Novel narrow band cyan-green phosphor LiK7[Li3SiO4]8:Eu2+ with enhanced suppression of second broad band emission

Novel Narrow Band Cyan-Green Phosphor LiK7[Li3SiO4]8:Eu2+with Enhanced Suppression of Second Broad Band Emission Daniel S. Wimmer,[a] Markus Seibald,[b] Dominik Baumann,[b] Simon Peschke,[b] Klaus Wurst,[a] Gunter Heymann,[a] Daniel Dutzler,[a] Amador Garcia-Fuente,[c, d] Werner Urland,[e] and Hubert Huppertz*[a] LiK7[Li3SiO4]8:Eu2+(LKLSO:Eu2+) was synthesized by a hightemperature solid-state reaction. It is accessible from K2CO3, Li2O, SiO2, and an Eu-source (Eu2O3) by conventional solid-state reaction in nickel crucibles under a constant flow of forming gas. The crystal structure of LKLSO has been determined by single-crystal X-ray diffraction and it crystallizes in the tetragonal space group I41=a(no. 88) with the lattice parameters a= 1560.83(5), c=1278.23(4) pm, and a volume of V= 0.31140(2) nm3. The compound is isostructural to NaK7[Li3SiO4]8: Eu2+but, in contrast, features an enhanced suppression of the second broad band emission through the specific channel filling in the crystal structure. Despite the observed positional disorder in the structure, the cyan-green phosphor shows a narrow band emission with a maximum at 511 nm and a fwhm of 45 nm (0.215 eV). Introduction In times of climate change, humankind is increasingly faced with important challenges. These include not only curbing global warming, but also the efficient use of energy and limited resources. In addition, worldwide energy consumption is growing annually, which is the reason for intensive research into energy-saving technologies. In the lighting sector, phosphor-converted light emitting diodes (pc-LEDs) were developed around the turn of the millennium, which are much more efficient than incandescent light bulbs and therefore consume much less energy.[1] For this application, a blue or UV-LED-chip is used and is coated with luminescent materials. These luminescent materials convert some of the blue light to higher wavelengths and hence, white light is then generated by additive color mixing.[2] Consequently, a lot of research has been done in the last decade in the field of phosphors and a significant number of novel phosphors were discovered and developed.[3,4] Especially alkaline earth (oxo)nitrido(alumo/magneso)silicates are scrutinized as red emitting phosphors such as CaAlSiN3:Eu2+(λmax =630 nm; fwhm=86 nm),[5] Ba[Mg3SiN4]:Eu2+(λmax =670 nm; fwhm=88 nm),[6] Sr4[LiAl11N14]:Eu2+(λmax =670 nm; fwhm=85 nm),[7] Li2Ca2[Mg2Si2N6]:Eu2+(λmax =638 nm; fwhm=86 nm),[8] and the ultra-narrow band red emitting phosphors SrLi2Al2O2N2:Eu2+ (SALON:Eu2+) (λmax =614 nm; fwhm=48 nm),[9] Sr[Mg3SiN4]:Eu2+ (λmax =615 nm; fwhm=43 nm),[10] and Sr[LiAl3N4]:Eu2+(SLA: Eu2+) (λmax =654 nm; fwhm=50 nm).[11] From the blue to the yellow spectral range, phosphors based on alkali lithosilicates, garnets, and alkaline earth (oxo)nitridoberyllates are developed.[12] The group of beryllate phosphors includes compounds such as AELi2[Be4O6]:Eu2+(AE=Sr, Ba) (λmax =454– 456 nm; fwhm=25 nm)[13] and b-SrBeO2:Eu2+(λmax =564 nm; fwhm=55 nm),[14] where Y3Al5O12:Ce3+(YAG:Ce3+) (λmax = �550 nm; fwhm=�100 nm) is an important representative of the garnet group.[15,16] The compound b-Si6-zAlzOzN8-z:Eu2+ (b-SiAlON:Eu2+) (λmax =536 nm; fwhm=63 nm) is often used as green phosphor.[17] From the field of alkali lithosilicates, which was already discovered by R. Hoppe in the late 1980s,[18–21] a relatively new substance class of Eu2+-doped luminescent materials (phosphors) emerged, containing compounds like RbLi[Li3SiO4]2:Eu2+(RLSO:Eu2+) (λmax =530 nm; fwhm= 42 nm),[22] RbNa3[Li3SiO4]4:Eu2+(λmax =471 nm; fwhm= 22.4 nm),[19,23,24] RbNa[Li3SiO4]2:Eu2+(λmax =523 nm; fwhm= 41 nm),[25] RbNa2K[Li3SiO4]4:Eu2+(λmax =480 nm; fwhm= 26 nm),[26] CsNa2K[Li3SiO4]4:Eu2+(λmax =485 nm; fwhm= [a] D. S. Wimmer, Dr. K. Wurst, Assoc. Prof. Dr. G. Heymann, Dr. D. Dutzler, Prof. Dr. H. Huppertz Institut für Allgemeine, Anorganische und Theoretische Chemie, Leopold-Franzens-Universität Innsbruck Innrain 80–82, A-6020 