Cobalt minimisation in violet Co3P2O8 pigment

  Citation: Tena, M.Á.; Mendoza, R.; Trobajo, C.; García-Granda, S. Cobalt Minimisation in Violet Co3P2O8Pigment. Materials 2022,15, 1111. https://doi.org/10.3390/ ma15031111 Academic Editors: Teofil Jesionowski and Gerald Anthony Murphy Received: 19 November 2021 Accepted: 27 January 2022 Published: 31 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Cobalt Minimisation in Violet Co3P2O8Pigment Mª Ángeles Tena 1,*, Rafael Mendoza 2, Camino Trobajo 3and Santiago García-Granda 4,* 1Inorganic Chemistry Area, Inorganic and Organic Chemistry Department, Jaume I University, P.O. Box 224, 12006 Castellón, Spain 2Physical Chemistry Area, Scientific and Technical Services, Oviedo University-CINN, 33006 Oviedo, Spain; [email protected] 3Inorganic Chemistry Area, Organic and Inorganic Chemistry Department, Oviedo University-CINN, 33006 Oviedo, Spain; [email protected] 4Physical Chemistry Area, Physical and Analytical Chemistry Department, Oviedo University-CINN, 33006 Oviedo, Spain *Correspondence: [email protected] (M.Á.T.); [email protected] (S.G.-G.) Abstract: This study considers the limitations of cobalt violet orthophosphate, Co 3 P 2 O 8 , in the ceramic industry due to its large amount of cobalt. Mg x Co 3−x P 2 O 8 (0 ≤ x ≤ 3) solid solutions with the stable Co 3 P 2 O 8 structure were synthesised via the chemical coprecipitation method. The formation of solid solutions between the isostructural Co 3 P 2 O 8 and Mg 3 P 2 O 8 compounds decreased the toxically large amount of cobalt in this inorganic pigment and increased the melting point to a temperature higher than 1200 ◦ C when x ≥ 1.5. Co 3 P 2 O 8 melted at 1160 ◦ C, and compositions with x ≥ 1.5 were stable between 800 and 1200 ◦ C. The substitution of Co(II) with Mg(II) decreased the toxicity of these materials and decreased their price; hence, the interest of these materials for the ceramic industry is greater. An interesting purple colour with a* = 31.6 and b* = − 24.2 was obtained from a powdered Mg 2.5 Co 0.5 P 2 O 8 composition fired at 1200 ◦ C. It considerably reduced the amount of cobalt, thus improving the colour of the Co 3 P 2 O 8 pigment (a* = 16.2 and b* = − 20.1 at 1000 ◦ C). Co 3 P 2 O 8 is classified as an inorganic pigment (DCMA-8-11-1), and the solid solutions prepared were also inorganic pigments when unglazed. When introducing 3% of the sample (pigment) together with enamel, spreading the mixture on a ceramic support and calcining the whole in an electric oven, a colour change from violet to blue was observed due to the change in the local environment of Co(II), which could be seen in the UVV spectra of the glazed samples with the displacement of the bands towards higher wavelengths and with the appearance of a new band assigned to tetrahedral Co(II). This blue colour was also obtained with Co 2 SiO 4 , MgCoSiO 4 or Co 3 P 2 O 8 pigments containing a greater amount of cobalt. Keywords: minimisation of toxicity; Co3P2O8;α-Mg3P2O8; solid solutions; pigments 1. Introduction In the ceramic industry, dark blue is obtained by using compounds or solid solutions containing cobalt. The coordination number of the Co(II) cation is different among the crystalline structures used, and the colouration of powdered compositions changes between blue (CoAl 2 O 4 with a spinel structure and 88% tetrahedral Co(II), ICSD-260589; Co x Zn 2−x SiO 4 (0.005 ≤ x ≤ 1) with a willemite structure, and 100% tetrahedral Co(II) ICSD-186367) and violet, blue, or purple (Co 2 SiO 4 with olivine structure and 100% octahedral Co(II) ICSD-260092; Co 3 P 2 O 8 with a related olivine structure and 100% octahedral Co(II) ICSD-9850; stable Co 3 P 2 O 8 with 1/3 octahedral Co(II) and 2/3 Co(II) in C.N. = 5 ICSD-38259) [1]. Cobalt violet orthophosphate, Co 3 P 2 O 8 , is a pigment included in the DCMA Classification of Mixed Metal Oxide Inorganic Colour Pigments (DCMA-8-11-1) [ 2 ]. Its use in the ceramic industry is limited because of the large amount of cobalt in this compound. The Materials 2022,15, 1111. https://doi.org/10.3390/ma15031111 