding more advanced applications of swelling sodium fluorohectorite. Additionally, sodium brittle mica with the ideal composition Na4[Mg6]Si4Al4O20(OH,F)4 shows a swelling behavior although it has high layer charge (4 negative charge per formula unit). Recently sodium brittle mica has drawn attention of scientists due to its high cation exchange capacity (CEC). Most of previous publications concerning the synthesis of sodium brittle mica reported a synthesis procedure which produces different types of sodium brittle mica (different layer charge) and small particle size. However, the crystal structure of the one-layer hydrate of sodium brittle mica is still not well defined and an efficient synthesis procedure needs to be established. The preparation of synthetic 2:1-layer silicates in a melt procedure produces well defined clay with much enhanced properties such as:(i) homogeneity of chemical composition and layer charge density (ii) colorless (iii) high aspect ratio (iv) large particles sizes (v) highly pure material (vi) uniform intracrystalline reactivity. The variation of interlayer chemistry gives a broad range of possible functionalizations and exfoliation via osmotic swelling. The exfoliation of layer silicates by osmotic swelling or external mechanical shear stress produces nano-platelets with extremely large aspect ratios which may be used in different applications[29,30]. For industrial applications an economically and scalable method is highly desirable without having to accept any compromises in the final materials properties regarding aspect ratios and homogeneity of charge density. Designing a melt synthesis process of swelling 2:1-layer silicates of Na0.6[Mg2.4Li0.6]Si4O10F2, and lithium fluorohectorite with a perfectly uniform distribution of isomorphous substitution in an unsealed container was not previously reported. Synopsis 13 3.2. Single crystal structure of hydrate of sodium fluorohectorite Swelling probably is the most important feature of expandable 2:1-layer silicates. The dominant driving force for water uptake is the hydration enthalpy of interlayer cations that of course varies with charge density and type of interlayer cation. For true solid solution type clays where the charge density is homogenous the intercalation occurs in well defined steps as a function of water activity [31-34]. The hydrate structures of 2:1-layer silicates were studied applying different methods such as NMR spectroscopy, neutron scattering, X-ray diffraction, and computer simulation[35-38]. The computer simulation was applied to understand the hydration geometry and location of the interlayer cation in respect to the lower and upper tetrahedral sheet [10,39,39-42]. However, there is a limited number of “single crystal” refinements of hydrated phases available in the literature applying some rare occasions of semi-ordered vermiculites (Santa Olalla, Spain and Carl Moss Ranch, Llano County, Texas) [36,43]. In order to study the structure of hydrate fluorohectorite, Na0.85[Mg2.15Li0.85]Si4O10F2 was synthesized. The synthesis was done as described elsewhere by Breu et al. [44], only the starting materials was changed, instead of LiF and NaF the lithium metasilicate (Li2SiO3) and sodium orthosilicate (Na2O-2SiO2) (glass) were used. The synthetic sodium fluorohectorite was characterized via powder X-ray diffraction PXRD, wavelength dispersive X-ray WDX and inductively coupled plasma atomic emission spectroscopy ICP-AES. The synthetic sodium fluorohectorite showed a pure phase and high crystallinty, uniform intracrystalline reactivity whereas the 001 peak of the one-layer hydrate of sodium fluorohectorite at 12.5 Å was observed as shown in Figure 4. Synopsis 14 Figure 4. Powder X-ray diffraction pattern of one-layer hydrate of synthetic sodium fluorohectorite Surprisingly, the PXRD of two-layer hydrate of synthetic sodium fluorohectorite showed features indicating ordered stacking, the 02l and 11l peaks observed were relatively sharp (Figure 5). Figure 5. Powder X-ray diffraction pattern of the two-layer hydrate of synthetic sodium fluorohectorite The single crystal structure refinement of the one-layer hydrate of synthetic sodium fluorohectorite showed two planes of interlayer cations (sodium) along the [100] (Figure 6A). The sodium interlayer cations are located approximately above the centre of hexagonal cavity Synopsis 15 at the m3 site. In the case of two-layer hydrate the sodium is located in the middle of interlayer (Figure 6B). The observed distance between the oxygen of water and the basal oxygen atoms of the tetrahedral sheet was 2.90 -3.03Å Figure 7. Figure 6. Structure of one-, and two-layer hydrate of sodium fluorohectorite projected along [100] indicating in particular the location of interlayer cations Figure 7. The structure of two-layer hydrate of synthetic sodium fluorohectorite present the hydrogen bonding between interlayer sodium water complex and the tetrahedral sheets fixing the stacking order. Details and further discussion: Appendix 1: Crystal structure of the hydrate of synthetic sodium-fluorohectorite. Synopsis 16 3.3. Large scale melt-synthesis of sodium fluorohectroite For the purpose of synthesizing a 2:1-layer silicate with an ideal formula Na0.6[Mg2.4Li0.6]Si4O10F2 in a scalable commercially interesting way, melt synthesis was carried out in an open glassy carbon crucible. The synthesis procedure afforded three steps; (i) synthesis of glass with composition Na2O-Li2O-6SiO2 at 1075 °C (ii) decarboxylation and dehydration of MgCO3 ·Mg(OH)2 x H2O and silicic acid hydrate, SiO2 ·x H2O with a molar ratio of MgO/SiO2 = 1.4/2.2 (iii) mixing, grinding, and melting of 0.3 mole of the synthetic glass with MgO/SiO2 = 1.4/2.2 and 1 mole of magnesium fluoride in an open glassy carbon crucible at 1265 °C for 15 min. The synthesis equipment (Figure 8), including the crucible, the cooling system of the furnace, and the coil of the high frequency furnace, was developed to reproduce the materials at low cost. Figure 8. The furnace used for scaling the synthesis of the sodium fluorohectorite. Synopsis 17 The powder X-ray diffraction of synthetic sodium fluorohectorite (Figure 9) showed uniform intracrystalline reactivity, the basal spacing at relative humidity 30 % was d001=12.3 Å. Figure 9. PXRD pattern of synthetic Na0.6[Mg2.4Li0.6]Si4O10F2 (one layer hydrate, d001 =12.3 Å). 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Mcconnell, The Structure of Interlayer Water in Vermiculite, Journal of Chemical Physics 1991, 94, 5751-5760. [38] N. T. Skipper, A. K. Soper, M. V. Smalley, Neutron-Diffraction Study of Calcium Vermiculite - Hydration of Calcium-Ions in A Confined Environment, Journal of Physical Chemistry 1994, 98, 942-945. [39] N. T. Skipper, K. Refson, J. D. C. Mcconnell, Computer-Simulation of Interlayer Water in 2-1 Clays, Journal of Chemical Physics 1991, 94, 7434-7445. [40] N. T. Skipper, F. R. C. Chang, G. Sposito, Monte-Carlo Simulation of Interlayer Molecular-Structure in Swelling Clay-Minerals .1. Methodology, Clays Clay Miner. 1995, 43, 285-293. [41] F. R. C. Chang, N. T. Skipper, G. Sposito, Computer-Simulation of Interlayer Molecular-Structure in Sodium Montmorillonite Hydrates, Langmuir 1995, 11, 27342741. [42] N. T. Skipper, Computer simulation of aqueous pore fluids in 2 : 1 clay minerals, Mineralogical Magazine 1998, 62, 657-667. [43] A. Arguelles, M. Leoni, J. A. Blanco, C. Marcos, Semi-ordered crystalline structure of the Santa Olalla vermiculite inferred from X-ray powder diffraction, American Mineralogist 2010, 95, 126-134. [44] J. Breu, W. Seidl, A. J. Stoll, K. G. Lange, T. U. Probst, Charge homogeneity in synthetic fluorohectorite, Chemistry Of Materials 2001, 13, 4213-4220. [45] A. R. Mermut, G. Lagaly, Baseline studies of The Clay Minerals Society Source Clays: Layer-charge determination and characteristics of those minerals containing 2 : 1 layers, Clays and Clay Minerals 2001, 49, 393-397. [46] M. K. Murthy, F. A. Hummel, Phase Equilibria in the System Lithium Metasilicate - Forsterite-Silica, Journal of the American Ceramic Society 1955, 38, 55-63. [47] S. Taruta, T. Ichinose, T. Yamaguchi, K. Kitajima, Preparation of transparent lithiummica glass-ceramics, Journal Of Non-Crystalline Solids 2006, 352, 5556-5563. [48] T. Kodama, Y. Harada, M. Ueda, K. Shimizu, K. Shuto, S. Komarneni, W. Hoffbauer, H. Schneider, Crystal-size control and characterization of Na-4-mica prepared from kaolinite, Journal of Materials Chemistry 2001, 11, 1222-1227. Bibliography 27 [49] M. D. Alba, M. A. Castro, M. Naranjo, E. Pavon, Hydrothermal reactivity of Na-nmicas (n=2, 3, 4), Chemistry Of Materials 2006, 18, 2867-2872. [50] M. Park, D. H. Lee, C. L. Choi, S. S. Kim, K. S. Kim, J. Choi, Pure Na-4-mica: Synthesis and characterization, Chemistry Of Materials 2002, 14, 2582-2589. [51] M. Gregorkiewitz, J. A. Rausellcolom, Characterization and Properties of A New Synthetic Silicate with Highly Charged Mica-Type Layers, American Mineralogist 1987, 72, 515-527. [52] T. Kodama, S. Komarneni, Na-4-mica: Cd2+, Ni2+, Co2+, Mn2+ and Zn2+ ion exchange, Journal of Materials Chemistry 1999, 9, 533-539. [53] T. Kodama, S. Komarneni, W. Hoffbauer, H. Schneider, Na-4-mica: simplified synthesis from kaolinite, characterization and Zn, Cd, Pb, Cu and Ba uptake kinetics, Journal of Materials Chemistry 2000, 10, 1649-1653. [54] W. J. Paulus, S. Komarneni, R. Roy, Bulk Synthesis and Selective Exchange of Strontium Ions in Na4Mg6Al4Si4O20F4 Mica, Nature 1992, 357, 571-573. [55] W. H. Baur, Computer-Simulated Crystal-Structures of Observed and Hypothetical Mg2SiO4 Polymorphs of Low and High-Density, American Mineralogist 1972, 57, 709-&. [56] W. Borchert, K. Jürgen, Beiträge zur Reaktionsfähigkeit der Silikate bei niedrigen Temperaturen, Contributions to Mineralogy and Petrology 1947, 1, 17-30. Individual contributions to Joint Publications 28 5. Individual contributions to Joint Publications: The publications/manuscripts, which are presented in the appendix, were obtained in cooperation with other co-workers at different departments. My contributions to each publication are specified below and the asterisk denotes the corresponding author (s). 