Innsbruck, Austria E-mail: [email protected] https://www.uibk.ac.at/aatc/mitarbeiter/hub/ [b] Dr. M. Seibald, Dr. D. Baumann, Dr. S. Peschke OSRAM Opto Semiconductors GmbH, Mittelstetter Weg 2, 86830 Schwabmünchen, Germany [c] A. Garcia-Fuente Departamento de Física Universidad de Oviedo Calle Federico García Lorca 18, 33007 Oviedo, Spain [d] A. Garcia-Fuente Nanomaterials and Nanotechnology Research Center CINN CSIC-Universidad de Oviedo Avda de la Vega 4–6, 33940 El Entrego, Spain [e] W. Urland Private Institute of Theoretical Chemical Physics Via Antonio Sciaroni 2, 6600 Muralto, Switzerland © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. Full Papers doi.org/10.1002/ejic.202100550 4470Eur. J. Inorg. Chem. 2021, 4470–4481 © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Freitag, 12.11.2021 2143 / 225114 [S. 4470/4481] 1 26 nm),[26] Na2K2[Li3SiO4]4:Eu2+(λmax =486 nm; fwhm= 20.7 nm),[27] K[Li3SiO4]:Eu2+(KLSO:Eu2+) (λmax =604 nm; fwhm= 150 nm), NaK7[Li3SiO4]8:Eu2+(NKLSO:Eu2+) (λmax =515 nm and 598 nm; fwhm=49 nm and 138 nm), Na[Li3SiO4]:Eu2+ (NLSO:Eu2+) (λmax =469 nm; fwhm=32 nm),[21,28–30] RbKLi2[Li3SiO4]4:Eu2+(λmax =474 nm and 532 nm; fwhm= 24.8 nm and 43.5 nm)[18,31] and Cs4-x-y-zRbxNayLiz[Li3SiO4]:Eu2+ (CRNLLSO:Eu2+) (λmax =473 nm and 531 nm; fwhm=25.2 nm and 58 nm).[32] These compounds exhibit mainly narrow band but also broad band single or double emissions in the blue and green spectral range with promising luminescence properties. In this article, we present a new phosphor of the family of alkali lithosilicates with the composition LiK7[Li3SiO4]8:Eu2+ (LKLSO:Eu2+). By incorporating additional Li+cations into the structure, it was possible to shift the emission band to shorter wavelengths and almost completely suppress the undesired broad band emission around 600 nm compared to NaK7[Li3SiO4]8:Eu2+(NKLSO:Eu2+). The effect of the additional Li+ cations in the channels of the structure and the resulting wavelength shift is discussed in later sections. The new compound shows a narrow band emission in the cyan-green spectral region, which is a novelty for this substance class, representing the first example with such interesting luminescence properties. Results and Discussion Crystal structure LiK7[Li3SiO4]8crystallizes in the space group I41=a(no. 88) and is isostructural to the phase NaK7[Li3SiO4]8(NKLSO)[28] and generally a structural variant of the UCr4C4-structure type.[33] In comparison to the compound NaK7[Li3SiO4]8, the unit cell of LKLSO with the lattice parameters a=1560.83(5), c= 1278.23(4) pm, and a volume of V=0.31140(2) nm3is larger than the one of the already known NKLSO. The lattice parameters in NKLSO are a=1555.57(8), c=1274.71(7) pm, and a volume of V=0.30845(4) nm3. The larger unit cell of LKLSO is probably the result of the occurring positional disorder, which will be explained in detail later. Details on the crystal-structure refinement are given in Table 1. Atomic coordinates, displacement parameters, and interatomic distances are listed in the Table 2, Table 3, and Table 4. The highly condensed network in the structure of LKLSO is built up from SiO4tetrahedra and LiO4tetrahedra, where the SiO4tetrahedra are isolated from each other and connected only via LiO4tetrahedra. Figure 1 shows that these tetrahedra share common corners and edges forming infinite channels along the crystallographic c-axis. The LiO distances within the tetrahedral units of the Li1, Li2, and Li5 positions vary from 191.3(2) to 216.3(2) pm and also the SiO bond lengths, which vary from 163.23(8) to 165.52(8) pm, are in good agreement with the sum of the ionic radii.