https://www.mdpi.com/journal/materials Materials 2022,15, 1111 2 of 14 formation of solid solutions between the isostructural Co 3 P 2 O 8 and Mg 3 P 2 O 8 compounds could be used to avoid the toxically large amount of cobalt in this pigment. In compositions that are rich in magnesium, these solid solutions could decrease the amount of cobalt, thus increasing the interest of these materials for the ceramic industry. The substitution of Co(II) with Mg(II) decreases the toxicity of these materials and decreases their price. The Co 3 P 2 O 8 compound melts at 1160 ◦ C [ 3 , 4 ]. Magnesium orthophosphate melts at 1357 ◦ C [ 5 ]. The melting point of some compositions of the solid solutions could be higher than 1200 ◦C. The formation of solid solutions through the substitution of ions in a crystalline structure changes the bond strength and modifies the colour of the materials. The colour blue is usually obtained from a tetrahedral CoO 4 geometry. The colour purple is obtained from LiZn 1−x Co x PO 4 (0 ≤ x ≤ 0.4) compositions with a LiZnPO 4 structure, which can be explained by the highly distorted geometry in the CoO 4 tetrahedra. The shorter CoO bonds increase the ligand field strength and lead to a blue-shifted absorption, thus developing an excellent purple pigment [ 6 ]. The increase in the amount of Co in these compositions decreases the negative b* value of the CIE L* a* b* parameters and the colour change to a violet hue due to the presence of the LiCoPO 4 phase with Co(II) ions in octahedral coordination together with the LiZnPO 4 phase with Co(II) ions in tetrahedral coordination [6]. The stable polymorph of the Co 3 P 2 O 8 compound with the monoclinic Mg 3 P 2 O 8 structure ( α phase) contains Co(II) ions in both a square planar pyramid and an octahedral coordination, Co1 in the 4e site and Co2 in the 2a site (ICSD-38259) [ 7 ]. A polymorph of the Mg 3 P 2 O 8 compound with the Ni 3 P 2 O 8 structure ( β phase) has also been reported (ICSD-9849) [1,8]. The transition temperature from β -Mg 3 P 2 O 8 to α -Mg 3 P 2 O 8 (ICSD-261231) is 1055 ◦ C [ 5 ]. In the stable Co 3 P 2 O 8 structure, all of the Co(II) ions are distributed in layers (bc planes), and these layers are joined by PO 4 tetrahedra. Figure 1 shows two unit cells of the stable Co 3 P 2 O 8 structure, the projection of nine unit cells in the (001) plane and the details of the oxygens around Co1 (CN = 5) and Co2 (CN = 6), with an edge shared by both polyhedra (two oxygens, O1 and O2, shared between Co1 and Co2). Two Co1-to-Co2 distances—2.896 and 3.165 Å—are shorter than the other Co-to-Co distances (ICSD-38259) [7]. The structure was drawn with the FPStudio program [9–11]. Materials 2022, 14, x FOR PEER REVIEW 3 of 15 Figure 1. Stable Co3P2O8 structure. The colour scheme to represent the atoms is O (-II): red, P(V): green, Co(II): blue with CN = 5, and lilac with CN = 6. In Co2−xZnxSiO4 solid solutions comprising Co2SiO4 and Zn2SiO4 compounds with olivine and willemite structures, the intense blue colour is kept with a considerably lower amount of cobalt [15]. In the same way, the colour of some compositions of solid solutions with the structure of cobalt orthophosphate with a small amount of cobalt could also be similar to the colour of the Co3P2O8 compound. It was expected that the change in the coordination number of the Co(II) ion would modify the colouration of the material with respect to that obtained with the olivine and willemite structures. The aim of this study was the formation of solid solutions from compositions comprising Co3P2O8 and Mg3P2O8 in order to obtain information about the composition and temperature with which the desired colour is developed and to minimise the toxic and expensive amounts of cobalt. 