5.1Appendix 1. This work was submitted to RSC Advance under the title “Single Crystal Structure Refinement of Oneand Two-layer Hydrate of Sodium-Fluorohectorite”. By Hussein Kalo, Wolfgang Milius, Josef Breu*. ¾ I have performed the synthesis, characterization, and single crystal measurement in addition to writing the manuscript. ¾ Dr. Wolfgang Milius performed the refinement of one-layer hydrate of sodium fluorohectorite and also contributed to the scientific discussion. ¾ Prof. Josef Breu contributed to the scientific discussion. 5.2Appendix 2: This work was published in Applied Clay Science under the title “Large scale meltsynthesis in an open crucible of Na-fluorohectorite with superb charge homogeneity and particle size”. By Hussein Kalo, Michael W. Möller, Mazen Ziadeh, David Dolejš, Josef Breu*. ¾ I have performed the synthesis, characterization of sodium fluorohectorite, and the scaling procedure in addition to writing the manuscript. ¾ Michael W. Möller performed the swelling evaluation procedure in the humidity chamber. ¾ David Dolejš performed the thermodynamics calculation and evaluation of the phase diagram. ¾ Mazen Ziadeh provided language help. ¾ Prof. Josef Breu contributed to the scientific discussion. 5.3Appendix 3: This work submitted to Nanoscale under the title “How to Maximize the Aspect Ratio of Clay Nanoplatelets”. By Hussein Kalo, Michael W. Möller, Daniel A. Kunz, and Josef Breu*. Individual contributions to Joint Publications 29 ¾ I have performed the synthesis, characterization of sodium fluorohectorite and subsequently the scaling procedure and the scientific writing. ¾ Michael W. Möller conducted the swelling evaluation in the humidity chamber. ¾ Daniel A. Kunz made the atomic force microscope AFM measurement. ¾ Prof. Josef Breu contributed to the scientific discussion. 5.4Appendix 4: This work submitted to Journal of Solid State Chemistry under the title “Synthesis and Single Crystal Structure of the One-layer hydrate of Sodium Brittle Mica“. By Hussein Kalo, Wolfgang Milius, Michael Bräu and Josef Breu*. ¾ I have performed the synthesis, characterization, single crystal measurements and refinements, in addition to the scientific writing. ¾ Dr. Wolfgang Milius contributed to the refinement discussion. ¾ Dr. Michael Bräu contributed to the twinning discussion. ¾ Prof. Josef Breu contributed to the scientific discussion. Appendix 30 Appendix 1 Single Crystal Structure Refinement of Oneand Two-layer Hydrate of Sodium-Fluorohectorite Hussein Kalo, Wolfgang Milius, Josef Breu* Department of Inorganic chemistry I, University of Bayreuth, D-95440 Bayreuth, Germany Run title: crystal structure of hydrate Sodium-Fluorohectorite Corresponding author: Prof. Dr. Josef Breu Universitätsstr. 30 95440 Bayreuth Germany * E-mail address: [email protected] RSC Advance, DOI: 10.1039/C2RA20457F Crystal structure of hydrate Sodium-fluorohectorite 31 Single Crystal Structure Refinement of Oneand Two-layer Hydrate of SodiumFluorohectorite Hussein Kalo, Wolfgang Milius and Josef Breu* Lehrstuhl für Anorganische Chemie I, Universität Bayreuth, D-95440 Bayreuth, Germany * [email protected] Running title: structure refinement of oneand two-layer hydrate * Corresponding author: Prof. Dr. Josef Breu, Phone: 0049921552531 Fax: 0049921552788 E-mail: [email protected] Abstract Crystal structures of both, oneand two-layer hydrate of sodium fluorohectorite could be refined against single crystal data for the first time because melt synthesis yielded a sodium fluorohectorite showing little stacking disorder as compared to natural clays. In both hydrate phases, the relative shift of adjacent 2:1-layer is fixed by hydrogen-bonding between water molecules coordinated to interlayer cations and basal oxygen atoms of tetrahedral sheets encompassing the interlayer space. Despite some reminiscent diffuse scattering, a decent single crystal refinement of the semi-ordered structure of the one-layer hydrate could be achieved revealing structural details of the interlayer space for the first time. For the twolayer hydrate the structural model proposed for vermiculites could be confirmed but a different ordering pattern of interlayer [Na(H2O)6]+ is suggested. While in the two-layer hydrate sodium cations reside at the center of the interlayer space, in the one-layer hydrate sodium is displaced from the center of the interlayer space either towards the upper or towards the lower tetrahedral sheet. This displacement allows for coordination to the hexagonal cavity on one side while the coordination sphere of sodium is completed by three coordinating water molecules on the other side. These three water molecules in turn are involved in hydrogen bonding to the opposite tetrahedral sheet. Introduction Hydrated (swollen) clays of the 2:1 structure family (e.g. montmorillonite, hectorite, or vermiculite; for nomenclature of clay minerals see Martin et al.1) are among the most important industrial minerals. World production of bentonites (rocks rich in montmorillonite) Crystal structure of hydrate Sodium-fluorohectorite 32 in 2006 amounted to 13,700,000 metric tons which were mostly used as rheological additives for drilling fluids and civil engineering, as foundry sand binder, and as adsorptive for instance in pet litter.2 More recently, more advanced applications such as microporous hybrid materials,3,4 functional films for optoelectronic packaging,5 halogen-free flame retardants,6 and nanofillers in composites 7 had been established. All these applications depend crucially on the hydration state of the clays and swelling probably is the most important feature of expandable 2:1 layered silicates. Despite the industrial importance of hydrated clay phases and despite intensive research over decades, 8-17 details of the one-layer hydrate structure are under debate. This is due to two handicaps intrinsic to natural clays: Hydration is critically dependent on two factors, the hydration enthalpy of interlayer cations and the charge density. For natural 2:1-clays like montmorillonite isomorphic substitution responsible for the layer charge clusters into domains and the charge density is inhomogenous. As a consequence of charge heterogeneity, individual interlayers in any singular clay crystal will realize different states of hydration. It is common that at a given relative humidity (r.h.) zero-, one-, and two-layer hydrates are found concomitantly in the same crystal. The random interstratification of different basal spacings renders even a 1-dimensional Fourier analysis of the electron density distribution in the interlayer space difficult. Besides interstratification, structure solution is even more severely hampered by planar defects. Intercalated water acts as a kind of lubricant and this is why most hydrated clay phases are turbostratically disordered. Adjacent layer stacked into a crystal are randomly rotated or shifted, the phase is not fixed, a unit cell cannot be defined, and scattering is completely diffuse. A comprehensive description of the structure of these hydrated phases would have to deliver information about the coordination of interlayer cations, the relative position/phase relationship of adjacent 2:1-layer (interlayer displacement),18 and the “interaction” pattern between the interlayer species (cations and water) and the basal oxygen atoms comprising the interlayer. Although an amazingly detailed picture of the structure of the interlayer could be derived employing 1-dimensional Fourier synthesis of X-ray and neutron diffraction data, possibly in combination with NMR data and different computer simulation methods,19-22 experimental evidence for interlayer displacement and specific interactions between interlayer species and the silicate layer require 3-dimensionally or at least semi-ordered “crystals” that are only little affected by planar defects (stacking disorder). There is, however, Crystal structure of hydrate Sodium-fluorohectorite 33 only a limited number of “single crystal” refinements of hydrated phases available in the literature applying some rare occasions of semi-ordered vermiculites (Santa Olalla, Spain and Carl Moss Ranch, Llano County, Texas) showing significantly less stacking faults and consequently much reduced diffuse scattering.23,24 In addition to remaining stacking disorder, the electron density of these natural vermiculites is, however, affected by mixed occupation of all cationic positions (octahedral, tetrahedral, and interlayer). Consequently, crystal structure refinement in all cases had to be restricted to certain classes of reflections that are least affected by the stacking disorder patterns (k=3n). Disorder patterns and structures of two-layer hydrates of Naand Mg-vermiculites was comprehensively discussed by Suquet and Perzerat, and de la Calle et al. applying PXRD and Weissenberg-camera single crystal data.10,11 In these studies, the one-layer hydrate was found to be completely disordered and details of the interlayer structure could consequently not be resolved. The two-layer hydrate was found to be semi-ordered and a structural model of the interlayer space was deduced.Please note, that all these “single crystal” refinements used standard programs that are incapable to take diffuse scattering into account. Consequently, the information about disorder patterns hidden in the diffuse scattering is ignored and only the main structure of the 3-dimensionally ordered volume of the crystals is refined. More recently, the Rietveld refinement of a X-ray powder diffraction (PXRD) trace of semi-ordered Santa Olalla vermiculite was refined with DIFFaX+, a program that indeed also takes stacking disorder and diffuse scattering into account.25 To reduce the complexity we sought to synthesize a well ordered fluorovermiculite (vermiculite where the hydroxyl group is replaced by fluorine) of nominal composition, [Na0.85]inter[Mg2.15Li0.85]oct[Si4]tetO10F2, which we prefer to refer to as highly charged fluorohectorite (Na-hect) (hectorite where the hydroxyl group is replaced by fluorine) because the material lacks the Tschermak substitution typical for vermiculites. Fortunately, for this synthetic Na-hect stacking disorder and thus diffuse scattering are indeed reduced to a level that allows a full crystal structure refinement applying all hkl reflections delivering full structural details for both, the one-layer and the two-layer hydrates. Experimental Section The fluorohectorite used, was synthesized via melt synthesis.16,26,27 The high purity reagents (in total ~4 g) of SiO2 (Merck, fine granular, calcined), MgF2 (chempur, 99.99%), MgO (alfa aesar 99.95 %), Li2SiO3 (alfa aesar 99.95 %), and Na2O-2SiO2-glass were weighed into a molybdenum crucible in an Ar atmosphere in accordance with a stoichiometric composition Crystal structure of hydrate Sodium-fluorohectorite 40 these differences are due to the different type of clay and charge pattern or whether it indeed would suggest that the forcefield parameters applied would have to be further refined. Part of the difference can certainly also be attributed to the different temperatures that the data were collected at. Table 1 Crystallographic data and details of the single crystal structure refinements of 1WL and 2WL of synthetic Na-hect Crystal data Formula unit [Na0.7·xH2O] i nte r [Mg2.3Li0.7]oc t [Si4]te t O10F2 Formula weight = 387.22 (g/mole) (without interlayer water) MoKα radiation (λ = 0.71073 Å) graphite monochromator T = 173 K 1WL 2WL a = 5.2434(10) Å a = 5.2432(10) Å b = 9.0891(18) Å b = 9.0870(18) Å c = 12.165(2) Å c = 15.064(3) Å β = 93.92(3)° β = 96.42(3) ° V = 578.4(2) Å3 V = 713.2(2) Å3 Monoclinic. C2/m (No. 12) Monoclinic. C2/m (No. 12) Z = 2 Z = 2 Plate colorless Plate colorless 0.25mm x 0.20mm x 0.02mm 0.25mm x 0.20mm x 0.02mm Data collection STOE IPDS I diffractometer Completeness of 2θ= 0.98 Completeness of 2θ = 0.98 2006 measured reflections 2477 measured reflections 597 independent reflections 734 independent reflections 384 reflections with I>2σ(I) 426 reflections with I>2σ(I) R in t = 0.181 Rin t = 0.226 θmax = 25.90° θmax = 25.93° θmin = 3.36° θmin = 2.72 h = -5 → 6 h = -5 → 6 k = -11 → 10 k = -10 → 10 l = -14 → 14 l = -18 → 18 Refinement applying SHELXTL 5.1 (Bruker AXS). Refinement on F2 Refinement on F2 R[F2 > 2σ(F2)] = 0.109 R[F2 > 2σ(F2)] = 0.1150 wR(F2) = 0.2881 wR(F2) = 0.2787 S = 1.110 S = 1.042 w = 1/[σ2(Fο2) + (0.20000P)2+0.000P] where P = (Fο2 + 2 Fc2)/3 w = 1/[σ2(Fο2) + (0.20000P)2+0.000P] where P = (Fο2 + 2 Fc2)/3 Δ ρmax = 1.58 eÅ3 Δ ρmax = 1.12 eÅ3 Δ ρmin = -0.71 eÅ3 Δ ρmin = -0.76 eÅ3 61 parameters 73 parameters Crystal structure of hydrate Sodium-fluorohectorite 41 Contrary to the 1WL, for the 2WL the interlayer cations reside at the central plane of the interlayer space as has been suggested by others23,31 before and also supported by the Fo-map of the interlayer space (see Fig. S4). The relative position of lower and upper tetrahedral sheet encompassing the interlayer region (stacking order) as viewed along c* is presented in (Fig. 3A) and (Fig. 3B) for 2WL and 1WL, respectively. The observed location of sodium cations for 1WL and 2WL relative to the lower tetrahedral sheet is depicted in (Fig. 4). For 2WL upper and lower hexagonal cavity are stacked face to face. Contrary to mica structures where this arrangement is assured by interlayer cations in the center intruding into the hexagonal cavities on both sides, the cavities are not occupied by interlayer cations in the 2WL structure. Instead, the Na+ resides above the tetrahedrons (see also (Fig. S4)). Fig. 3 Comparison of the relative position of lower (black) and upper (grey) tetrahedral sheet encompassing the interlayer region (stacking order) as viewed a long c*: 2WL (A) and 1WL (B). As pointed out by Beyer and von Reichenbach31 two sets of Na+ sites depicted as m1 and m2 in (Fig. 4A) exist. With a charge density of x = 1.0 p.f.u. only half of these possible Na+ sites are occupied, with x = 0.7 p.f.u., as we found for Na-hect, even less than half are occupied. Not surprisingly, structure refinement results in an equal statistical occupation of both sites, m1 and m2 as is also obvious from the Fo-map (see Fig. S5). Beyer and von Reichenbach31 have proposed that m1 and m2 sites are concomitantly occupied in the same interlayer space resulting in zig-zag-chains of edge-sharing octahedral running along a. Beyer and von Reichenbach31 had to propose this ordering pattern in order to match octahedral coordination of Na+ interlayer cations with a H2O/Na molar ratio of only 4. While this motif is widespread for µ-hydroxy-bridges we are not aware of other examples of µ-aquo-bridges of Na+ Crystal structure of hydrate Sodium-fluorohectorite 42 in the literature. Moreover, we determined a H2O/Na molar ratio of close to six allowing to propose isolated [Na(H2O)6]+. We therefore suggest that the electron density seen in the Xray experiment is interpreted in an alternative way: For any individual interlayer space, either purely m1 or purely m2 sites are occupied and m1 and m2 interlayers are stacked randomly with equal probability. Fig. 4 Position of hydrated Na+ interlayer cations relative to lower hexagonal cavity; A: 2WL hydrate the position m1 and m2, B: 1WL hydrate position m3 along c* projection. Please note that non-occupied m3 sites host water molecules coordinated to Na+ in contact with the upper tetrahedral sheet. This packing pattern of [Na(H2O)6]+ complex cations is shown in (Fig. 4A). Each Na+ in the interlayer is coordinated by six oxygen of water (Ow) (the average of Na-Ow bond length 2.45 Å). The coordinating oxygen atoms are located in two slightly corrugated planes above and below the plane of Na+ (Fig. 2). As pointed out by Beyer and von Reichenbach,31 with x = 1.0 pf.u. two densely packed planes of interlayer water result (Fig. S6). However, with x = 0.7 p.f.u., some Na+ sites are not occupied and the then non-coordinated water molecules are expected to relax. This might explain why Ferrage et al.15 had to apply a Gaussian-shaped distribution in their 1-dimensional Fourier synthesis. Similarly, Argülles et al.25 introduced interstitial, non-coordinated water sites. The stacking order of adjacent 2:1 layer is assured by well defined hydrogen bonding motifs between interlayer [Na(H2O)6]+ and the silicate layers. Each coordinated water molecule is able to connect to one basal oxygen atom of the tetrahedral sheet via hydrogen bonding (Fig. 5). The distance between the two oxygen atoms connected by hydrogen bonding was 2.90 - 3.03Å. Crystal structure of hydrate Sodium-fluorohectorite 43 Fig. 5 Hydrogen bonding between interlayer [Na(H2O)6]+ and tetrahedral sheets fixing the stacking order in 2WL Na-hect. The stacking order puts the basal oxygen atoms of the two tetrahedral sheets encompassing the interlayer space on top of each other. In order to allow for concomitant connection of the octahedral interlayer species to both sides of the interlayer space, the two triangles of oxygen atoms forming [Na(H2O)6]+ need to be rotated relative to the triangle of the basal oxygens of the tetrahedral sheets. As was already obvious from the PXRD trace, the 1WL suffers more severely from stacking faults as compared to the 2WL. This is in line with observations for vermiculite where the 1WL material has been labeled disordered.39 Consequently, the knowledge of interlayer structures is much more limited as compared to the 2WL. However, as shown in the inset of (Fig. 1), some comparatively sharp symmetric reflections are superimposed on the asymmetric λ-shaped 11/02-band indicating that the bulk material also contained a view much better ordered crystals. Applying these maxima a unit cell could be indexed and refined even for the 1WL (a= 5.2430 Å, b= 9.0851 Å, c=12.2150 Å, β=94.24°). By screening a large number of crystals, we were able to identify relatively well ordered individuals that allowed a single crystal structure refinement. The unit cell found for this single crystal is in close agreement with the one refined from PXRD. As became already obvious from the metric of the unit cell the stacking order found for Na-hect differed significantly from what has been Crystal structure of hydrate Sodium-fluorohectorite 44 reported for the 1WL of vermiculite.39 While these authors report a monoclinic angle of 90°, we get β = 93.92(3)° clearly indicating that the relative position of tetrahedral sheets encompassing the interlayer space was different (Fig. 3B). The hexagonal cavities are not arranged opposite of each other but are shifted by 2.62 Å relative to each other corresponding to an interlayer displacement of a/2. Na+ occupies m3 positions (Fig. 4B). As will be explained next, this shifting is required for the coordination of the interlayer cation by basal oxygen atoms of the two tetrahedral sheets encompassing