[28,31,32,34] The LiO distances within the tetrahedra of the Li6, Li7, and Li8 position range from 190.9(2) to 262.2(3) pm. The corresponding coordination polyhedra can also be described as elongated trigonal pyramids exhibiting a 3+1 coordination, where the Li+cations are slightly displaced from the centers of the “LiO4tetrahedra”. In the unit cell, there are two different channels, designated as CH1 and CH2, which can be distinguished by their central cations. Due to the special order of these channels in the structure, a third, empty channel is formed, whose cavities are probably too small to host any metal cations (see Figure 1a). The channel CH1 is solely occupied with K+cations, represented by only one crystallographic potassium site (K1). In the center of the channel, there is a 41screw axis, which is not aligned with the K1 position leading to a slight offset of the central cation. Each K1 position is surrounded by two vierer rings,[35] consisting of three LiO4tetrahedra (orange polyhedra) and one SiO4tetrahedron (blue polyhedron). Due to the 41 screw axis, these vierer rings are connected via corners in such a way that the SiO4tetrahedra form a helix-like substructure along the crystallographic c-axis (see Figure 1b). The slight offset of the K1 position along the central axis leads to a variance of the KO bond lengths to overall seven O2anions with distances of 264.65(8) pm to 287.25(8) pm. The KO bond length to the eighth O2anion is 339.89(8) pm, resulting in a 7 +1 coordination of the K1 position. The channel CH2 contains three crystallographically distinguishable potassium sites (K2, K3, and K4) and two lithium sites (Li3 and Li4). In this channel, a complex positional disorder of the K2, K3, K4, Li3, and Li4 sites occurs (see Figure 1b). To Table 1. Crystal data and structure refinement of tetragonal LKLSO. Empirical formula LiK7[Li3SiO4]8 Molar mass/g·mol11183.87 Crystal system tetragonal Space group I41/a Powder data Powder diffractometer STOE Stadi P Radiation Mo-Kα1(λ=70.93 pm) a/pm 1556.32(4) c/pm 1278.62(5) V/nm30.3100(21) Single-crystal data Single-crystal diffractometer Bruker D8 Quest Kappa Radiation Mo-Kα(λ=71.073 pm) a/pm 1560.83(5) c/pm 1278.23(4) V/nm30.31140(2) Formula units per cell Z 4 Calculated density/g·cm32.525 Crystal size/mm30.100×0.060×0.060 Temperature/K 183(2) Detector distance/mm 40 Exposure time 0.5°/frame; 40 s/frame Absorption coefficient/mm112.944 F(000)/e 2304 Ø-range/deg 2.61–32.49 Range in hkl �23, �23, �19 Reflections total/independent 40679/2823 Rint 0.0169 Reflections with I�2σ(I) 2551 Rσ0.0367 Data/ref. parameters 2823/174 Absorption correction multi-scan Final R1/wR2[I�2σ(I)] 0.0211/0.0542 Final R1/wR2(all data) 0.0251/0.0557 Goodness of fit on F21.115 Largest diff. peak/hole/e·Å30.35/0.995 Full Papers doi.org/10.1002/ejic.202100550 4471Eur. J. Inorg. Chem. 2021, 4470–4481 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Freitag, 12.11.2021 2143 / 225114 [S. 4471/4481] 1 maintain charge electroneutrality of the structure, these specific sites cannot be fully occupied. The crystal structure refinement results in specific occupancies (site occupation factors) of K2= 0.38, K3=0.25, K4=0.12, Li3=0.11, and Li4=0.14. To confirm the occupations of these sites, four crystals from three different syntheses were measured by single-crystal X-ray diffraction. All crystal structure refinements showed the same results with minimal deviations so that the site occupation factors have been fixed to these values. Due to the local distances between the K+and the Li+cations, only five possibilities, on how these positions can be occupied, exist. The five possibilities are (K2, K4, K2), (K2, Li3a, K2), (K2, Li3b, K2), (Li4a, Li3a, K2), and (K2, Li3b, Li4b), leading to distances of about 250 pm between the K+and Li+cations. Otherwise, the distances between the K+ and Li+cations would be less than 200 pm, resulting in steric repulsions, because the sum of the ionic radii of the Li+and the K+cation is 238 pm (r(Li+(CN=4))=73 pm, r(K+(CN=8))= 165 pm).[36] The exact elucidation, on how these channels are occupied in the real structure by the five possibilities may vary from channel to channel and from unit cell to unit cell, but on average, the Li positions in the channels are occupied with 25% probability. Another possibility could be that Li-rich domains are formed in the real structure, as has already been postulated for other known alkali lithosilicates such as RbKLi2[Li3SiO4]4:Eu2+ and CRNLLSO:Eu2+.