2. Experimental Methods MgxCo3−xP2O8 (0 ≤ x ≤ 3) compositions were synthesised from MgCl2·6H2O (Scharlau, extra pure), Co(NO3)2·6H2O (Acros Organic, 99%) and H3PO4 (Merck, 99%) via the chemical coprecipitation method. Stoichiometric amounts of MgCl2·6H2O, Co(NO3)2·6H2O and a 0.5 M solution of H3PO4 in water were added to 100 mL of water. Samples were vigorously stirred for 20 h at room temperature. Then, an aqueous ammonia solution (Panreac, 25%) was added under continuous stirring until reaching pH = 10. The experimental parameters were chosen in order to obtain precipitates of the cations before drying the material. pH = 10 was chosen because, although Co(OH)2 precipitates at pH > 7, Mg(OH)2 precipitates at pH > 9.5. Under these conditions, the materials were coprecipitated and dried in a stove at 65 °C to evacuate only the water. The Mg:Co:P molar ratio of the starting materials was preserved in this process. The dry samples were fired at 300, 600, 800, 1000 and 1200 °C for 6 h at each temperature. The development of the crystalline phases at different temperatures was studied by using XRD. The resulting materials were examined using a Panalytical X-ray diffractometer (Malvern Panalytical, Almelo, The Netherlands) with CuKα radiation. A structure Figure 1. Stable Co 3 P 2 O 8 structure. The colour scheme to represent the atoms is O (-II): red, P(V): green, Co(II): blue with CN = 5, and lilac with CN = 6. Materials 2022,15, 1111 3 of 14 Cobalt orthophosphate is soluble in Mg 3 (PO 4 ) 2 at about 1100 K (827 ◦ C) over the whole range of compositions [ 12 ]. Information about (Co, Mg) 3 (PO 4 ) 2 solid solutions with the α -Mg 3 P 2 O 8 structure prepared from mixtures of Co 3 P 2 O 8 and Mg 3 P 2 O 8 orthophosphates and fired at 800 ◦ C can be found in the bibliography [ 13 ]. The catalytic behaviour of Mg 3−x Co x (PO 4 ) 2 solid solutions (compositions fired at 500–700 ◦ C) shows that the substitution of magnesium with cobalt in the Mg 3 (PO 4 ) 2 structure leads to active and selective phases in the oxidative dehydrogenation of ethane and propane [ 14 ]. It seems possible to increase the thermal stability of Co 3 P 2 O 8 through the formation of these solid solutions with the substitution of Co(II) ions by Mg(II) ions. As far as we know, no information about MgxCo3−xP2O8solid solutions at T ≥1000 ◦C has been reported. In Co 2−x Zn x SiO 4 solid solutions comprising Co 2 SiO 4 and Zn 2 SiO 4 compounds with olivine and willemite structures, the intense blue colour is kept with a considerably lower amount of cobalt [ 15 ]. In the same way, the colour of some compositions of solid solutions with the structure of cobalt orthophosphate with a small amount of cobalt could also be similar to the colour of the Co 3 P 2 O 8 compound. It was expected that the change in the coordination number of the Co(II) ion would modify the colouration of the material with respect to that obtained with the olivine and willemite structures. The aim of this study was the formation of solid solutions from compositions comprising Co 3 P 2 O 8 and Mg 3 P 2 O 8 in order to obtain information about the composition and temperature with which the desired colour is developed and to minimise the toxic and expensive amounts of cobalt. 2. Experimental Methods Mg x Co 3−x P 2 O 8 (0 ≤ x ≤ 3) compositions were synthesised from MgCl 2· 6H 2 O (Scharlau, extra pure), Co(NO 3 ) 2· 6H 2 O (Acros Organic, 99%) and H 3 PO 4 (Merck, 99%) via the chemical coprecipitation method. Stoichiometric amounts of MgCl 2· 6H 2 O, Co(NO 3 ) 2· 6H 2 O and a 0.5 M solution of H 3 PO 4 in water were added to 100 mL of water. Samples were vigorously stirred for 20 h at room temperature. Then, an aqueous ammonia solution (Panreac, 25%) was added under continuous stirring until reaching pH = 10. The experimental parameters were chosen in order to obtain precipitates of the cations before drying the material. pH = 10 was chosen because, although Co(OH) 2 precipitates at pH > 7, Mg(OH) 2 precipitates at pH > 9.5. Under these conditions, the materials were coprecipitated and dried in a stove at 65 ◦ C to evacuate only the water. The Mg:Co:P molar ratio of the starting materials was preserved in this process. The dry samples were fired at 300, 600, 800, 1000 and 1200 ◦ C for 6 h at each temperature. The development of the crystalline phases at different temperatures was studied by using XRD. The resulting materials were