the interlayer space. In total 4 Na+ sites, all partially occupied (≈ 0.35) are located in the unit cell. Clearly, some of the distances to water sites are too short to be occupied concomitantly in the same interlayer space. It would not be expected that ordering of interlayer species in a particular interlayer space influences the ordering in adjacent interlayer spaces. However, the X-ray beam averages within the coherence length over all possible positions and the electron density is artificially superimposed into an average interlayer space. A tentative assignment of superimposed electron densities into an ordering pattern that makes sense chemically is attempted in the following (Fig. 6): The coordination observed for Na+ is unusual. It involves both basal oxygen atoms of the tetrahedral sheet and interlayer water. In the direction of the displacement of the interlayer cation towards the tetrahedral sheet, Na+ resides above the hexagonal cavity and is coordinated by 6 basal oxygens with distances ranging between 3.22 Å and 3.24 Å. On the opposite side of the interlayer region a single siloxan bridge is located at a rather long distance of 3.52 Å which was therefore not regarded to be part of the coordination sphere. The coordination of Na+ is rather completed by water molecules residing at the central plane resulting in 9-fold coordination ([Na(Ob)6(H2O)3]+). In (Fig. 6) we choose the three closest water (2.45-2.55 Å) positions to be coordinated. These three oxygen atoms are involved in hydrogen bonding to basal oxygens (2.72-3.60 Å). While this interaction pattern explains why the interlayer space is bridged in a well defined mode resulting in the interlayer displacement of a/2, some arbitrariness remains in the assignment due to the many alternative oxygen positions generated by the four alternative Na+ sites clearly visible in the Fo-map of 1WL (Fig. S3). Also, it was not clear whether indeed adjacent upper and lower Na+ sites are occupied concomitantly in the same interlayer or whether these alternate sites are rather segregated into different interlayer spaces. Crystal structure of hydrate Sodium-fluorohectorite 45 Fig. 6 Tentative coordination of interlayer Na+ and connecting mode of adjacent silicate layers for 1WL of Na-hect. Disorder mode of 1WL Na-hect As has been pointed out in the literature, stacking faults may significantly alter properties and in consequence applications of clays.40,41 Therefore, we briefly focus on the diffuse scattering containing the information on such disorder modes. As pointed before, the 2WL structure is only little effected by disorder and only little diffuse scattering is apparent in the reciprocal lattice (Fig. S1, S2). In agreement with the shape of the PXRD traces (Fig. 1), in the reciprocal lattice of 1WL diffuse scattering is much more prominent as compared to the 2WL. Moreover, only some hk-rods are affected by the disorder while others only show sharp Bragg reflections, suggesting that the disorder patterns are commensurate with the lattice (for comparison see (Fig.1) in Slade and Stone42). Clearly, 1WL represents a semi-ordered structure. The most prominent semi-ordered stacking is observed for micas where ±b/3 shifts occur frequently and reflections with k=3n remain sharp. Please note that tetrahedral rotation for the synthetic Na-hect is close to zero generating an ideal hexagonal pseudo-symmetry for interlayer species. Therefore, disorder modes will be available that are not feasible for vermiculites.24,25 In 1WL a different disorder mode was realized as clearly indicated by the distribution of diffuse scattering in the reciprocal lattice space: 02l, 42l, 24l, 06l, and 46l are diffuse, while 22l, 62l, 04l, 44l, and 26l are sharp. As shown in (Fig. 7), translations of the upper layer by combinations of ±a/4 and ±b/4 create the same environment for the interlayer cations suggesting that random shifts of that type will generate energetically degenerate modes of stacking. This degeneracy in turn is responsible for the stacking disorder observed. Crystal structure of hydrate Sodium-fluorohectorite 46 Fig. 7 Illustration of energetically degenerate stacking modes for semi-ordered 1WL explaining the observed diffuse scattering. Translation of upper tetrahedral sheet (grey) relative to lower tetrahedral sheet (black) with [(+a/4) + (+b/4)] or [(-a/4) + (+b/4)] provide a similar environment for interlayer species. Conclusion By melt synthesis swelling Na-hect could be synthesized that upon hydration converts to semi-ordered (1WL) and ordered (2WL) hydrates. The significant reduction of stacking faults in these synthetic hydrates allowed for the first time to solve the structure of the one-layer hydrate. The interlayer cations reside off the central plane in a 9-fold coordination of 6 basal oxygen atoms and three water molecules. The stacking order is determined by hydrogen bonding of the latter to the second tetrahedral sheet. For the two-layer hydrate structures proposed for vermiculites in the literature were in large confirmed. However, modifications in details like ordering of alternative sites for [Na(H2O)6]+ were suggested. Acknowledgements The authors thank the Bayerisches Geoinstitut, Bayreuth, Germany, for the WDX measurement. This work was supported financially by the graduate school ‘Structure, Reactivity and Properties of Oxide Materials’ within the Elitenetzwerk Bayern, the Deutsche Forschungsgemeinschaft (SFB 840), and the University of Aleppo. Crystal structure of hydrate Sodium-fluorohectorite 47 1 R. T. Martin, S. W. Bailey, D. D. Eberl, D. S. Fanning, S. Guggenheim, H. Kodama, D. R. Pevear, J. Srodon and F. J. Wicks, Clays Clay Miner., 1991, 39, 333-335. 2 D. D. Eisenhour and R. K. Brown, Elements, 2009, 5, 83-88. 3 A. Baumgartner, K. Sattler, J. Thun and J. Breu, Angew. Chem. Int. Ed., 2008, 47, 16401644. 4 M. Stöcker, W. Seidl, L. Seyfarth, J. Senker and J. Breu, Chem. Commun., 2008, 629631. 5 M. W. Möller, T. Lunkenbein, H. Kalo, M. Schieder, D. A. Kunz and J. Breu, Adv. Mater., 2010, 22, 5245-5249. 6 M. R. Schutz, H. Kalo, T. Lunkenbein, A. H. Groschel, A. H. E. Muller, C. A. Wilkie and J. Breu, J. Mater. Chem., 2011, 21, 12110-12116. 7 M. W. Möller, D. Hirsemann, F. Haarmann, J. Senker and J. Breu, Chem. Mater., 2010, 22, 186-196. 8 J. F. Alcover and L. Gatineau, Clay Miner., 1980, 15, 193-203. 9 J. F. Alcover and L. Gatineau, Clay Miner., 1980, 15, 25-35. 10 H. Suquet and H. Pezerat, Clays Clay Miner., 1987, 35, 353-362. 11 C. Delacalle, H. Suquet and C. H. Pons, Clays Clay Miner., 1988, 36, 481-490. 12 N. Malikova, E. Dubois, V. Marry, B. Rotenberg and P. Turq, Z. Phys. Chem., 2010, 224, 153-181. 13 M. W. Möller, U. A. Handge, D. A. Kunz, T. Lunkenbein, V. Altstadt and J. Breu, Acs Nano, 2010, 4, 717-724. 14 T. J. Tambach, P. G. Bolhuis, E. J. M. Hensen and B. Smit, Langmuir, 2006, 22, 12231234. 15 E. Ferrage, B. Lanson, N. Malikova, A. Plancon, B. A. Sakharov and V. A. Drits, Chem. Mater., 2005, 17, 3499-3512. 16 H. Kalo, M. W. Möller, M. Ziadeh, D. Dolej and J. Breu, Appl. Clay Sci., 2010, 48, 3945. 17 V. Marry, E. Dubois, N. Malikova, S. Durand-Vidal, S. Longeville and J. Breu, Environ. Sci. Technol., 2011, 45, 2850-2855. Crystal structure of hydrate Sodium-fluorohectorite 48 18 S. Guggenheim, J. M. Adams, F. Bergaya, M. F. Brigatti, V. A. Drits, M. L. L. Formoso, E. Galan, T. Kogure, H. Stanjek and J. W. Stucki, Clays Clay Miner., 2009, 57, 134-135. 19 N. T. Skipper, K. Refson and J. D. C. Mcconnell, J. Chem. Phys., 1991, 94, 7434-7445. 20 F. R. C. Chang, N. T. Skipper and G. Sposito, Langmuir, 1995, 11, 2734-2741. 21 E. S. Boek, P. V. Coveney and N. T. Skipper, J. Am. Chem. Soc., 1995, 117, 1260812617. 22 N. T. Skipper, F. R. C. Chang and G. Sposito, Clays Clay Miner., 1995, 43, 285-293. 23 P. G. Slade, P. A. Stone and E. W. Radoslovich, Clays Clay Miner., 1985, 33, 51-61. 24 H. Shirozu and S. W. Bailey, Amer. Mineral., 1966, 51, 1124-1143. 25 A. Arguelles, M. Leoni, J. A. Blanco and C. Marcos, Amer. Mineral., 2010, 95, 126-134. 26 K. Kitajima, F. Koyama and N. Takusagawa, Bull. Chem. Soc. Jpn., 1985, 58, 1325-1326. 27 M. Stöcker, L. Seyfarth, D. Hirsemann, J. Senker and J. Breu, Appl. Clay Sci., 2010, 48, 146-153. 28 J. Breu, W. Seidl, A. J. Stoll, K. G. Lange and T. U. Probst, Chem. Mater., 2001, 13, 4213-4220. 29 S. W. Bailey, Amer. Mineral., 1982, 67, 394-398. 30 J. Breu, W. Seidl and A. Stoll, Z. Anorg. Allg. Chem., 2003, 629, 503-515. 31 J. Beyer and H. G. von Reichenbach, Clay Miner., 2002, 37, 157-168. 32 G. M. Sheldrick, Acta Crystallogr. A, 2008, 64, 112-122. 33 C. Delacalle and H. Suquet, Rev. Mineral., 1988, 19, 455-496. 34 C. Delacalle, A. Plancon, C. H. Pons, J. Dubernat, H. Suquet and H. Pezerat, Clay Miner., 1984, 19, 563-578. 35 A. Berghout, D. Tunega and A. Zaoui, Clays Clay Miner., 2010, 58, 174-187. 36 N. T. Skipper, A. K. Soper and J. D. C. Mcconnell, J. Chem. Phys., 1991, 94, 5751-5760. 37 R. P. Tenorio, M. Engelsberg, J. O. Fossum and G. J. da Silva, Langmuir, 2010, 26, 97039709. 38 E. Ferrage, B. A. Sakharov, L. J. Michot, A. Delville, A. Bauer, B. Lanson, S. Grangeon, G. Frapper, M. Jimenez-Ruiz and G. J. Cuello, J. Phys. Chem. C, 2011, 115, 1867-1881. Crystal structure of hydrate Sodium-fluorohectorite 49 39 C. Delacalle, H. Suquet and H. Pezerat, Clay Miner., 1985, 20, 221-230. 40 A. Plancon, Clay Miner., 2001, 36, 1-14. 41 T. Kogure, J. Elzea-Kogel, C. T. Johnston and D. L. Bish, Clays Clay Miner., 2010, 58, 62-71. 