[31,32] This positional disorder in LKLSO is represented in the situation that the K+cations (K2, K3, and K4) possess an eightfold coordination sphere by the O2anions with KO bond Table 2. Wyckoff positions, atomic coordinates, and equivalent isotropic displacement parameters Ueq (Å2) of LKLSO (standard deviations in parentheses). Atom Wyckoff-Position x y z Ueq SOF K1 16f0.76306(2) 0.48998(2) 0.01369(2) 0.0106(6) 1 K2 8e1/2 1/4 0.65111(4) 0.01305(9) 0.38 K3 4b1/2 3/4 1/8 0.0248(2) 0.25 K4 4a1/2 1/4 3/8 0.0071(2) 0.12 Si1 16f0.59924(2) 0.56985(2) 0.12741(2) 0.00458(6) 1 Si2 16f0.56996(2) 0.39597(2) 0.50282(2) 0.00429(6) 1 O1 16f0.69502(5) 0.61535(5) 0.12563(6) 0.0060(2) 1 O2 16f0.55310(5) 0.60186(5) 0.01930(6) 0.0077(2) 1 O3 16f0.62410(5) 0.30576(5) 0.50739(6) 0.0076(2) 1 O4 16f0.59526(5) 0.44929(5) 0.60824(6) 0.0081(2) 1 O5 16f0.46719(5) 0.37366(5) 0.50420(6) 0.0082(2) 1 O6 16f0.55260(5) 0.60666(5) 0.23298(6) 0.0076(2) 1 O7 16f0.59644(5) 0.44337(5) 0.39357(6) 0.0078(2) 1 O8 16f0.60818(5) 0.46566(5) 0.13138(6) 0.0088(2) 1 Li1 16f0.5643(2) 0.3998(2) 0.2491(2) 0.0096(3) 1 Li2 16f0.5687(2) 0.4138(2) 0.7494(2) 0.0106(3) 1 Li3 8e1/2 1/4 0.453(2) 0.021(3) 0.11 Li4 8e1/2 1/4 0.526(2) 0.018(2) 0.14 Li5 16f0.6675(2) 0.6824(2) 0.2526(2) 0.0107(3) 1 Li6 16f0.5894(2) 0.5606(2) 0.8838(2) 0.0137(4) 1 Li7 16f0.6817(2) 0.3195(2) 0.3727(2) 0.0157(4) 1 Li8 16f0.5715(2) 0.5628(2) 0.3722(2) 0.0219(5) 1 Table 3. Anisotropic displacement parameters Uij (Å2) of LKLSO (standard deviations in parentheses). Atom U11 U22 U33 U23 U13 U12 K1 0.0083(1) 0.0123(1) 0.0111(1) 0.00535(8) 0.00068(7) 0.00179(7) K2 0.0118(2) 0.0084(2) 0.0190(2) 0 0 0.0006(2) K3 0.0051(2) 0.00516(2) 0.0642(4) 0 0 0 K4 0.0071(2) 0.0071(2) 0.0072(4) 0 0 0 Si1 0.0046(2) 0.0060(2) 0.0032(2) 0.00002(8) 0.00003(8) 0.00107(9) Si2 0.0045(2) 0.0046(2) 0.0038(2) 0.00035(8) 0.00001(8) 0.00020(8) O1 0.0054(3) 0.0071(3) 0.0056(3) 0.0000(2) 0.0004(2) 0.0011(2) O2 0.0064(3) 0.0126(3) 0.0041(3) 0.0006(2) 0.0008(2) 0.0002(3) O3 0.0091(3) 0.0070(3) 0.0065(3) 0.0003(2) 0.0005(2) 0.0032(2) O4 0.0123(3) 0.0075(3) 0.0044(3) 0.0008(2) 0.0003(3) 0.0009(3) O5 0.0047(3) 0.0116(3) 0.0082(3) 0.0017(3) 0.0002(2) 0.0006(2) O6 0.0070(3) 0.0119(3) 0.0041(3) 0.0002(2) 0.0008(2) 0.0001(3) O7 0.0117(3) 0.0074(3) 0.0042(3) 0.0005(2) 0.0005(2) 0.0014(3) O8 0.0135(3) 0.0065(3) 0.0064(3) 0.0004(2) 0.0003(3) 0.0032(3) Li1 0.0084(8) 0.0117(9) 0.0089(8) 0.0009(7) 0.0005(6) 0.0007(6) Li2 0.0091(8) 0.0137(9) 0.0090(8) 0.0013(7) 0.0003(7) 0.0009(7) Li3 0.034(9) 0.010(6) 0.019(9) 0 0 0.004(6) Li4 0.013(5) 0.013(5) 0.029(7) 0 0 0.003(4) Li5 0.0150(9) 0.0087(8) 0.0083(8) 0.0004(6) 0.0018(7) 0.0015(7) Li6 0.022(2) 0.0109(9) 0.0080(8) 0.0002(7) 0.0013(7) 0.0051(8) Li7 0.0130(9) 0.027(2) 0.0071(8) 0.0013(7) 0.0011(7) 0.0072(8) Li8 0.044(2) 0.015(1) 0.0072(9) 0.0014(7) 0.0018(9) 0.013(1) Full Papers doi.org/10.1002/ejic.202100550 4472Eur. J. Inorg. Chem. 2021, 4470–4481 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Freitag, 12.11.2021 2143 / 225114 [S. 4472/4481] 1 lengths between 259.13(8) and 288.37(8) pm, while the Li+ cations Li3 and Li4 prefer an approximately square-planar one. In LKLSO, the LiO bond lengths in this square-planar coordination range from 201.5(2) to 223.5(5) pm and are in good agreement with the coordination observed in the channels of RbKLi2[Li3SiO4]4:Eu2+.[19,31] The K4 site is surrounded by four SiO4and four LiO4tetrahedra, while the Li+cations Li3 and Li4 in the same cavity are enclosed by two SiO4and two LiO4tetrahedra. The K2 site shows an asymmetry in its surrounding. The K2 cation is incorporated in the cavity surrounded by two LiO4and six SiO4tetrahedra, dislocated from its central position away from the nearby Li+cations and SiO4 tetrahedra. The K3 site is the only position solely surrounded by LiO4tetrahedra. In comparison to CH1, CH2 has a different channel symmetry. K3 is positioned on a � 4 inversion axis and all the cation sites are aligned along this axis. A superstructure Table 4. Interatomic distances (Å) in LKLSO (standard deviations in parentheses). K1O1 2.6465(8) Si1O8 1.6330(8) Li4O5 2.015(2) 2× K1O8 