examined using a Panalytical X-ray diffractometer (Malvern Panalytical, Almelo, The Netherlands) with CuK α radiation. A structure profile refinement was carried out using the Rietveld method (Fullprof.2k computer program) [9–11]. Diffraction patterns ranging between 6 and 110 (2 θ ) were collected by employing monochromatic CuK α radiation, a step size of 0.02 (2 θ ) and a sampling time of 10 s. The unit cell parameters, interatomic distances and Co(II) ion occupation in the two M1 and M2 sites in the stable Co 3 P 2 O 8 structure were determined in order to investigate the possible formation of solid solutions under these synthesis conditions. The initial structural information was taken from the Inorganic Crystal Structure Database [1]. The Co(II) ion sites and the transfer charge bands in the samples were studied by using UV–vis–NIR spectroscopy (diffuse reflectance). The ultraviolet–visible–near-infrared (UV–vis–NIR) spectra in the range of 200 to 2500 nm were obtained using a Jasco V-670 spectrophotometer and BaSO4as reference substance. The CIEL*a*b* colour parameters for the fired samples—L* is the lightness axis (black (0) → white (100)), a* is the green ( − ) → red (+) axis and b* is the blue ( − ) → yellow (+) axis [ 16 ]—were obtained with an X-Rite spectrophotometer (SP60, standard illuminant: D65, an observer of 10◦, and a reference sample of MgO). Materials 2022,15, 1111 4 of 14 To test their possible utility in the ceramic industry, the compositions fired at 1200 ◦ C were enamelled at 3% weight with a commercial glaze (SiO 2 –Al 2 O 3 –PbO–Na 2 O–CaO glaze) onto commercial ceramic biscuits. Many pigments were dissolved in this glaze. The colour of the material was lost when this occurred. Glazed tiles were fired for 15 min at 1065 ◦C, and subsequently, their UV–vis–NIR spectra and their CIEL*a*b* colour parameters were obtained. 3. Results and Discussion Table 1shows the evolution of the crystalline phases in the Mg x Co 3−x P 2 O 8 (0.0 ≤ x ≤ 3.0) compositions according to composition and temperature. A stable Co 3 P 2 O 8 structure was developed at 800 ◦ C, although small amounts of Mg 2 P 2 O 7 were also detected when x≥2.0 at this temperature. This crystalline phase was the only crystalline phase detected in all of the compositions at 1000 ◦ C and when x ≥ 1.5 at 1200 ◦ C. Compositions with x < 1.5 melted at 1200 ◦C, and they could not be removed from the crucible. Table 1. Evolution of crystalline phases with temperature in the Mg x Co 3−x P 2 O 8 (0.0 ≤ x ≤ 3.0) compositions. X 800 ◦C 1000 ◦C 1200 ◦C 0.0 C(s) C(s) 0.5 C(s) C(s) 1.0 C(s) C(s) 1.5 C(s) C(s) C(s) 2.0 C(s), M(vw) C(s) C(s) 2.5 C(s), M(vw) C(s) C(s) 3.0 C(s), M(w) C(s) C(s) Crystalline phases: C = stable Co 3 P 2 O 8 , Mg 3 P 2 O 8 , or solid solutions with the same structure; M = Mg 2 P 2 O 7 . Diffraction peak intensity: s = strong, vw = very weak. The unit cell parameters, volumes and interatomic distances in the stable Co 3 P 2 O 8 structure (isostructural with the α -Mg 3 P 2 O 8 structure) were obtained from the Mg x Co 3−x P 2 O 8 (0 ≤ x ≤ 3) compositions by using Rietveld’s method. Three examples of graphical results are shown in Figure 2. The changes in intensities due to compositional differences can be observed in this figure. Table 2includes the unit cell parameters and volume, Table 3 includes the M-O (M = Mg, Co) distances, and Table 4includes the P-O distances. The variations in the unit cell parameters in this structure at temperatures between 800 and 1200 ◦ C are shown in Figures 3and 4shows the variations in the M-O and P-O distances at 1000 ◦C. For the variations in the unit cell parameters with temperature, only small differences were detected when x > 1.5 (Figure 3). The decrease in the b unit cell parameter with x was consistent with the replacement of the Co(II) ion by the smaller Mg(II) ion (Table 2 and Figure 3). A slight negative departure of Vegard’s law in the b parameter associated with short-range cation ordering [ 17 ] can be observed in Figure 3. The small increases obtained in the a and c unit cell parameters