Abstract
The new quaternary lithogallate Sr2LiGaO4 was prepared by conventional solid-state synthesis in a welded tantalum ampoule at T = 850 °C. Single-crystal X-ray diffraction was used to elucidate the crystal structure of the compound, which crystallizes in the orthorhombic space group Pnma (no. 62) with the lattice parameters a = 11.2434(4), b = 5.6879(2), and c = 6.6983(2) Å. The phase composition of the powder sample was determined by Rietveld refinement based on X-ray diffraction data. The crystal structure is composed of layers of corner-sharing LiO4 and GaO4 tetrahedra with alternating orientation parallel to the crystallographic bc plane. The eightfold coordinated strontium atoms are arranged in a zigzag manner between the layers forming two types of double-capped trigonal prismatic SrO8 units with common faces. The crystal structure of Sr2LiGaO4 is isotypic to that of the lithoaluminate derivate Sr1.85Ba0.15LiAlO4.
1 Introduction
Solid-state lighting (SSL) technologies based on semiconductor diodes and inorganic phosphors are directed to an enhancement of luminous efficacy, energy efficiency, durability, and design, paving the way for gradually replacing the prevailing incandescent and discharge lamps [1], [2], [3], [4], [5], [6], [7], [8], [9]. In the last decade, high-efficiency red- and green-emitting phosphors in the substance class of rare earth (RE)-activated alkaline earth lithoaluminates have been discovered, exhibiting excellent luminescence properties for future applications in phosphor-converted light-emitting diodes (pc-LEDs) [10], [11], [12], [13], [14]. In a search of new RE-activated phosphors, the Ce3+- or Eu2+-activated layered compound Sr2LiAlO4 was published in 2018 along with theoretical calculations and reports on its photoluminescence performance [15]. The crystal structure of monoclinic Sr2LiAlO4 (space group P21/m) derived from density-functional theory calculations and powder X-ray diffraction data was confirmed by single-crystal X-ray diffraction in 2019 [15, 16]. Polymorphism and cationic or anionic substitution of the lithoaluminate yielded the isostructural compounds Sr2LiAlO4:Eu2+ (space group Cmcm), Sr1.85Ba0.15LiAlO4:Eu2+ (space group Pnma), and Sr2Li0.9Al1.1O3.8N0.2:Eu2+ (space group Cmcm) showing green-to-red emissions upon blue-light excitation due to the electric dipole allowed 4f 65d 1 ↔ 4f 7 transitions of the Eu2+ ion [16, 17]. The effect of activator concentration on the emission intensity and the temperature-dependent luminescence of Sr2LiAlO4:Ce3+/Eu2+ were investigated, including co-activation of the Eu- and Ce-activated compound [15, 23]. Several substitutional derivates of RE-activated alkaline earth aluminates and the corresponding gallates were discussed in the literature describing effects on the photoluminescence properties as a result of cationic or anionic substitution [18], [19], [20], [21], [22]. The distribution of Eu2+ over two crystallographically distinct Sr sites in the monoclinic polymorph (space group P21/m) of Sr2LiAlO4 leads to the observation of an asymmetric double emission band peaking at approximately 512 and 559 nm, when excited by UV to blue light [15, 16, 23, 24]. According to results of density-functional theory calculations and of temperature-dependent luminescence spectroscopic data, the lower-energy emission most likely originates from Eu2+ occupying the thermally less stable SrO8 polyhedron with shorter average interatomic distances, which is supported by the deconvolution spectrum of Sr2LiAlO4:Eu2+, showing an intensity decrease of the longer wavelength emission at room temperature [15]. The lower-energy emission is associated with Eu2+ occupying the thermally less stable Sr2 site in the P21/m polymorph of Sr2LiAlO4 as was also observed for the barium-containing lithoaluminate derivate [16]. Recently, spectral tailoring was assigned to partial cationic substitution in Sr2Li(Al1−x Ga x )O4:Eu2+ leading to a narrow-band emission peaking at 512 nm that is caused by the compression of the Sr2O8 polyhedron and forces the Eu2+ ion to occupy exclusively the Sr1 site [24]. The existence of the gallium-containing derivates of Sr2Li(Al1−x Ga x )O4 and the associated effects on the luminescence properties of the Eu2+-activated compounds motivated the studies on the complete replacement of Al3+ by Ga3+ in Sr2LiAlO4. The new lithogallate Sr2LiGaO4 was now prepared by conventional solid-state synthesis in a welded tantalum ampoule and its crystal structure was elucidated by single-crystal X-ray diffraction. The structural relation between the title compound and the previously published lithoaluminate derivate Sr1.85Ba0.15LiAlO4 is at the heart of discussion in this contribution [16].