2.6837(8) Si1O2 1.6365(8) Li4O3 2.136(2) 2× K1O4 2.6925(8) Si1O6 1.6373(8) Ø Li4O 2.076 K1O1 2.7611(8) Si1O1 1.6552(8) K1O7 2.8701(8) Ø Si1O 1.6405 Li5O8 1.917(2) K1O7 2.8721(8) Li5O2 1.929(2) K1O8 2.8725(8) Si2O4 1.6323(8) Li5O1 1.977(2) K1O4 3.3989(8) Si2O7 1.6334(8) Li5O6 2.163(2) Ø K1O 2.8497 Si2O5 1.6415(8) Ø Li5O 1.997 Si2O3 1.6433(8) K2O5 2.7411(8) Ø Si2O 1.6376 Li6O2 1.932(2) K2O5 2.7412(8) Li6O1 1.941(2) K2O6 2.8062(8) 2× Li1O3 1.918(2) Li6O3 1.982(2) K2O3 2.8078(8) 2× Li1O8 1.946(2) Li6O4 2.282(2) K2O2 2.8837(8) 2× Li1O7 2.031(2) Ø Li6O 2.034 Ø K2O 2.8097 Li1O5 2.048(2) Ø Li1O 1.986 Li7O4 1.909(2) K3O2 2.8033(8) 3× Li7O5 1.944(2) K3O2 2.8034(8) Li2O4 1.932(2) Li7O7 2.363(3) K3O6 2.7540(8) 4× Li2O6 1.934(2) Li7O7 2.604(3) Ø K3O 2.7787 Li2O1 1.975(2) Ø Li7O 2.205 Li2O2 2.141(2) K4O5 2.5913(8) 4× Ø Li2O 1.996 Li8O7 1.924(2) K4O3 2.7153(8) 4× Li8O5 1.961(2) Ø K4O 2.6533 Li3O5 2.102(5) 2× Li8O8 2.569(3) Li3O3 2.235(5) 2× Li8O4 2.622(3) Ø Li3O 2.169 Ø Li8O 2.269 Figure 1. Crystal structure of LiK7[Li3SiO4]8. Figure 1a illustrates the layered arrangement of the different channels (CH1, CH2) along the crystallographic c-axis. Figure 1b shows CH1 (top) occupied exclusively by potassium and CH2 (bottom) occupied by potassium and lithium including the positional disorder along the crystallographic a-axis. The LiO4tetrahedra are displayed in orange and the SiO4tetrahedra are displayed in blue. Full Papers doi.org/10.1002/ejic.202100550 4473Eur. J. Inorg. Chem. 2021, 4470–4481 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Freitag, 12.11.2021 2143 / 225114 [S. 4473/4481] 1 consisting of compositely modulated structures, as is found in the compound SrxLi2+xAl2-xO4:Eu2+,[37] can be excluded, since no reflections of diffuse scattering were observed in a regular distance between the main reflections in the reciprocal space. The difference in the structures of LKLSO und NKLSO runs in the occupation of CH2 and the resulting positional disorder. In NKLSO, there are two crystallographically distinguishable potassium sites and one sodium site, corresponding to the three crystallographically distinguishable potassium sites K3, K2, and K4 in LKLSO (see Figure 2a and Figure 2b). The sodium site in NKLSO corresponds to the K4 site in LKLSO possessing an eightfold coordination sphere of O2anions. The absence of the Li+cations in these channels induce no positional disorder in NKLSO. In fact, these structural differences affect the luminescence properties, which are discussed in the next section. Luminescence Most of the examinations of the luminescence properties were carried out on single-crystals. For the excitation spectrum and the determination of the thermal quenching behavior as well as the quantum efficiency, a powder sample had to be used. This could not be synthesized in pure phase, but is impurified with 6.5 wt% Li2SiO3(see Figure 3). At a nominal activator concentration of 2 mol% Eu2+, this compound can be excited with near-UV to blue light resulting in cyan-green emitting crystals of LiK7[Li3SiO4]8:Eu2+exhibiting a narrow band emission (λmax =511 nm) with a full width at half maximum (fwhm) of 45 nm (1734 cm1/0.215 eV) and a small broad band emission around 550 nm with a full width at half maximum (fwhm) around 100 nm (see Figure 4a). For comparison, the single-crystal emission spectrum of NKLSO:Eu2+[28] is additionally displayed in Figure 4b. Furthermore, an excitation spectrum of a powder sample, which exhibits the cyan-green emission at λmax =509 nm with a full width at half maximum (fwhm) of 42 nm (1651 cm1/ 0.204 eV), including the small broad band emission around λmax =550 nm, was measured. In the CIE-xy color space, this corresponds to the value x=0.192(1) and y=0.608(1). The excitation spectrum monitored at λem =509 nm exhibits a maximum at 377 nm (see Figure 5a and 5b). The measurements of the thermal quenching behavior (TQ) and the quantum efficiency (QE) were carried out on the same powder sample. In addition to an acceptable TQ behavior, the first unoptimized sample showed a QE of 56%, which is quite comparable