indicate a structural distortion produced when Mg x Co 3−x P 2 O 8 solid solutions were formed with the stable Co 3 P 2 O 8 structure. This structural distortion is in accordance with the opposite variation in the M1-O distances with the variation in the composition (x) that can be observed in Figure 4. The most remarkable variations were the decrease in M1-O1 (the longest distance) and the increase in M1-O3 with x when x > 1.0. At 1000 ◦ C, the M-O distances were close to the Co-O distances in the Co 3 P 2 O 8 compound when x ≤ 1.5, and the M-O distances were close to the Mg-O distances in the Mg 3 P 2 O 8 compound when x > 1.5. When the amount of Mg(II) ions increased, the difference between the shortest and longest distances was smaller than in compositions in which the amount of Co(II) ions was greater. A gradual variation in the M-O distances with x was obtained at 800 and 1000 ◦ C, as well as in the compositions that did not melt at 1200 ◦ C. The great distortion in the position with C.N. = 5 in Co 3 P 2 O 8 (with Co1-O distances between 1.940 and 2.286 Å according to the ICSD-38259 data) was kept in the Materials 2022,15, 1111 5 of 14 compositions in which 0.0 ≤ x ≤ 1.5 at 800 and at 1000 ◦ C. This distortion in the M1 site decreased in accordance with the values of the Mg1-O distances in Mg 3 P 2 O 8 (with Mg1-O distances between 1.969 and 2.150 Å according to the ICSD-261231 data). The differences in the electron configuration with the seven electrons in the 3d orbitals in the Co(II) ion and with a small orbital penetration effect could explain the fact that the Co-O distance (1.940 Å) was shorter than the Mg-O distance (1.969 Å), although the ionic radius was greater in the Co(II) ion than in the Mg(II) ion. The greater covalence in the Co-O bond than in the Mg-O bond could explain the shorter Co-O distances. Materials 2022, 14, x FOR PEER REVIEW 6 of 15 Figure 2. The diffraction profile refinement obtained with Rietveld’s method for the Mg 0.5 Co 2.5 P 2 O 8 (x = 0.5), Mg 1.5 Co 1.5 P 2 O 8 (x = 1.5) and Mg 2.5 Co 0.5 P 2 O 8 (x = 2.5) compositions fired at 1000 °C. Figure 2. The diffraction profile refinement obtained with Rietveld’s method for the Mg 0.5 Co 2.5 P 2 O 8 (x = 0.5), Mg1.5Co1.5P2O8(x = 1.5) and Mg2.5Co0.5P2O8(x = 2.5) compositions fired at 1000 ◦C. Materials 2022,15, 1111 6 of 14 Table 2. Variations in unit cell parameters in the stable Co 3 P 2 O 8 structure obtained from fired MgxCo3−xP2O8(0.0 ≤x≤3.0) compositions. T (◦C) x a (Å) b (Å) c (Å) β(◦)V (Å3) 800 0.0 5.0629(1) 8.3618(2) 8.7888(2) 121.003(1) 318.92(1) 800 0.5 5.0655(1) 8.3377(2) 8.7938(2) 120.882(1) 318.75(1) 800 1.0 5.0691(1) 8.3131(1) 8.7999(2) 120.840(1) 318.40(1) 800 1.5 5.07242(9) 8.2887(1) 8.8076(1) 120.8274(9) 318.018(9) 800 2.0 5.07424(8) 8.2691(1) 8.8169(1) 120.8303(8) 317.675(8) 800 2.5 5.0734(1) 8.2505(2) 8.8237(2) 120.854(1) 317.07(1) 800 3.0 5.07529(9) 8.2370(1) 8.8316(2) 120.862(1) 316.93(1) 1000 0.0 5.06288(8) 8.3631(1) 8.7894(1) 120.9897(8) 319.027(8) 1000 0.5 5.06491(9) 8.3386(1) 8.7933(2) 120.8665(9) 318.78(1) 1000 1.0 5.0677(1) 8.3143(2) 8.7981(2) 120.820(1) 318.35(1) 1000 1.5 5.07022(8) 8.2914(1) 8.8056(1) 120.7983(8) 317.975(8) 1000 2.0 5.07251(6) 8.2704(1) 8.8146(1) 120.813(7) 317.588(7) 1000 2.5 5.07328(5) 8.25084(9) 8.82223(9) 120.8395(5) 317.073(6) 1000 3.0 5.07521(4) 8.23586(8) 8.83144(7) 120.8663(5) 316.860(5) 1200 1.5 5.07299(7) 8.2876(1) 8.8068(1) 120.8024(8) 318.034(7) 1200 2.0 5.07408(4) 8.26660(7) 8.81422(8) 120.8155(5) 317.520(5) 1200 2.5 5.07550(3) 8.24883(5) 8.82347(5) 120.8519(4) 317.139(3) 1200 3.0 5.07675(2) 8.23098(4) 8.83270(4) 120.8969(3) 316.712(3) Table 3. Variations in the M-O (M = Co, Mg) distances in the stable Co 3 P 2 O 8 structure with temperature in the MgxCo3−xP2O8compositions. T (◦C) X M1-O1 (Å) M1-O2 (Å) M1-O3 (Å) M1-O4 (Å) M1-O4 (Å) M2-O1 (x2) (Å) M2-O2 (x2) (Å) M2-O3 (x2) (Å) 800 0.0 2.274(6) 1.973(5) 2.065(7) 1.960(5) 1.996(5) 2.052(8) 2.195(4) 2.174(4) 800 0.5 2.273(6) 1.970(5) 2.076(7) 1.985(5) 1.992(5) 2.089(7) 2.184(4) 2.173(4) 800 1.0 2.253(5) 1.969(5) 2.070(7) 1.969(4) 1.999(4) 