2 Experimental section
2.1 Synthesis
Powder and micrometer-sized crystals of Sr2LiGaO4 were synthesized from a stoichiometric ratio of the metal nitrate hydrates Sr(NO3)2·4 H2O (51.19 mg, 0.18 mmol; Merck), LiNO3·3 H2O (11.10 mg, 0.09 mmol; Merck), and Ga(NO3)3·9 H2O (37.71 mg, 0.09 mmol; Merck). The starting materials were mixed in an agate mortar, transferred into a tantalum ampoule, and sealed using a tungsten inert gas welding system. The sealed ampoule was placed in a silica tube filled with 400 mbar argon to avoid oxidation of tantalum. The sample setup was placed in a horizontal tube furnace and heated to 850 °C with 3 K min−1. After calcinating for 6 h, the sample was cooled to 700 °C with 0.1 K min−1 and then the tube furnace was turned off. As depicted in Figure 1, grey colored powder and rod-shaped transparent crystals of Sr2LiGaO4 were obtained from the tube-furnace approach.
Grey-colored powder (left) and rod-shaped transparent crystals (right) of Sr2LiGaO4.
2.2 Single-crystal X-ray diffraction
Single crystals of Sr2LiGaO4 were isolated under a stereomicroscope (Leica M125, Wetzlar, Germany) equipped with integrated polarization filters. The crystal structure was determined from single-crystal X-ray diffraction data recorded on a D8 Quest diffractometer (Bruker Corporation, Billerica, USA). An IμS microfocus X-ray system (Incoatec, Geesthacht, Germany) was used as an X-ray source (MoKα, λ = 0.7107 Å) and the diffracted radiation was detected on a Photon 100 CMOS detector. Data acquisition and processing were performed with Apex3 including the global unit cell refinement with Saint, and the multi-scan detector scaling and absorption correction using Sadabs [25], [26], [27]. The crystal structure was solved with Shelxt using intrinsic phasing and refined with Shelxl, both algorithms implemented in the Olex2 suite [28], [29], [30]. The crystal structure and the determined symmetry were checked in Platon using the ADDSYM routine [31, 32]. The graphical representations of the crystal structure were created in Diamond [33].
Detailed information on the crystal structure of Sr2LiGaO4 is available at Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (Fax: +49-7247-808-666; E-mail: crysdata@fiz-karlsruhe.de/, http://www.fizinformationsdienste.de/en/DB/icsd/depot_anforderung.html) on quoting the deposition number CSD-2266500.
2.3 Powder X-ray diffraction
The composition of powder samples was investigated on a STOE Stadi P diffractometer (STOE & Cie. GmbH, Darmstadt, Germany) using Ge(111)-monochromatized primary X-ray radiation (MoKα 1, λ = 0.7093 Å) and a Mythen 1K (Dectris AG, Baden-Daettwil, Switzerland) PSD detector. Data acquisition and processing were carried out in Winxpow and the integrated data mining software Icdd pdf-2 provided the comparative data for the phase identification [34, 35]. A Rietveld refinement based on the powder X-ray diffraction data was performed in Topas [36].
2.4 Bond valence and MAPLE calculations
Bond length-bond strength (BLBS), charge distribution (CHARDI), and Madelung Part of Lattice Energy (MAPLE) calculations were performed to support the refined crystal structure model. The BLBS model is based on the hypothesis that the bond length directly correlates with the sum of ionic radii within the first coordination sphere of an atom. The calculations are based on the bond valence model for inorganic compounds from Brown, and the universal constant and bond valence parameters were taken from Brese and O’Keeffe [37, 38]. The CHARDI method combines Pauling´s concept of bond length-bond strength with the effective coordination numbers obtained from the mean fictive ionic radii (ECoNs-MEFIR). The concept is based on weighted interatomic distances and applies only one arbitrarily chosen constant compared to the BLBS model, which uses a number of empirically determined parameters [39], [40], [41]. In addition to the Coulomb fraction of the lattice energies, the MAPLE calculations provided the effective coordination numbers used in the CHARDI concept.