to other reports in the literature also knowing that the QE can be dramatically increased by synthesis optimization. Although the maximum absorption of the powder sample is significantly shorter than 440 nm, the maximum quantum efficiency occurs at this higher wavelength. At T=225°C, the relative integrated intensity is reduced to half of the value at room temperature (see Figure 6). The compound NaK7[Li3SiO4]8:Eu2+has a quantum efficiency of 54% and the relative integrated intensity is reduced to 25% at a temperature of 225°C. Despite the fact that the quantum efficiency and thermal behavior of the new compound LKLSO:Eu2+is better with respect to the compound NaK7[Li3SiO4]8:Eu2+, which is Figure 2. Difference between NKLSO and LKLSO in CH2 alongside [� 100]. Figure 2a shows the positional disorder of lithium and potassium cations in LKLSO. Figure 2b represents the arrangement of sodium and potassium sites in NKLSO. The LiO4tetrahedra are displayed in orange and the SiO4tetrahedra are displayed in blue. Full Papers doi.org/10.1002/ejic.202100550 4474Eur. J. Inorg. Chem. 2021, 4470–4481 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Freitag, 12.11.2021 2143 / 225114 [S. 4474/4481] 1 possibly due the suppression of the broadband emission, especially the thermal behavior is much worse compared to the other known alkali lithosilicates. A possible reason for this could be the positional disorder in the structure on the one hand (variation of electron-phonon coupling in case of Eu2+-doping), and on the other hand, the Li+cations could be responsible for non-radiative transitions. It is conceivable that some of the energy is transferred to the mobile Li+cations and is no longer available for luminescence. In addition, charge balancing as a consequence of doping with the activator ion Eu2+creates vacancies/defects, which can also influence the overall electronic structure and the relative position of Eu2+energy levels. However, it should be taken into account that the measurements were performed on non-optimized samples and both the doping level with Eu2+, crystal size and quality, and the minor phases in the powder samples may also have an effect on these properties. In comparison, NKLSO:Eu2+has two different emission bands, one at 515 nm with a fwhm of 49 nm and another at 598 nm with a fwhm of 138 nm (see Figure 4b). For the double band emission, there exist three possible positions, which can be occupied by the activator ion Eu2+. The broad band emission at 598 nm is explained by the fact that the activator ion Eu2+ occupies the K1 site, also observed in the compound KLSO.[28] For the narrow band emission at 515 nm, there are two possible positions (K2 and K3). Both sites show a cube-like (K3) or a slightly deformed cube-like oxygen coordination (K2) and provide enough space hosting Eu2+. Since the coordination of these central alkali cations by the O2anions varies only less, the extended coordination sphere is considered for the luminescence properties. The Si4+cations have a much higher partial charge than the Li+cations, so they have a more distinct position in the anionic framework. Presumably, this leads to a stronger interaction with the central cation position. With regard to the extended coordination sphere, the K3 and K2 sites differ. K2 shows a trigonal prismatic and K3 a square planar surrounding by Si4+cations. Due to the narrow band emission, one of the two sites is obviously preferred, but the luminescence center cannot be localized. An occupation of the Na1 site by the activator ion Eu2+can be excluded, otherwise a blue emission would be observed as it is the case in Na[Li3SiO4]:Eu2+. Despite the positional disorder in the new compound LKLSO:Eu2+, only structurally analogue positions and the additional K4 site can be considered as luminescence centers. Since LKLSO:Eu2+shows a small broad band emission, an occupation of the K1 site as luminescence center seems reasonable. In relation to NKLSO:Eu2+, the broad band emission in LKLSO:Eu2+ shows a maximum around 550 nm, so the wavelength shift to shorter