2.059(7) 2.183(4) 2.164(4) 800 1.5 2.246(5) 1.972(4) 2.074(6) 1.961(4) 1.995(4) 2.054(6) 2.185(3) 2.155(4) 800 2.0 2.242(5) 1.979(4) 2.091(6) 1.941(4) 1.985(4) 2.061(6) 2.188(3) 2.153(4) 800 2.5 2.228(5) 1.967(4) 2.102(4) 1.946(4) 1.991(4) 2.050(5) 2.187(3) 2.150(3) 800 3.0 2.226(5) 1.950(5) 2.098(6) 1.941(4) 1.980(4) 2.058(5) 2.182(3) 2.153(3) 1000 0.0 2.270(6) 1.977(5) 2.064(7) 1.969(5) 1.999(4) 2.057(7) 2.189(4) 2.172(4) 1000 0.5 2.265(6) 1.970(5) 2.069(7) 1.983(4) 1.992(4) 2.070(7) 2.188(4) 2.173(4) 1000 1.0 2.265(6) 1.961(5) 2.075(7) 1.967(5) 2.002(5) 2.058(7) 2.182(4) 2.171(4) 1000 1.5 2.246(5) 1.964(4) 2.072(6) 1.970(4) 1.997(4) 2.045(6) 2.179(3) 2.163(4) 1000 2.0 2.242(5) 1.972(4) 2.093(6) 1.954(4) 1.987(4) 2.057(6) 2.180(3) 2.158(4) 1000 2.5 2.190(4) 1.971(4) 2.088(5) 1.958(3) 1.986(4) 2.041(4) 2.177(3) 2.140(3) 1000 3.0 2.171(3) 1.960(3) 2.088(4) 1.953(3) 1.982(3) 2.035(3) 2.172(2) 2.138(2) 1200 1.5 2.229(6) 1.979(5) 2.078(7) 1.950(5) 1.983(5) 2.036(8) 2.201(4) 2.157(4) 1200 2.0 2.231(5) 1.956(4) 2.067(6) 1.965(4) 2.009(4) 2.037(6) 2.190(3) 2.171(4) 1200 2.5 2.203(4) 1.968(4) 2.085(5) 1.961(3) 2.000(4) 2.042(4) 2.162(3) 2.151(3) 1200 3.0 2.166(3) 1.956(3) 2.081(4) 1.965(3) 1.988(3) 2.033(3) 2.172(2) 2.138(2) Materials 2022,15, 1111 7 of 14 Table 4. Variations in the P-O distances in the stable Co 3 P 2 O 8 structure with temperature from the MgxCo3−xP2O8compositions. T (◦C) X P1-O1 (Å) P1-O2 (Å) P1-O3 (Å) P1-O4 (Å) 800 0.0 1.944(9) 1.527(5) 1.561(5) 1.512(5) 800 0.5 1.521(9) 1.520(5) 1.566(5) 1.517(5) 800 1.0 1.512(9) 1.526(5) 1.550(5) 1.527(5) 800 1.5 1.516(8) 1.521(4) 1.546(4) 1.519(4) 800 2.0 1.518(7) 1.508(4) 1.537(4) 1.521(4) 800 2.5 1.516(7) 1.507(4) 1.534(3) 1.515(4) 800 3.0 1.524(6) 1.520(4) 1.544(3) 1.527(4) 1000 0.0 1.502(9) 1.524(5) 1.563(5) 1.517(5) 1000 0.5 1.513(9) 1.520(5) 1.560(5) 1.517(5) 1000 1.0 1.510(9) 1.518(5) 1.562(5) 1.520(5) 1000 1.5 1.513(7) 1.516(4) 1.548(4) 1.527(4) 1000 2.0 1.514(7) 1.508(4) 1.531(4) 1.515(4) 1000 2.5 1.519(5) 1.513(3) 1.543(3) 1.524(3) 1000 3.0 1.532(4) 1.522(3) 1.550(3) 1.532(3) 1200 1.5 1.516(9) 1.526(5) 1.522(5) 1.520(5) 1200 2.0 1.520(7) 1.483(4) 1.549(4) 1.526(4) 1200 2.5 1.505(5) 1.511(3) 1.537(3) 1.548(3) 1200 3.0 1.530(4) 1.516(3) 1.549(3) 1.527(3) Materials 2022, 14, x FOR PEER REVIEW 7 of 15 Figure 3. Unit cell parameters in the Co 3 P 2 O 8 structure from the Mg x Co 3−x P 2 O 8 (0.0 ≤ x ≤ 3.0) compositions fired at 800, 1000 and 1200 °C. Table 3. Variations in the M-O (M = Co, Mg) distances in the stable Co 3 P 2 O 8 structure with temperature in the Mg x Co 3−x P 2 O 8 compositions. T (°C) X M1-O1 (Å) M1-O2 (Å) M1-O3 (Å) M1-O4 (Å) M1-O4 (Å) M2-O1 (x2) (Å) M2-O2 (x2) (Å) M2-O3 (x2) (Å) 800 0.0 2.274(6) 1.973(5) 2.065(7) 1.960(5) 1.996(5) 2.052(8) 2.195(4) 2.174(4) 800 0.5 2.273(6) 1.970(5) 2.076(7) 1.985(5) 1.992(5) 2.089(7) 2.184(4) 2.173(4) 800 1.0 2.253(5) 1.969(5) 2.070(7) 1.969(4) 1.999(4) 2.059(7) 2.183(4) 2.164(4) 800 1.5 2.246(5) 1.972(4) 2.074(6) 1.961(4) 1.995(4) 2.054(6) 2.185(3) 2.155(4) 800 2.0 2.242(5) 1.979(4) 2.091(6) 1.941(4) 1.985(4) 2.061(6) 2.188(3) 2.153(4) 800 2.5 2.228(5) 1.967(4) 2.102(4) 1.946(4) 1.991(4) 2.050(5) 2.187(3) 2.150(3) 800 3.0 2.226(5) 1.950(5) 2.098(6) 1.941(4) 1.980(4) 2.058(5) 2.182(3) 2.153(3) 1000 0.0 2.270(6) 1.977(5) 2.064(7) 1.969(5) 1.999(4) 2.057(7) 2.189(4) 2.172(4) 1000 0.5 2.265(6) 1.970(5) 2.069(7) 1.983(4) 1.992(4) 2.070(7) 2.188(4) 2.173(4) 1000 1.0 2.265(6) 1.961(5) 2.075(7) 1.967(5) 2.002(5) 2.058(7) 2.182(4) 2.171(4) 1000 1.5 2.246(5) 1.964(4) 2.072(6) 1.970(4) 1.997(4) 2.045(6) 2.179(3) 2.163(4) 1000 2.0 2.242(5) 1.972(4) 2.093(6) 1.954(4) 1.987(4) 2.057(6) 2.180(3) 2.158(4) 1000 2.5 2.190(4) 1.971(4) 2.088(5) 1.958(3) 1.986(4) 2.041(4) 2.177(3) 2.140(3) 1000 3.0 2.171(3) 1.960(3) 2.088(4) 1.953(3) 1.982(3) 2.035(3) 2.172(2) 2.138(2) 1200 1.5 2.229(6) 1.979(5) 2.078(7) 1.950(5) 1.983(5) 2.036(8) 2.201(4) 2.157(4) 1200 2.0 2.231(5) 1.956(4) 2.067(6) 1.965(4) 2.009(4) 2.037(6) 2.190(3) 2.171(4) 1200 2.5 2.203(4) 1.968(4) 2.085(5) 1.961(3) 2.000(4) 2.042(4) 2.162(3) 2.151(3) 1200 3.0 2.166(3) 1.956(3) 2.081(4) 