3 Results and discussion
3.1 Crystal structure
Isostructural phases of monoclinic (space group P21/m) and orthorhombic (space group Cmcm) polymorphs of Sr2LiAlO4, including the lithoaluminate derivate Sr1.85Ba0.15LiAlO4 (space group Pnma), were previously discussed in detail [16]. The aforementioned alkaline earth lithoaluminates are composed of layers consisting of corner-sharing LiO4 and AlO4 tetrahedra with alternating orientation. The centers of the tetrahedra are alternately occupied by lithium and aluminum or exhibit mixed occupancy, as in the case of the Cmcm polymorph of Sr2LiAlO4. The alkaline earth (AE) atoms (AE = Sr, Ba) are eightfold coordinated by oxygen atoms forming double-capped trigonal prismatic SrO8 units that are arranged in a zigzag manner between the layers [15, 16]. The eightfold coordination of the AE atoms was alternatively referred as a [6 + 2] coordination because the two capping oxygen atoms are located farther away from the center of the polyhedron [16]. The lithoaluminate derivate Sr1.85Ba0.15LiAlO4 crystallizes in the centrosymmetric space group Pnma exhibiting seven crystallographically distinct sites within the crystal structure. The partial substitution of Sr2+ by Ba2+ on the AE1 site leads to a lengthening of the average interatomic distance in the mixed-occupied AE1 polyhedron (Ø [Sr1/Ba1–O]: 2.721(9) Å) compared to the AE1 polyhedra of the polymorphs of Sr2LiAlO4 (Ø [Sr1–O]: 2.694(5) Å [space group P21/m] and 2.668(6) Å [space group Cmcm]) due to the larger space demand of Ba2+ ion [16, 42]. The average interatomic distances of the AE2 polyhedra of Sr1.85Ba0.15LiAlO4 and the P21/m polymorph, including the single Sr site in the Cmcm polymorph, are almost equal within the range of the standard deviations of the interatomic distances (Ø [Sr2–O]: 2.658(5) Å [space group Pnma], 2.667(6) Å [space group P21/m], 2.668(6) Å [space group Cmcm]) [16]. As depicted in Figure 2, the complete replacement of Al3+ by Ga3+ at the tetrahedrally coordinated Al1 site yields the quaternary lithogallate Sr2LiGaO4, which is isotypic to the lithoaluminate derivate Sr1.85Ba0.15LiAlO4. The lithogallate crystallizes in the orthorhombic crystal system in the centrosymmetric space group Pnma (no. 62) with the lattice parameters a = 11.2434(4), b = 5.6879(2), c = 6.6983(2) Å, and a unit cell volume of 428.37(2) Å3 at T = 300(2) K.
Top: isotypic crystal structures of Sr1.85Ba0.15LiAlO4 and Sr2LiGaO4 with corner-sharing LiO4 (orange) and AlO4/GaO4 (grey) tetrahedra. Bottom: coordination polyhedra of the crystallographically distinct AE sites (red) with the Sr sites (red spheres), mixed Sr/Ba site (brown sphere), and the O sites (blue spheres).
As in the case of Sr1.85Ba0.15LiAlO4, seven crystallographically distinct sites are present in Sr2LiGaO4. The corner-sharing LiO4 and GaO4 tetrahedra are arranged parallel to the bc plane and the layers are stacked along the crystallographic a axis. The eightfold or [6 + 2] coordination of the strontium atoms can also be applied to Sr2LiGaO4, but slightly distorted due to the unequal interatomic Sr–O distances and the resulting different edge lengths of the polyhedra. An overview of the average interatomic cation-anion distances in monoclinic Sr2LiAlO4, orthorhombic Sr1.85Ba0.15LiAlO4, and orthorhombic Sr2LiGaO4 is given in Table 1 [16]. In order to estimate the standard deviations of the average interatomic distances, the maximum values of the respective deviations for the individual cation-anion distances were taken from the corresponding crystallographic information files of the listed compounds. Structural information of Sr2LiGaO4 based on single-crystal X-ray diffraction data is given in Table 2. The atomic coordinates, Wyckoff positions, displacement parameters, and non-averaged interatomic cation-anion distances are listed in Tables 3–5. The interatomic distances are within the expected range compared to the lithoaluminates described above. The anisotropic displacement parameters along the principal axes U 11 and U 33 for both strontium sites indicate that the strontium atoms are deflected parallel to the layers and toward the lighter lithium atoms.