wavelengths could be caused by larger average interatomic distances between the K+cation and the O2 anions resulting in an enlarged Eu2+coordination. Since the broad band emission is strongly suppressed (see Figure 4a and Figure 3. Rietveld fit of the experimental powder diffraction data (black), the calculated pattern based on single-crystal data of LiK7[Li3SiO4]8is shown in red and the difference plot is shown in blue. The reflection positions of LiK7[Li3SiO4]8are shown in green and the reflection positions of the secondary phase Li2SiO3are represented in black. The reflections marked with an asterisk refer to an unidentified byproduct. Full Papers doi.org/10.1002/ejic.202100550 4475Eur. J. Inorg. Chem. 2021, 4470–4481 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Freitag, 12.11.2021 2143 / 225114 [S. 4475/4481] 1 Figure 4b), other positions are obviously much more preferred as doping sites for the narrow band emission. It is more likely that the activator ion Eu2+occupies the K3, K2, or the K4 position. In the case of K3 and K2, the Si4+cations form a square planar or a trigonal prismatic coordination sphere, which is relatively unaffected by the occupation of the surrounding cavities, implying a narrow band emission. Additionally, the activator ion Eu2+replaces monovalent cations inducing vacancies due to charge neutrality. Hence, one of the neighboring cation sites is not occupied and it seems more realistic that the charge neutrality is compensated by the absence of a neighboring Li+cation in the channel than a K+cation as observed in the already known compounds RbKLi2[Li3SiO4]4:Eu2+ and CRNLLSO:Eu2+. In comparison to NKLSO, the K4 site has also to be considered as luminescence center. Due to the superposition of K+and Li+cations, indicating a domain formation with Li-rich channels, it is conceivable that the activator ion Eu2+may also occupy the K4 position and charge neutrality is realized by the absence of two Li+cations in the square planar coordination resulting in a narrow band emission (see Figure 7). An occupation of the K4-like position by the activator ion Eu2+would probably lead to a structural relaxation resulting in a red-shifted emission compared to NLSO:Eu2+.[28,29] Therefore, all three sites could act as doping sites for the narrow band emission, however, it is assumable that only one of these sites is preferred, otherwise a broad or split emission profile should be observed. Figure 8 summarizes the four possible positions that can be occupied by the activator ion Eu2+including the distances to the O2anions. Based on SCXRD, an exact localization of the luminescence center in LKLSO:Eu2+is not possible due to the specific filling of CH2. According to the theory of Fang et al.,[39] the luminescence properties are determined by the cuboid size, in which the activator ion Eu2+is incorporated. In the substance class of the alkali lithosilicate phosphors, there are three types of regions, (I) �40 Å3, (II) 29–34 Å3, and (III) 26–27 Å3classified with respect to the cuboid size. They assume that the activator ion Eu2+is unable to dope in region (I), because these sites are too large. The cyan (blue) and green emission results from the incorporation of the activator ion Eu2+in the region (II) and region (III), respectively. In the new phosphor LKLSO:Eu2+, there are three sites K2, K3, and K4, which could act as doping sites for the narrow band emission. The cuboid sizes of the K2, K3, and K4 sites are calculated to be about 33.84, 32.42, and 28.81 Å3, respectively. The program Vesta 3[40] was used to calculate the cuboid size. In relation to the other alkali lithosilicates, the K4 site in LKLSO is smaller than the K sites normally observed in this substance class, which typically range from 31.5 to 34.2 Å3 resulting in a blue emission between 470 and 490 nm. Region (III) includes a range between 26–27 Å3, where a green emission between 520–530 nm is observed. The observed emission at 511 nm could be the result of the K4 site (cuboid size: 28.81 Å3) being situated between region (II) and region (III). So, considering