42 P. G. Slade and P. A. Stone, Clays Clay Miner., 1984, 32, 223-226. Large scale melt-synthesis of Sodium-fluorohectorite 56 Large scale melt-synthesis of Sodium-fluorohectorite 57 Large scale melt-synthesis of Sodium-fluorohectorite 58 Large scale melt-synthesis of Sodium-fluorohectorite 59 Large scale melt-synthesis of Sodium-fluorohectorite 60 Large scale melt-synthesis of Sodium-fluorohectorite 61 Large scale melt-synthesis of Sodium-fluorohectorite 62 Appendix 63 Appendix 3 How to Maximize the Aspect Ratio of Clay Nanoplatelets Hussein Kalo†, Michael W. Möller†, Daniel A. Kunz† and Josef Breu†* †Lehrstuhl für Anorganische Chemie I, University of Bayreuth, D-95440 Bayreuth, Germany * Corresponding author: Prof. Dr. Josef Breu, Phone: 0049921552531 Fax: 0049921552788 E-mail: [email protected] Nanoscale, DOI: 10.1039/C2NR31322G Synthesis of Lithium-fluorohectorite 64 How to Maximize the Aspect Ratio of Clay Nanoplatelets Hussein Kalo, Michael W. Möller, Daniel A. Kunz, and Josef Breu* Lehrstuhl für Anorganische Chemie I, University of Bayreuth, D-95440 Bayreuth, Germany * Corresponding author: Prof. Dr. Josef Breu, Phone: 0049921552531 Fax: 0049921552788 E-mail: [email protected] Abstract Melt-synthesis yielded Lithium-fluorohectorite (Li-hectx) with variable layer charge (x = 0.4, 0.6, 0.8, 1.0). Counter intuitively, both tactoid diameter and intracrystalline reactivity increased concomitantly with increasing layer charge. This way hectorites with very large diameters were obtained (d50% = 48 μm) that nevertheless still spontaneously delaminate when immersed into water and nano-platelets with huge aspect ratios (> 10000) are formed. Melt-synthesis of Li-hectx has been performed in an open glassy carbon crucible allowing for easy scaling to batches of 500g. These unprecedented huge aspect ratio fillers promise great potential for flame retardants and barrier applications. Keywords: Li-fluorohectorite, high aspect ratio, spontaneous delamination, barrier filler, osmotic swelling. Synthesis of Lithium-fluorohectorite 65 1. Introduction Aspect ratio of fillers is the key factor in improving properties of polymer nanocomposites.1-3 Fortunately, for clays the intracrystalline reactivity allows for extensive variation of properties and functions post synthesis.4-7 For instance the ionic conductivity may be optimized for battery applications,8 the mechanical properties of clay platelets may be tuned by controlled exfoliation,9 or aspect ratios may be maximized via osmotic swelling.10 Exfoliation into thinner tactoids or delamination into singular 2:1-lamellae represents an anisotropic top-down process (Figure 1a) that delivers nano-platelets with large aspect ratios α, which for instance may be used in gas barrier applications.11,12 Exfoliation may be triggered by applying mechanical force (platelets are sheared apart), which, however, concomitantly always will induce some breakage of tactoids.3 Aspect ratios will thus only increase during mechanical agitation if shearing wins over breakage (Figure 1b). Due to comparatively high hydration enthalpies of selected interlayer cations, for 2:1 clay minerals spontaneous delamination via osmotic swelling represents an attractive alternative to mechanical force for maximizing the aspect ratio. Numerous publications focused on the influence of interlayer cations such as Li+, Na+, K+, Mg2+, Ca2+, Sr2+, and Ba2+ on the hydration behaviour of swelling layered silicates,13,14 and usually the highest degree of hydration was observed with Li+. In that line, commercially available synthetic Li-hectorites (laponite-type clays) are well known for spontaneous delamination. Unfortunately, hydrothermal synthesis of these laponites yields very small tactoids (< 30 nm in diameter) and therefore even complete delamination delivers only mediocre aspect ratios. Therefore, these materials are very good rheological additives because the number of independent particles per mass is high and edge-face interactions are dominant interparticle interactions in suspension triggering gelling. Their performance as fillers is however dreadful because of the low maximum aspect ratios (< 30). For high aspect ratio nano-platelets, the diameter of tactoids have first to be significantly increased (Figure 1c) followed by a maximization of α by delamination (Figure 1d). We have previously shown that melt synthesis yields Na-hectorites with large tactoid sizes.15,16 While these Na-hectorites easily desagglomerated and exfoliated in platelets of some 10 nm thickness, this material did not spontaneously delaminate. Na-hectorites can of coarse easily be converted to Li-hect by repeated exchange with Li+ and this way the swelling in water can be improved. This ion exchange is, however, time consuming and we therefore sought after a direct access to Li-hect. Synthesis of Lithium-fluorohectorite 72 desired formulae (cited in parenthesis) while in the text the different samples are referred to by their nominal stoichiometry. Table 1. Chemical compositions of washed synthetic Li-hectx as determined via AAS and ICP-AES (after cation exchange with Co(en)3Cl3·3H2O). sample Mg wt% Li wt% Si wt% Co wt% Li-hect0.4 15.45 (15.48) 1.63 (0.68) 25.00 (27.52) 1.30 (1.93) Li-hect0.6 14.55 (13.86) 1.72 (0.99) 25.60 (26.70) 1.45 (2.80) Li-hect0.8 13.35 (12.35) 1.01 (1.28) 24.35 (25.93) 2.26 (3.54) Li-hect1.0 12.15 (10.91) 1.04 (1.56) 25.95 (25.21) 2.41 (4.41) 3.3. Cation exchange capacity (CEC) Deviations from the nominal formula as seen in the chemical analysis will of course also greatly influence the CEC. Table 2 lists the experimentally observed CECs together with expected values as calculated applying the desired formulae. The pronounced discrepancies between experimental and calculated CECs again were attributed to both, crystalline and possible amorphous impurities. Moreover, additionally a minor contribution might originate from the influence of layer edges that represent stoichiometric defects. There are numerous methods available for the determination of the CEC,32-34 the most convenient being colorimetric methods applying cationic dyes with high extinction coefficients. Please note that we could not use the standard complex cation, [Cu(trien)]2+, since according to Ammann et. al.34 [Cu(trien)]2+ is limited to medium and low layer charge for steric reasons. We therefore rather used [Co(en)3]3+ as dye that due to its higher charge is capable of also matching higher layer charge densities in a monolayer arrangement. Although CECs deviated significantly from nominal values (Table 2) the absolute values nevertheless consistently increased with nominal layer charge. The CEC of Li-hect0.6 is close to the value reported for the Corning hectorite (122 meq/100g ) for which a formula of [Li0.56]inter[Mg2.44Li0.56]oct[Si4]tetO10F2, has been given. As described in the patent US 4339540, the Corning material was made in a similar approach: After grinding the raw material in a ball mill, the mixture was melted at 1450 °C for 5 h without stirring. Then the melt was allowed to crystallize at 700 °C within 4 h. The product was purified by immersing the crystallized material in water while stirring, to remove soluble impurities by washing. Unfortunately, the patent gives no information what means had been taken to limit and or counterbalance the loss of Li by volatilization. Given the rather long reaction times, the effect must have been pronounced. Synthesis of Lithium-fluorohectorite 73 The maximum CEC of 185 meq/100 g observed for Li-hect1.0 is among the highest reported CEC in the literature for swelling 2:1 layered silicates. For comparison, a commercial Nafluorotetrasilicic mica (nominal layer charge x = 1.0 p.f.u) with an ideal formula of NaMg2.5Si4O10F2 (COOP Chemicals Co. Ltd., Somasif ME-100) was reported to have a CEC of around 120 meq/100g.35 Table 2. CECs of synthetic Li-hectx. Samples Li-hect0.4 Li-hect0.6 Li-hect0.8 Li-hect1.0 CECexp. (meq/100g) 70 118 130 185 CECcalc. (meq/100g) 106 159 213 268 3.4. Hydration behaviour Since our ultimate goal was to obtain high aspect ratio nanoplatelets by spontaneous delamination of Li-hectx in water; their intracrystalline reactivity was investigated in some detail by studying hydration in situ in a humidity chamber. Figure 3 illustrates the evolution of the basal spacings (d-values of the 001-reflection) of Li-hectx as a function of r.h. recorded at 25°C. It is commonly accepted that intracrystalline reactivity is diminished with increasing layer charge. Besides the coulomb attraction between interlayer species and silicate lamellae, the state of hydration is, however, determined by a second as important factor, the hydration enthalpy of the interlayer cation. Both factors will increase with layer charge, because the hydration enthalpy will scale with the number of interlayer cations. With interlayer cations having high hydration enthalpies like Li+ it is therefore a priori not clear which of the two factors is going to win and whether hydration will decrease as commonly assumed or rather increase with increasing layer charge. Synthesis of Lithium-fluorohectorite 74 Figure 3. Water desorption experiments of freeze-dried Li-hectx a) x=0.4, b) x=0.6, c) x=0.8 and d) x=1.0) at 25 °C as monitored in a humidity chamber via in-situ PXRD. The 2θ-range, where the 001-reflection of the different hydrates is expected, is recorded as function of the relative humidity while diffraction intensities are given as colour code. Arrows mark the dvalues for one-, two-, and three-layer hydrates. Generally, the transition between the different states of hydration for all synthetic materials was rather sharp and occurred in steps at well defined r.h. (Figure 3). The absence of randomly interstratified intermediates, as frequently observed for natural montmorillonites,36 indicated a uniform intracrystalline reactivity which in turn supported a homogenous