Averaged interatomic cation-anion distances in Sr2LiAlO4, Sr1.85Ba0.15LiAlO4, and Sr2LiGaO4.
| Compound | Sr2LiAlO4 | Sr1.85Ba0.15LiAlO4 | Sr2LiGaO4 |
|---|---|---|---|
| Space group | P21/m | Pnma | Pnma |
| Ø (Sr1/Ba1–O)/Å | 2.694(5) | 2.721(9) | 2.708(2) |
| Ø (Sr2–O)/Å | 2.667(6) | 2.658(5) | 2.674(2) |
| Ø (Al1/Ga1–O)/Å | 1.775(5) | 1.776(8) | 1.856(2) |
| Ø (Li1–O)/Å | 2.012(2) | 2.07(2) | 2.001(6) |
In accordance with the findings for the Sr1 site in the substitution series of Sr2Li(Al1–x Ga x )O4 (x = 0.1–0.4) and the above-mentioned polymorphs of Sr2LiAlO4, the average interatomic Sr1–O distances remain approximately unchanged when either polymorphism occurs, and the Al1–site is partially substituted, or Al3+ is completely replaced by Ga3+ [16, 24]. Only the partial substitution of Sr2+ by Ba2+ leads to a lengthening of the interatomic Sr1/Ba1–O distances compared to orthorhombic Sr2LiGaO4 and monoclinic Sr2LiAlO4 [16]. The lengthening is about twice as large for the barium-containing lithoaluminate derivate as for the new lithogallate Sr2LiGaO4 with respect to the P21/m polymorph of Sr2LiAlO4. On the other hand, the trend of shortening of the average interatomic Sr2–O distances in the gallium-containing derivates of Sr2Li(Al1–x Ga x )O4 obtained from powder X-ray diffraction data (Ø [Sr2–O]: 2.603(7) Å for x = 0.4) is not supported by the single-crystal diffraction data of Sr2LiGaO4, which show only minor differences in the average Sr2–O distances [24]. Starting from the crystal structure of monoclinic Sr2LiAlO4, the partial substitution of strontium by barium and the complete replacement of aluminum by gallium lead to a comparable structural effect, since the new lithogallate and the barium-containing lithoaluminate crystallize in the same space group type. The general tetrahedron linkage pattern of Sr2LiGaO4 is shown in Figure 3, where each LiO4 tetrahedron is connected to four GaO4 tetrahedra via shared corners and vice versa.
Tetrahedron linkage pattern in Sr2LiGaO4 with the LiO4 tetrahedra (orange) and GaO4 tetrahedra (grey) showing the Li sites (orange spheres), Ga sites (green spheres), and the O sites (blue spheres).
Crystal structure refinement of Sr2LiGaO4 derived from single-crystal X-ray diffraction data.
| Empirical formula | Sr2LiGaO4 |
| Formula weight/g∙mol−1 | 315.90 |
| Temperature/K | 300(2) |
| Crystal system | Orthorhombic |
| Space group | Pnma (no. 62) |
| a/Å | 11.2434(4) |
| b/Å | 5.6879(2) |
| c/Å | 6.6983(2) |
| Volume/Å3 | 428.37(2) |
| Z | 4 |
| Crystal size/mm3 | 0.08 × 0.05 × 0.04 |
| ρ calc/g∙cm−3 | 4.90 |
| μ/mm−1 | 30.9 |
| F(000)/e | 568 |
| Single-crystal diffractometer | Bruker D8 Quest |
| Radiation; wavelength/Å | MoKα; 0.71073 |
| Detector | Photon 100 CMOS |
| Absorption correction | Multi-scan |
| 2θ range/deg | 3.54 < θ < 37.81 |
| Index range hkl | −19 ≤ h ≤ 19 |
| −8 ≤ k ≤ 9 | |
| −11 ≤ l ≤ 11 | |
| Reflections collected (total) | 15791 |
| Independent reflections | 1239 |
| R int; R σ | 0.0295; 0.0160 |
| Data; parameters | 1239; 46 |
| Goodness-of-fit on F 2 | 1.181 |
| Final indices R 1; wR 2 [I > 2 σ(I)] | 0.0211; 0.0483 |
| Indices R 1; wR 2 (all data) | 0.0296; 0.0508 |
| Largest diff. peak and hole/e Å−3 | 0.89; −1.15 |
The interatomic Ga–O distances of Sr2LiGaO4 are elongated compared to the average Al–O distances of Sr1.85Ba0.15LiAlO4 and monoclinic Sr2LiAlO4. As a result of the elongated Ga–O distances, the oxygen atoms move closer to the lithium center leading to a compression of the neighboring LiO4 tetrahedra compared to the monoclinic lithoaluminate and the barium-containing lithoaluminate derivate. The crystal structure of Sr2LiGaO4 compensates for the described elongation and compression through the evasion of the two capping oxygen atoms of the SrO8 polyhedra of both Sr sites, which exhibit longer interatomic Sr–O distances compared to the corresponding lithoaluminates. As depicted in Figure 4, there are three distinct O sites present in the crystal structure of Sr2LiGaO4, as is also found for the coordination of the oxygen atoms in the lithoaluminate derivate Sr1.85Ba0.15LiAlO4 [16].