the cuboid size of all three sites, the K4 site seems to be preferred as doping site for the activator ion Eu2+, otherwise a blue emission should be observed. To confirm this hypothesis, we have performed simulations based on DFT of the LKLSO structure doped with Eu2+on the different K sites available. We have considered periodic supercells containing ~300 atoms and with several initial structures to take into account the different local surroundings of the sites due to partial occupations. We have found however that the doping energies are mainly dependent of the particular doping site, and they are mostly independent of other structural factors. After relaxation, the total energy of the system indicates that K4 site is preferred as doping site, with K1, K2 and K3 sites presenting energies 143, 362 and 160 meV above the K4 site, respectively. Figure 4. Single-crystal emission spectra of LKLSO:Eu2+and NKLSO:Eu2+. Figure 4a shows the single-crystal emission spectrum of LKLSO:Eu2+(black line) in combination with two Gaussian curves (red) to describe the total emission. Figure 4b displays the single-crystal emission spectrum of NKLSO:Eu2+(black line) in combination with two Gaussian curves (red) to describe the total emission. Full Papers doi.org/10.1002/ejic.202100550 4476Eur. J. Inorg. Chem. 2021, 4470–4481 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Freitag, 12.11.2021 2143 / 225114 [S. 4476/4481] 1 Figure 5. a The excitation spectrum (powder measurements) for the cyan-green emission peak (λem =509 nm) is represented by the blue line. The green line shows the luminescence spectrum of the powder sample recorded at an excitation wavelength of 440 nm. The red line shows the luminescence spectrum of a crystal recorded at an excitation wavelength of 440 nm. Figure 5b: shows the color point of the LKLSO:Eu2+powder sample in the CIE-diagram.[38] Figure 6. Representation of the thermal quenching behavior of LKLSO:Eu2+as the relative integral photoluminescence intensity to that at 25°C measured in steps of 25°C up to 225°C. Full Papers doi.org/10.1002/ejic.202100550 4477Eur. J. Inorg. Chem. 2021, 4470–4481 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Freitag, 12.11.2021 2143 / 225114 [S. 4477/4481] 1 UV/Vis As mentioned before, the synthesis of LKLSO with Eu2+as an activator ion results in an intense yellow color of the product, which does not occur, when the synthesis is carried out without doping. Therefore, diffuse reflectance measurements were performed on the doped and undoped phase (see Figure 9). The doped compound features a yellow hue with the color values of a*=2.9, b*=9.0, and L*=90.7. In contrast, the undoped compound shows color values of a*=0.3, b* =0.3, and L*=93.8. Conclusion This work contains a detailed discussion of the crystal structure and luminescence properties of the new phosphor LiK7[Li3SiO4]8:Eu2+. In addition to the known compounds NLSO and NKLSO, this is an additional substitution variant in the field of alkali lithosilicates, which crystallizes analogically in the space group I41=a. The structure consists of a highly condensed network of SiO4and LiO4tetrahedra, forming channels along [00� 1], in which K+and Li+cations are incorporated. Despite the positional disorder of Li+and K+cations in the channels of LKLSO, it is isostructural to NKLSO. Although the positional disorder is not yet understood in detail and needs to be clarified in much deeper investigations, the compound provides Figure 7. Possible occupation of the activator ion Eu2+in the domains of the Li-rich channels and the resulting charge neutrality by absence of two Li+ cations. The LiO4tetrahedra are displayed in orange and the SiO4tetrahedra are displayed in blue. Figure 8. Representation of the four possible coordination environments of the activator ion Eu2+including the distances to the O2anions. Full Papers doi.org/10.1002/ejic.202100550 4478Eur. J. Inorg. Chem. 2021, 4470–4481 www.eurjic.org © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH Wiley VCH Freitag, 12.11.2021 2143 / 225114 [S. 4478/4481] 1