charge density. Interestingly, only the material with the highest layer charge Li-hect1.0 adopted the 3 WL state of hydration above 80 % r.h. (d =18.6 Å) (Figure 3d). Even with prolonged equilibration times at 100 % r. h., the 3WL-hydrate could not be achieved for the three materials with lower charge densities (For completeness, we note that for Li-hect0.8 barely visible traces of 3WL-hydrate were observed (Figure 3c)). The maximum hydration state achieved for Lihect0.4-0.8 was the 2WL-hydrate (d= 15.5 Å) (Figure 3a-c). At ambient conditions (r.h. < 40 %) all materials adopted the 1WL-hydrate (d≈12 Å). Moreover, comparing Li-hect0.6 with Lihect0.8 , the latter showed the higher swelling power as indicated by the fact that the transition from the 1WLto the 2WL-hydrate was observed at lower r.h. These results prove that for Li- Synthesis of Lithium-fluorohectorite 75 hect the swelling power indeed increases with the number of interlayer cations due to the high hydration enthalpy of the small lithium cation. Figure 4. Swelling of Li-hect1.0 at higher water activities. PXRD patterns of varying H2O:Lihect1.0 ratios are shown. To extend the swelling studies for the most reactive hectorite to higher water activities, freeze-dried Li-hect1.0 was mixed with varying amounts of water. Figure 4 shows PXRD patterns of these H2O:Li-hect1.0-mixtures. With increasing amount of water available in the mixtures, the 00l peaks were shifted to lower diffraction angle (higher d-values). This gradual shift of d-values with increasing water activity available, indicated, that Li-hect1.0 might show osmotic swelling when suspended in an excess of water. Similar to what has been reported by Tamura et al., 37 even the highly hydrated states showed an integral series of 00l reflections. The d00l series for the mixture with a ratio of water to Lihectx (H2O:Li-hect1.0) of 3:1 was quite rational (d001= 70.1 Å, d002= 35.2 Å, and d003 = 23.8 Å). With ratios of H2O:Li-hect1.0 of 5:1 and higher the 001 peak is out of the 2θ-range that can be measured, but the 002 peak continued to be shifted to higher d-values with increasing amounts of water available (for 5:1: d002=44 Å). Synthesis of Lithium-fluorohectorite 76 Figure 5. AFM image (20 µm × 20 µm scan) of a typical sample of synthetic Li-hect1.0. For H2O:Li-hect1.0– ratios > 10 basal reflections could no longer be observed, indicating that the tactoids might have delaminated. AFM images were in line with this interpretation of the PXRD pattern as mostly delaminated platelets could be seen after suspending Li-hect1.0 in deionized water (Figure 5) (please see also Fig S1 in supporting information). Beginning exfoliation/delamination might have already been indicated with ratios of H2O:Li-hect1.0 < 10 by the much broadened basal reflections as compared to the 1WL-hydrate (Figure 2) that has not been in contact with liquid water. For completely delaminated fluorohectorite a specific surface area of 800 m2.g-1 would be expected. It is, however, well known that high vacuum conditions required in N2-physisorption experiments induce massive re-aggregation into band-like structures and only a minor part of the potential surface is accessible for the probe gas. In line with this, we measured a BET-surface area as low as 12.3 m2.g-1 for a freeze dried sample of Li-hect10.38 Synthesis of Lithium-fluorohectorite 77 3.5. Particle size distribution. Aspect ratios are not only determined by the thickness of tactoids but of course also by their diameter (Figure 1c). Usually the diameter of tactoids of layered silicates increases with increasing layer charge p.f.u.38,39 The synthetic Li-hectx clearly followed this trend. As indicated by SLS measurements (Table 3 and Figure 6), a correlation was observed between median particle size and layer charge. SEM images of the materials confirmed this trend (Figure 7). The SLS measurements were performed in aqueous dispersions; the PSDs therefore are representative for the bulk material. Moreover, it has been shown by Goossens40 that the lateral extensions of tactoids correlate well with the hydrodynamical radius obtained from SLS. Considering the observed median values of 20 to 40 µm and assuming complete delamination by osmotic swelling, average aspect ratios of these hectorites are certainly significantly above 1000 and may range up to more than 10000 as confirmed by the AFM-images of a typical sample shown in Figure 5. Table 3. Median particle sizes of synthetic Li-hectx as measured by SLS. Sample Li-hect0.4 Li-hect0.6 Li-hect0.8 Li-hect1.0 d50% Particle size μm (SLS) 19 27 30 48 Figure 6. Particle size distributions of the Li-hectx as analyzed by SLS. Synthesis of Lithium-fluorohectorite 78 Figure 7. SEM images of pristine Li-hect x as obtained by synthesis (a: x = 0.4; b: x = 0.6; c: x = 0.8; d: x =1.0). 4. Conclusion Melt-synthesis yielded Li-hect x of variable layer charge. Luckily and somewhat counter intuitively, both tactoid diameter and intracrystalline reactivity increased concomitantly with increasing layer charge. This way hectorites with huge tactoid diameters were obtained (d 50% = 48 μm) that spontaneously delaminated by osmotic swelling when immersed into deionized water. Nano-platelets consisting of a singular 2:1 lamella of approximately 1 nm thickness and a diameter of more than 40 μm could thus easily be obtained. These unprecedented huge aspect ratio fillers promise great potential for flame retardants and barrier application. Proof of concept papers on barrier properties and flame retardancy have already been published. 2,42,43 Acknowledgements This work was financially supported by the Deutsche Forschungsgemeinschaft (SFB 840) and the University of Aleppo (Syria). References 1 K. Tamura, S. Yokoyama, C. S. Pascua and H. Yamada, Chem. Mater., 2008, 20, 22422246. 2 M. W. Möller, T. Lunkenbein, H. Kalo, M. Schieder, D. A. Kunz and J. Breu, Adv. Mater., 2010, 22, 5245-5249. 3 T. Cao, P. D. Fasulo and W. R. Rodgers, Appl. Clay Sci., 2010, 49, 21-28. 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Nakazawa, Langmuir, 1999, 15, 5509-5512. 38 F.Bergaya, B.K.G.Theng and G.Lagaly, Handbook of Clay Science, Elsevier, Amsterdam, 2006, vol 1. 39 S. Taruta, R. Obara, N. Takusagawa and K. Kitajima, J. Mater. Sci., 2005, 40, 5597-5602. 40 A. Meunier, Clay Miner., 2006, 41, 551-566. 41 D. Goossens, Sedimentology, 2008, 55, 65-96. 42 M. R. Schütz, H. Kalo, T. Lunkenbein, J. Breu and C. A. Wilkie, Polymer, 2011, 52, 3288-3294. 43 M. W. Möller, D. A. Kunz, T. Lunkenbein, S. Sommer, A. Nennemann and J. Breu, Adv. Mater., 2012, 24, 2142-2147. Synthesis of Lithium-fluorohectorite 81 TOC: By a novel melt synthesis hectorites with very large diameters were obtained that spontaneously delaminate when immersed into water. These nano-platelets with unprecedented huge aspect ratio (> 10000) promise great potential as fillers for flame retardants and barrier applications. Crystal structure of hydrate Sodium Brittle Mica 88 Fig. 1 A; Cut’s through the reconstructed reciprocal space. (A) hk0 showing the three domains 1 (blue), 3 (red), and 5 (black) that are related by the 3-fold rotation axis || [001]. (B) h0l exemplarily showing how each of these three domains is related to a corresponding domain by a 2-fold rotation axis || [001], where the 2-fold twin domains is completely overlap with h=3n. The composition of the synthetic Na4-F-mica was determined by (WDX) with acceleration voltage 15 kV, and a beam spot diameter of 1 μm on a Joel JXA 8200 spectrometer, which was calibrated against certified mineral standards (Si - andradite Ca3Fe2Si3O12, O - SiO2, F - fluorite CaF2, Na - albite NaAlSi3O8,AlSpinel MgAl2O4, Mgsynthetic Enstatit Mg2[Si2O6]). The counting time was 20 s at the peak position and 10 s on each side of the peak position. The very same crystal that has been used for structure refinement (Table 1) was fixed in resin on a glass slide and coated with carbon. The composition of the synthetic layered silicate was normalized to Mg6 per formula unit. Additionally, the Na+, Mg2+, and Al3+ content of the bulk material was confirmed independently by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Two samples of about 20 mg of dry synthetic Na4F-mica were weighed into a clean Teflon flask of 150 mL volume. After addition of 1.5 mL 30 wt. % HCl (Merck), 0.5 mL of 85 wt. % H3PO4 (Merck), 0.5 mL 65 wt. % HNO3 (Merck) and 1 mL of 48 wt. % HBF4 (Merck) the sample was digested in a MLS 1200 Mega microwave digestion apparatus for 6.5 min and heated at 600W (MLS GmbH, MikrowellenLabor-Systeme, Leutkirch, Germany). The closed sample container was cooled to room temperature and the clear solution was diluted to 100 mL in a volumetric flask and analyzed. The water content of one-layer hydrated of Na4-F-mica was determined gravimetrically. Approximately 100 mg of equilibrated one-layer hydrated samples were dried at 250 °C at reduced pressure (2.0*10-2 mbar) for 24 h. The weight loss during drying was attributed to Crystal structure of hydrate Sodium Brittle Mica 89 interlayer water. The cation exchange capacity (CEC) of synthetic Na4-F-mica was determined by the potassium chloride method. A 0.2 g of synthetic Na4-F-mica was suspended in 25 mL of distilled water and 1 M potassium chloride solution was added. The mixture was shaken for 48h and then centrifuged, and the supernatant solution was collected. To ensure complete exchange of sodium, the cation exchange was repeated four more times. The Na+ content of the collected solutions was determined by Atomic Absorption Spectroscopy (AAS). SEM pictures are taken on a LEO 1530 FE-SEM at an operation voltage of 2 kV using in-lens detection modes. 