1.965(3) 1.988(3) 2.033(3) 2.172(2) 2.138(2) Figure 3. Unit cell parameters in the Co 3 P 2 O 8 structure from the Mg x Co 3−x P 2 O 8 (0.0 ≤ x ≤ 3.0) compositions fired at 800, 1000 and 1200 ◦C. Materials 2022,15, 1111 8 of 14 Materials 2022, 14, x FOR PEER REVIEW 8 of 15 Table 4. Variations in the P-O distances in the stable Co3P2O8 structure with temperature from the MgxCo3−xP2O8 compositions. T (°C) X P1-O1 (Å) P1-O2 (Å) P1-O3 (Å) P1-O4 (Å) 800 0.0 1.944(9) 1.527(5) 1.561(5) 1.512(5) 800 0.5 1.521(9) 1.520(5) 1.566(5) 1.517(5) 800 1.0 1.512(9) 1.526(5) 1.550(5) 1.527(5) 800 1.5 1.516(8) 1.521(4) 1.546(4) 1.519(4) 800 2.0 1.518(7) 1.508(4) 1.537(4) 1.521(4) 800 2.5 1.516(7) 1.507(4) 1.534(3) 1.515(4) 800 3.0 1.524(6) 1.520(4) 1.544(3) 1.527(4) 1000 0.0 1.502(9) 1.524(5) 1.563(5) 1.517(5) 1000 0.5 1.513(9) 1.520(5) 1.560(5) 1.517(5) 1000 1.0 1.510(9) 1.518(5) 1.562(5) 1.520(5) 1000 1.5 1.513(7) 1.516(4) 1.548(4) 1.527(4) 1000 2.0 1.514(7) 1.508(4) 1.531(4) 1.515(4) 1000 2.5 1.519(5) 1.513(3) 1.543(3) 1.524(3) 1000 3.0 1.532(4) 1.522(3) 1.550(3) 1.532(3) 1200 1.5 1.516(9) 1.526(5) 1.522(5) 1.520(5) 1200 2.0 1.520(7) 1.483(4) 1.549(4) 1.526(4) 1200 2.5 1.505(5) 1.511(3) 1.537(3) 1.548(3) 1200 3.0 1.530(4) 1.516(3) 1.549(3) 1.527(3) Figure 4. M-O (circles) and P-O (squares) distances in the stable Co3P2O8 structure at 1000 °C. Figure 5 shows the variations in the occupation of Co(II) ions in the 2a (M1) and 4e (M2) sites for the results of the DRX profile refined with the Rietveld method. The initial occupation of each position was calculated by dividing the multiplicity of this position by Figure 4. M-O (circles) and P-O (squares) distances in the stable Co3P2O8structure at 1000 ◦C. Figure 5shows the variations in the occupation of Co(II) ions in the 2a (M1) and 4e (M2) sites for the results of the DRX profile refined with the Rietveld method. The initial occupation of each position was calculated by dividing the multiplicity of this position by the multiplicity of the general position in the space group and multiplying that quotient by the value of (3 − x)/3. In accordance with the multiplicity of the sites, a random distribution of Co(II) ions between the M1 and M2 sites corresponded to a ratio of 4/4 = 1 (2/3 of the total Co(II) ions in the M1 site; general 4e position with C.N. = 5) and 2/4 = 0.5 (1/3 of the total Co(II) ions in the M2 site; special 2a position with C.N. = 6). The calculated values were refined, and the results appear in Figure 5as E1 (experimental occupation in the M1 position) and E2 (experimental occupation in the M2 position). The experimental distribution of Co(II) between the two positions indicates that the Co(II) ion presents a higher preference for the site with C.N. = 5 (4e) than the Mg(II) ion at 800, 1000 and 1200 ◦ C. The octahedral positions (2a) were mostly occupied by Mg(II) ions. This result is in agreement with the literature on compositions at 800 ◦C [12,13]. Figure 6shows the UV–vis–NIR spectra of Mg x Co 3−x P 2 O 8 fired at 800, 1000 and 1200 ◦ C. The three bands at 1100, 580 and 500 nm from the fired Mg x Co 3−x P 2 O 8 composition were assigned to Co(II) in the octahedral site with ∆ /B < 13. These bands could be assigned to the first 4 T 1→4 T 2 (F) transition, to the second 4 T 1→4 A 2 (F) transition and to the third 4 T 1→4 T 1 (P) transition [ 18 ]. The bands at 1700–1717, 890 and 480 nm were assigned to Co(II) in a square-planar pyramid coordination ( 4 A 2→4 A 1 (F), 4 A 2→4 E(F) and 4 A 2→ 4E(P) transitions) according to the stable Co3P2O8solid solutions detected by XRD. Materials 2022,15, 1111 9 of 14 Materials 2022, 14, x FOR PEER REVIEW 9 of 15 the multiplicity of the general position in the space group and multiplying that quotient by the value of (3 − x)/3. In accordance with the multiplicity of the sites, a random distribution of Co(II) ions between the M1 and M2 sites corresponded to a ratio of 4/4 = 1 (2/3 of the total Co(II) ions in the M1 site; general 4e position with C.N. = 5) and 2/4 = 0.5 (1/3 of the total Co(II) ions in the M2 site; special 2a position with