Coordination spheres of the three distinct O sites (blue spheres) in Sr2LiGaO4 with the Sr sites (red spheres), Li sites (orange spheres), and the Ga sites (green spheres).
Wyckoff positions, atomic coordinates, and isotropic displacement parameters U iso (/Å2) for Sr2LiGaO4.
| Site | Wyckoff position | x | y | z | U iso |
|---|---|---|---|---|---|
| Ga1 | 4c | 0.13528(2) | 1/4 | 0.37793(6) | 0.00445(6) |
| Li1 | 4c | 0.1479(4) | 1/4 | 0.868(1) | 0.0078(8) |
| O1 | 8d | 0.0471(2) | 1/4 | 0.1410(3) | 0.0095(3) |
| O2 | 4c | 0.2419(1) | 0.0026(2) | 0.3728(2) | 0.0071(2) |
| O3 | 4c | 0.0471(2) | 1/4 | 0.6119(3) | 0.0096(3) |
| Sr1 | 4c | 0.38443(2) | 1/4 | 0.62333(5) | 0.00715(5) |
| Sr2 | 4c | 0.36588(2) | 1/4 | 0.12499(5) | 0.00669(5) |
Anisotropic displacement parameters U ij (/Å2) for Sr2LiGaO4.
| Site | U 11 | U 22 | U 33 | U 23 | U 13 | U 12 |
|---|---|---|---|---|---|---|
| Ga1 | 0.0040(1) | 0.0044(2) | 0.0050(1) | 0 | −0.0001(1) | 0 |
| Li1 | 0.002(2) | 0.007(2) | 0.014(2) | 0 | 0 | 0 |
| O1 | 0.0074(6) | 0.0154(8) | 0.0057(7) | 0 | −0.0010(6) | 0 |
| O2 | 0.0075(4) | 0.0052(5) | 0.0085(5) | 0.0007(3) | 0.0010(4) | 0.0022(4) |
| O3 | 0.0076(6) | 0.0150(8) | 0.0063(7) | 0 | 0.0011(7) | 0 |
| Sr1 | 0.00598(8) | 0.0107(1) | 0.00473(9) | 0 | 0.00002(8) | 0 |
| Sr2 | 0.00515(8) | 0.0103(1) | 0.00460(9) | 0 | 0.00014(8) | 0 |
Non-averaged interatomic cation-anion distances in Sr2LiGaO4.
| Bond | Distance/Å | Bond | Distance/Å |
|---|---|---|---|
| Ga1–O2 | 1.849(2) | Li1–O2 | 1.898(3) |
| Ga1–O2 | 1.849(2) | Li1–O2 | 1.898(3) |
| Ga1–O3 | 1.854(2) | Li1–O1 | 2.057(6) |
| Ga1–O1 |
1.871(2) |
Li1–O3 |
2.150(6) |
| Sr1–O2 | 2.546(2) | Sr2–O2 | 2.527(2) |
| Sr1–O2 | 2.548(2) | Sr2–O2 | 2.527(2) |
| Sr1–O2 | 2.622(2) | Sr2–O1 | 2.571(2) |
| Sr1–O2 | 2.622(2) | Sr2–O3 | 2.583(2) |
| Sr1–O1 | 2.714(2) | Sr2–O2 | 2.585(2) |
| Sr1–O3 | 2.714(2) | Sr2–O2 | 2.585(2) |
| Sr1–O1 | 2.9486(5) | Sr2–O3 | 3.0088(6) |
| Sr1–O1 | 2.9486(5) | Sr2–O3 | 3.0088(6) |
3.2 Powder analysis
To approximate the phase composition of the bulk sample, a powder X-ray diffraction measurement was carried out and a Rietveld refinement was performed, as shown in Figure 5. The related refinement data are listed in Table 6. The overall diffraction pattern is explained by the presence of the new lithogallate Sr2LiGaO4 and the by-products SrO (CSD-1603938), Sr(OH)2 (CSD-1596121), and Sr3Ga2(OH)12 (CSD-1715947). The high mass fraction of the two hydroxides most likely originates from the crystal water content of the metal nitrate hydrates used in the tube furnace synthesis. According to the Rietveld analysis, the phase composition is equal to 57.3 wt% of the lithogallate and 42.7 wt% of by-products.