3. Results and Discussions 3.1. Synthesis of Na4-F-mica Fig. 2 shows the PXRD pattern of zero-, and one-layer hydrate of synthetic Na4-F-mica. Two crystalline impurities could be identified: magnesium orthosilicate (Mg2SiO4) [25] and sodium aluminum silicate Na6Al4Si4O17 [26]. Indexing of both diffractograms was straight forward and gave the following unit cell parameters: a= 5.3322 Å, b=9.2477 Å, c=10.0621 Å, β=99.717° and a= 5.3248 Å, b=9.2688 Å, c=12.1597 Å, β=99.054° for zero-, and one-layer hydrate, respectively. The parameters are in close agreement with what had been published by Gregorkiewitz and Rausellcolom [18]: a= 5.34 Å, b=9.24 Å, c=9.97 Å, β=100.3° and a= 5.35 Å, b=9.24 Å, c=12.32 Å, β=98.5 for zero-, and one-layer hydrate, respectively. Good quality crystals with little diffuse scattering could quite easily be found for the onelayer hydrate allowing for structure refinements. Screening several “single crystals” of the zero-layer hydrate, however, showed massive diffuse scattering suggesting that extended planar disorder might have prevented successful structure refinements, much similar to what has been reported by Gregorkiewitz and Rausellcolom [18]. Obviously the stacking order could be improved significantly with hydration suggesting that the interlayer water helped to bridge the interlayer space in a more defined way and thus fixing the phase of adjacent 2:1silicate layers. Despite the planar defects, the zero-layer hydrate of Na4-F-mica nevertheless showed uniform intracrystalline reactivity. At a relative humidity of 43 % a rational 00l series with a basal spacing of d001= 12.1 Å was observed. Neither was residual intensity at the positions of the zero-layer hydrate nor was any signs of random interstratifications found indicating a uniform one-layer hydrate of synthetic Na4-F-mica. Further hydration steps of synthetic Na4-F-mica were not accessible (see discussion in crystal structure part). Crystal structure of hydrate Sodium Brittle Mica 90 Fig. 2 PXRD of zeroand onelayer hydrate pf Na4-F-mica. The one-layer water hydrate was measured at a relative humidity of 43%. Ticks indicate the impurity phases: AS: Sodium aluminum silicate Na6Al4Si4O17, MS: Magnesium silicate Mg2SiO4. The average chemical composition of synthetic Na4-F-mica as determined by WDX for three different crystal [Na3.3]inter[Mg6]oct[Si4.7Al3.3]tetO20F4. Using ICP-AES the ratio of Na+ : Al3+ : Mg2+ was found to be 3.3 : 6: 3.3. The CEC of synthetic Na4-F-mica as determined via Na+ in the supernatant after K+ exchange was 2.40 meq.g-1. SEM micrographs showed that melt synthesis of Na4-F-mica not only yielded highly crystalline but also coarse grained material (Fig. 3), contrary to established synthesis routes that reported particle sizes too small to pick single crystals (~5 µm) [5,6]. The water content as determined gravimetrically was H2O:Na= 1.6, a ratio in close agreement with what has been reported by Kodama et. al. (1.5) [27]. Furthermore, this ratio was confirmed by the occupation factors obtained in the structure refinement (see discussion below). Fig. 3 SEM images of synthetic Na4-F-mica. 3.2. Single crystal structure refinement Crystallographic data, experimental details of the structure refinement, and details of the single crystal structure refinement of one-layer water hydrate of synthetic Na4-F-mica are included in Table 1. Although some diffuse intensity was visible in reciprocal space, a Crystal structure of hydrate Sodium Brittle Mica 91 standard single crystal refinement program was applied that is not capable of handling diffuse (non-Bragg) scattering. Diffuse scattering had to be ignored but refinement was performed against the complete data set. A comprehensive description of the structure of hydrated phases of layered silicates has to deliver information about the coordination of interlayer cations, the relative position/phase relationship of adjacent 2:1-layer (interlayer displacement) [28], and the “interaction” pattern between the interlayer species (cations and water) and the basal oxygen atoms comprising the interlayer. For Na4-F-mica initial structural models cannot be deduced from known mica structure because it is not obvious where the additional interlayer cations would fit in. Certainly, the extra interlayer cations will have a crucial influence the distortion of tetrahedral sheet of sodium brittle mica and relative position of upper and lower tetrahedral sheet. To reduce the complexity of structure solution and to increase the precision of the structure refinement, we sought to synthesize a well ordered Na4-F-mica. Furthermore, the synthesis was optimized in order to yield large crystals Fig. 3 by applying high melting procedures and decreasing the cooling rate (as described in experimental section). Fortunately, for this synthetic Na4-F-mica stacking disorder and thus diffuse scattering are indeed reduced to a level that allows a full crystal structure refinement applying all hkl reflections delivering full structural details for the one-layer hydrate. Although the structure could be solved using the diffraction of the data of the main a stable and reliable refinement was possible only after having identified all twins triggered by the high pseudo symmetry of the interlayer. Gregorkiewitz and Rausellcolom [18] had already noticed that absences are consistent with the assumption of a twin of two individuals with a 1M cell and rotation of 120° between them. They, however, missed the second type of twinning operations based on the 2-fold rotation axis. Moreover, they noticed that Laue symmetry 2/m is violated which they interpreted reduced symmetry (1Tc) instead of the monoclinic polytype (1M). As Löwenstein´s rule would require an at least locally ordered distribution for an approximately 1:1 ratio of Al : Si, we reduced the symmetry to C 2 applying such an ordered model. Taking into account Löwenstein´s principle of avoidance of Al-O-Al geometries, the Si4+ and Al3+ substitution in the tetrahedral sheet was therefore assumed to be arranged in an ordered manner allowing each Al3+-tetrahedral to be surrounded by three Si4+–tetrahedral and vice versa when the Si : Al ratio is 1:1 [29,30]. An ordered arrangement would be expected to be corroborated by Si-O being significantly shorter than Al-O distances (1.63 and 1.78 Å, respectively). We see smaller differences in these distances in our refinement which we Crystal structure of hydrate Sodium Brittle Mica 92 attributed to be related to twinning because twin operations interrelate Al3+ and Si4+ sites. Additionally, the real composition showed a Al3+ : Si4+-ratio < 1 : 1 end hence some Si4+ must reside on Al3+ -sites. Alternatively, the ordering might be long range and the X-ray beam might be averaging over the ordered domains. The average observed Si-O and Al-O distance were 1.67 and 1.72 Å, respectively and clearly support an ordered tetrahedral sheet. Taking into account all six twin domains the refinement of the one-layer hydrate of Na4-Fmica was straight forward. Atomic displacement parameters (ADP) of all heavy atoms including the interlayer cations and the oxygen atoms of interlayer water could even be refined anisotropically. This underlines the good quality of the data sets. Occupancies of interlayer cations and water were freely refined. The water content as obtained by the refinement is in good agreed with values determined by gravimetric analysis. From the occupation factors obtained in the refinement the following compositions could be calculated: [Na3.4·6H2O]inter[Mg6]oct[Si4.6Al3.4]tetO20F4. Crystal structure of hydrate Sodium Brittle Mica 93 Table 1. Crystallographic data and experimental details of the structure refinement of onelayer water hydrate of synthetic Na4-F-mica. Crystal data Formula unit [Na3.4]inte r [Mg6]oc t [Si4.6Al3.4]te t O20F4·6H2O Formula weight (anhydrous) = 822.54 g/mol MoKα radiation (λ = 0.71073 Å) graphite monochromator T = 173 K a = 5.3520(11) Å b = 9.2700(19) Å c = 12.145(2) Å β = 98.35(3) ° V = 596.1(2) Å3 Monoclinic. C2 (No. 5) Z = 2 Plate colorless 0.2 mm x 0.2 mm x 0.05 mm Data collection STOE IPDS I diffractometer Completeness of 2theta= 0.89 1825 measured reflections 720 merged reflections 307 reflections with I>2σ(I) R int = 0.087 θmax = 25.33° θmin = 3.39° h = -6 → 6 k = -11 → 10 l = -14 → 14 Refinement SHELXTL Refinement on F2 R[F2 > 2σ(F2)] = 0.083 wR(F2) = 0.219 S = 1.31 w = 1/[σ2(Fο2) + (0.10000P)2+10.000P] where P = (Fο2 + 2 Fc2)/3 Δ ρmax = 1.691 eÅ3 Δ ρmin = -1.140 eÅ3 117 refined parameters Crystal structure of hydrate Sodium Brittle Mica 94 3.3. Structure of one-layer hydrate of Na4-F-mica The structure of one-layer hydrate Na4-F-mica projected along [100] is shown in Fig. 4. As suggested by Gregorkiewitz and Rausellcolom [18], the interlayer water was found to located in the middle of the interlayer space. Sodium interlayer cations were displaced from the center of the interlayer space towards the lower and upper tetrahedral sheet and on each side were located above the center of the distorted hexagonal cavity (Fig. 6). Fig. 4 Refined structure of one-layer hydrate of Na4-F-mica viewed along [100]. The relative position of the lower and the upper tetrahedral sheet was, however, crucially different from the structure purposed by Gregorkiewitz and Rausellcolom (compare Fig. 5 with Fig. 7 in ref. [18]). This is of course not surprising because the interlayer displacement cannot be determined reliably if massive disorder is present. Because of the limited quality of the crystals available to Gregorkiewitz and Rausellcolom the suggested interlayer displacement is erroneous. Unfortunately, this also affects details of the