C.N. = 6). The calculated values were refined, and the results appear in Figure 5 as E1 (experimental occupation in the M1 position) and E2 (experimental occupation in the M2 position). The experimental distribution of Co(II) between the two positions indicates that the Co(II) ion presents a higher preference for the site with C.N. = 5 (4e) than the Mg(II) ion at 800, 1000 and 1200 °C. The octahedral positions (2a) were mostly occupied by Mg(II) ions. This result is in agreement with the literature on compositions at 800 °C [12,13]. Figure 5. Variations in the occupation of Co(II) ions with composition in sites of the stable Co 3 P 2 O 8 structure from the fired Mg x Co 3−x P 2 O 8 compositions. E1: experimental occupation in pentacoordinated sites (M1), E2: experimental occupation in octahedral sites (M2), T1: theoretical random occupation in M1 sites and T2: theoretical random occupation in M2 sites. Figure 6 shows the UV–vis–NIR spectra of Mg x Co 3−x P 2 O 8 fired at 800, 1000 and 1200 °C. The three bands at 1100, 580 and 500 nm from the fired Mg x Co 3−x P 2 O 8 composition were assigned to Co(II) in the octahedral site with Δ/B < 13. These bands could be assigned to the first 4 T 1  4 T 2 (F) transition, to the second 4 T 1  4 A 2 (F) transition and to the third 4 T 1  4 T 1 (P) transition [18]. The bands at 1700–1717, 890 and 480 nm were assigned to Co(II) in a square-planar pyramid coordination ( 4 A 2  4 A 1 (F), 4 A 2  4 E(F) and 4 A 2  4 E(P) transitions) according to the stable Co 3 P 2 O 8 solid solutions detected by XRD. Figure 5. Variations in the occupation of Co(II) ions with composition in sites of the stable Co 3 P 2 O 8 structure from the fired Mg x Co 3−x P 2 O 8 compositions. E1: experimental occupation in pentacoordinated sites (M1), E2: experimental occupation in octahedral sites (M2), T1: theoretical random occupation in M1 sites and T2: theoretical random occupation in M2 sites. In the spectra at 1000 ◦ C, the absorbance at 1100 nm (the 4 T 1→4 T 2 (F) transition was assigned to Co(II) in the octahedral site) was slightly smaller than at 890 nm (the 4 A 2→ 4 E(F) transition was assigned to Co(II) in a square-planar pyramid coordination) when x < 1.0 (small amount of Mg(II) in the compositions), although the most noticeable change was the decrease in absorbance with the composition due to the decrease in the total Co(II) in samples. The slight changes in the Co-O distances with composition (Figure 4) changed the ligand field strength, and a gradation of purple to violet colour was obtained (Table 5). Purple and violet colours were obtained when the stable Co 3 P 2 O 8 structure was developed from these compositions. These colourations were kept at 1000 and 1200 ◦ C when x ≥ 1.5, so pigments with a smaller amount of Co(II) were obtained. According to the CIE L* a* b* parameters (Table 5), an increase in the amount of red colour could be detected starting at 600 ◦ C with a* > 20 when 1.0 ≤ x ≤ 2.5 at 1000 ◦ C. Figure 7shows the variations in a* and b* with composition (x) at T ≥ 800 ◦ C. All of the compositions with x 6= 3.0 were violet or purple with a positive a* (red amount) and negative b* (blue amount). The composition with x = 1.5 (Mg 1.5 Co 1.5 P 2 O 8 ) showed the greatest amounts of red and blue at 1000 ◦ C, and the composition with x = 2.5 (Mg 2.5 Co 0.5 P 2 O 8 ) did so at 1200 ◦ C. The positions of the third transition band of the octahedral Co(II) ion, 4 T 1→4 T 1 (P), and the 4 A 2→4 E(P) band of the Co(II) ion in a square-planar pyramid coordination could be related to the variation of the amount of red colour (positive a*). The amount of blue colour (negative b*) could be related to the second transition band of the octahedral Co(II) ion, 4 T 1→4 A 2 . The greatest amounts of red colour (positive a*) and blue colour (negative b*) were obtained when the totality of Co(II) was almost entirely in the pentacoordinated site (x > 1.0), with a great distortion at 1200 ◦ C (Table 3). The optimal compositions could be established when 2.0 ≤ x ≤ 2.5 (highest a* and lowest b*) at this temperature.