Rietveld refinement of the powder X-ray diffraction data showing the experimental data (black) and the calculated diffraction pattern (red) derived from the single-crystal data (red) of Sr2LiGaO4 and the by-products. The difference curve (grey) is plotted along with the theoretical reflection positions of Sr2LiGaO4 (blue), SrO (violet), Sr(OH)2 (green), and Sr3Ga2OH12 (orange).
Results of the Rietveld refinement of the Sr2LiGaO4 sample obtained from powder X-ray diffraction data.
| Powder X-ray diffraction data | |
|---|---|
| a/Å | 11.2269(4) |
| b/Å | 5.6809(1) |
| c/Å | 6.6872(2) |
| V/Å3 | 426.50(2) |
| 2θ/deg | 2.3–50.0 |
| 2θ step width/deg | 0.015 |
| R exp/% | 0.97 |
| R wp/% | 0.87 |
| R p/% | 0.69 |
| Goof | 0.90 |
3.3 Bond valence and MAPLE calculations
The results of the BLBS and CHARDI calculations for the new lithogallate are listed in Table 7. Except for the anionic sites, the numeric values calculated from the bond valence concepts are in good agreement with the formal oxidation states of the respective cations, which was to be expected since Sr2LiGaO4 exhibits a simple crystal structure without any complicated occupational or positional disorders. Each O1 and O3 is connected to two strontium atoms that are located farther away from the polyhedron center as part of a bicapped structure described in the introduction. The difference lies presumably in the reduced influence of the strontium atoms, which leads to smaller values for the valence states of the O1 and O3 sites. In case of the O2 site, the average interatomic distance is significantly shorter compared to the O1 and O3 sites, and four strontium atoms are in closer proximity leading to higher values for the valence state of O2.
Charge distribution in Sr2LiGaO4 obtained from BLBS (ΣV) and CHARDI (ΣQ) calculations.
| Atom | Ga1 | Li1 | Sr1 | Sr2 | O1 | O2 | O3 |
|---|---|---|---|---|---|---|---|
| ΣV | +2.90 | +0.98 | +1.75 | +1.99 | −1.67 | −2.12 | −1.71 |
| ΣQ | +3.05 | +0.97 | +2.02 | +1.96 | −1.73 | −2.27 | −1.73 |
The lattice energy of a hypothetical compound with the formula Sr2LiGaO4 was calculated from the reaction between 2 mol strontium oxide, 0.5 mol lithium oxide, and 0.5 mol gallium oxide, summing up to 17498 J∙mol−1. As shown in Table 8, the hypothetical value differs only by 0.43 % from the MAPLE value of 18058 J∙mol−1 calculated for the new lithogallate. The good agreement of the MAPLE values supports the refined structure model.
4 Conclusions
The complete replacement of aluminum by its heavier homologue gallium in Sr2LiAlO4 was successfully achieved, leading to the isostructural and hitherto unknown phase Sr2LiGaO4. Isotypic crystal structures in the centrosymmetric space group Pnma are obtained when either strontium is partially substituted by barium or aluminum is fully replaced by gallium in Sr2LiAlO4, as demonstrated by single-crystal X-ray diffraction. Experimental efforts to achieve photoluminescence in the visible spectrum upon blue-light excitation at room temperature by replacing strontium by europium or cerium in Sr2LiGaO4 were not successful.
Acknowledgments
We would like to thank Dr. K. Wurst for recording the SC-XRD data and for his support regarding the interpretation.
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Research ethics: Not applicable.
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Author contributions: All authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
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Competing interests: The authors declare no conflict of interest regarding this article.
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Research funding: None declared.
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Data availability: The raw data can be obtained on request from the corresponding author.
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