coordination of interlayer cations, and the “interaction” pattern between the interlayer species (cations and water) and the basal oxygen atoms comprising the interlayer. While Gregorkiewitz and Rausellcolom proposed the displacement to be +b/3, we found the lower tetrahedral sheet in relation to the upper tetrahedral sheet being shifted along [100] by approximately +a/2. As pointed out by Gregorkiewitz and Rausellcolom, having to accommodate 4 interlayer cations per unit cell excludes a face-to-face stacking of the hexagonal cavities of the tetrahedral sheets. Rather, adjacent layers must be displaced and consequently each hexagonal cavity becomes an independent site large enough to host a relatively small Na+. While both, shifts of +b/3 and +a/2 create four Na+ sites per unit cell, the coordination environment created for the interlayer cation will be different. Crystal structure of hydrate Sodium Brittle Mica 95 Fig. 5 Relative position of upper and lower tetrahedral sheet comprising the interlayer space as viewed along c* (please compare Fig. 4 for assignment of T1 and T2). Fig. 6 Structure of the hexagonal cavity of six connected tetrahedra showing the location of the Na+ interlayer cation (m3). The tetrahedron rotation angle (α) was calculated applying the equation:  ∑120°Φ   , α= 24°. As previously observed for non-swollen brittle mica, the hexagonal cavity of Na4-F-mica was found to be severely distorted (Fig. 6). This distortion of the hexagonal cavity is related to the pronounced misfit between the octahedral and the tetrahedral sheet and also reflects the field strength of the interlayer cation [31]. The α angle of tetrahedron rotation was calculated to be α ~24°, while the model suggested by Gregorkiewitz and Rausellcolom gives only α =14° [18]. The sodium cations are located in the m3 position above the center of the distorted hexagonal cavity. The sodium interlayer cations are coordinated to three basal oxygen atoms of the tetrahedral sheet (Ob) on one side and on the other side they are coordinated to three oxygen atoms (Ow) of intercalated water. The distance between Na+ and the three coordinated inner basal oxygen atoms of the tetrahedral sheet is ~ 2.50 Å, while due to the distortion the remaining three Crystal structure of hydrate Sodium Brittle Mica 96 oxygen atoms of the same hexagonal cavity are significantly further away (~3.2 Å). The distance between Na+ and the three oxygen atoms of water is ~ 2.57 Å, where the distance of Ow-Ow is 3.1 Å (Fig. 7A). The interlayer Na+ are thus coordinated by three planes of oxygen atoms, basal oxygens of lower and upper tetrahedral sheet and the interlayer water. Two sheets of isolated [Na(Ob)3(Ow)3] octahedra (blue and yellow in Fig. 7B) at different z-values are created, which are connected by shared water molecules. Blue and yellow each share one edge and one corner which gives a ratio of sodium : water of 1:1.5 which in turn corresponds to 5.1 water molecules per unit cell of Na4-F-mica (the sodium content is 3.4 mole per unit cell). This is in good agreement with the water content as determined gravimetrically (5.28 mole water per unit cell). Although the interlayer displacement is different the mixed Ow/ Ob coordination of the interlayer cation in some respects resembles motifs found for the onelayer hydrate of a highly charged Na-hectorite [32]. While these highly charged hectorites like vermiculites readily hydrate to the two-layer hydrate state at higher r.h., this was not observed for synthetic Na4-F-mica. This can certainly be related to the much higher Coulomb attraction. But it might also be related to crystal chemically reasons. In the two-layer hydrate of hectorites and vermiculites, the sodium cations in the central plane of the interlayer space. If one would have to pack four sodium cations per unit cell into one plane, the Na+ – Na+ distance would be much too short (3 Å). Fig. 7 Position of interlayer sodium cations, oxygens of the interlayer water and oxygens of the tetrahedral sheet along projection [001]. A) the lower octahedral sheet of [Na(Ob)3(Ow)3] (cyan color)and B) the lower and the upper octahedral sheet of [Na(Ob)3(Ow)3] (yellow color). Crystal structure of hydrate Sodium Brittle Mica 97 3.4. Stacking faults in one-layer hydrate Na4-F-mica As has been pointed out in the literature, stacking faults may significantly vary properties and in consequence applications of clays [33,34]. Clearly, scanning the reciprocal lattice space of the one-layer hydrate of Na4-F-mica revealed that the structure has some stacking faults on top of twinning. As observed by Gregorkiewitz and Rausellcolom [18] reflections with k=3n are sharp whereas all other hk-bands showed some diffuse intensity parallel to c* (Fig. 8). This pattern in reciprocal space is well known with micas and indicates a certain degree of ±b/3 stacking disorder in the one-layer hydrate of Na4-F-mica. As shown in Fig. 8, shifts of the upper layer by ±b/3 create the same environment for the interlayer cations suggesting that random shifts of that type will generate energetically degenerate modes of stacking. This degeneracy in turn is responsible for the stacking disorder observed. Moreover, since only some hk-rods are affected by the disorder while others only show sharp Bragg reflections, suggesting that the disorder patterns are commensurate with the lattice, even the domains of one-layer-hydrate of Na4-F-mica that suffer of disorder are not turbostratically disordered but still have a semi-ordered structure. The unusual high pseudo-symmetry reflected in several degenerate alternative stackings generating similar coordination of interlayer species that give rise to twinning and disorder is “probing the limits of the concept “crystal” as it was recently stated in a paper by Gerisch and Ruck [35]. Curriculum Vitae 104 Hussein Kalo Köllstr. 5 95447 Bayreuth Phone: 00499211612301 Mobile:004917664131900 [email protected] Personal Data Data of birth 04.06.1975 Place of birth Aleppo\Syria Family Status Married N ationality Syrian Study 03/ 2007 – present PhD student at department of inorganic chemistry I – university of Bayreuth. Title: „ M elt Synthesis, Structural, Characterization and Scaling of Swelling 2:1L ayer Silicate Materials” 05/2004 - 05/2006 Lecturer in Chemistry Department at the Faculty o f Science and Civil Engineering – university of Aleppo 11/2001 - 12/2003 M.Sc. Applied chemistry – faculty of science – university of Aleppo Title of project: “manufacturing of Silica refractory from Syrian sand” 09/2001 - 06/2000 diploma in applied Chemistry – faculty of science – University of Aleppo 09/1996 - 06/2000 Bachelor in applied Chemistry – faculty of science – University of Aleppo 09/1995 - 06/1996 Syrian Baccalaureate , scientific section -Aleppo - Syria Publication List 105 7. Publication List: The following publications are enclosed in this PhD thesis: 1Hussein Kalo, Wolfgang Milius and Josef Breu, Single Crystal Structure Refinement of Oneand Two-layer Hydrate of Sodium-Fluorohectorite. Journal of RSC Advance, DOI: 10.1039/C2RA20457F. 2Hussein Kalo, Michael W. Möller, Mazen Ziadeh, David Dolejš and Josef Breu, Large scale melt-synthesis in an open crucible of Na-fluorohectorite with superb charge homogeneity and particle size. Applied Clay Science, 2010, 48, 1-2, 1-290. 3Hussein Kalo, Michael W. Möller, Daniel A. Kunz and Josef Breu, How to Maximize the Aspect Ratio of Clay Nanoplatelets. Nanoscale, DOI: 10.1039/C2NR31322G. 4Hussein Kalo, Wolfgang Milius, Michael Bräu and Josef Breu, Synthesis and single crystal structure of hydrate sodium brittle mica, Hussein kalo, Manuscript submitted to Journal of Solid State Chemistry. 5Möller, M.W., Lunkenbein, T., Kalo, H., Schieder, M., Kunz, D.A. and Breu, J. (2010) Barrier Properties of Synthetic Clay with a Kilo-Aspect Ratio. Advanced Materials, 22, 5245-5249. 6Möller, M.W., Lunkenbein, T., Kalo, H., Schieder, M., Kunz, D.A. and Breu, J. (2010) Kilo Aspect Ratio clay platelets. Zeitschrift für anorganische und allgemeine Chemie. 2010, 636, 2113. 7Schütz, M.R., Kalo, H., Lunkenbein, T., Breu, J. and Wilkie, C.A. (2011a) Intumescentlike behavior of polystyrene synthetic clay nanocomposites. Polymer, 52, 3288-3294. 8Schütz, M.R., Kalo, H., Lunkenbein, T., Groschel, A.H., Muller, A.H.E., Wilkie, C.A. and Breu, J. (2011b) Shear stiff, surface modified, mica-like nanoplatelets: a novel filler for polymer nanocomposites. Journal of Materials Chemistry, 21, 12110-12116. 9Mazen Ziadeh, Bettina Chwalka, Hussein Kalo, Michael R. Schütz, Josef Breu, A Facile Approach for Producing High Aspect Ratio Fluorohectorite Nanoplatelets by Utilizing a Stirred Media Mill. Submit to Clay Minerals. 10Markus M. Herling, Hussein Kalo, Sebastian Seibt, Rainer Schobert, Josef Breu, Tailoring the Pore Sizes of microporous pillared interlayered clays. Manuscript will submit to microporous and mesoporous materials. Publication List 106 Patent: 1WO/2010/034408A1, synthetic Phyllosilicates Not Capable of swelling for Polymer Phyllosilicate Nanocomposite. Nennemann Arno, Bahnmueller Stefan, Breu Josef, Möller Michael, Kalo Hussein. 2WO/2011/089089A1, Method for producing Phyllosilicate Platelet having a High Aspect Ratio Breu Josef, Möller Michael, Kalo Hussein, Nennemann Arno. Erklärung 107 8. Erklärung Hiermit versichere ich, die vorliegende Arbeit selbstständig verfasst und keine anderen als die von mir angegebenen Quellen und Hilfsmittel benutzt zu haben. Ferner erkläre ich, dass ich weder an der Universität Bayreuth, noch an einer anderen Hochschule versucht habe, eine Dissertation einzureichen, oder mich einer Promotionsprüfung zu unterziehen. Bayreuth den, 8/10/2012 Hussein kalo