Isomorphism of Sr[Li3AlO4] and Sr[Li3GaO4] – Syntheses, Crystal Structure, and Europium(II) Luminescence

While highly efficient red-emitting inorganic phosphors have been discovered in the substance class of alkaline earth oxo(nitrido)lithoaluminates, new narrow-band green- and yellow-emitting components are being sought to improve the performance of phosphor-converted light-emitting diodes (pc-LEDs). Various solid-state reactions were carried out under protective gas atmosphere in nickel crucibles and sealed tantalum ampules to synthesize Sr[Li3AlO4], Sr[Li3GaO4], and five substitutional derivates of Sr[Li3(Al1–xGax)O4] at moderate temperatures. The observation of a linear increase in the unit cell parameters as a function of the increasing gallium mole fraction x in Sr[Li3(Al1–xGax)O4] revealed Vegard behavior in the solid-solution series, which was derived from powder X-ray diffraction data. The isomorphic crystallization of the new oxolithogallate Sr[Li3GaO4] and the known oxolithoaluminate Sr[Li3AlO4] in an ordered variant of the U[Cr4C4] aristotype was verified on the basis of powder and single-crystal X-ray diffraction data. Photoluminescence spectroscopy was used to investigate the narrow-band emissions in the substitution series of Eu2+-activated Sr[Li3(Al1–xGax)O4] under blue-light excitation. The emission maximum was shifted to higher energies as the gallium mole fraction increased. Peak wavelengths were observed at λem = 572 nm (fwhm equals 47 nm, 1446 cm–1, 0.18 eV) for yellow-emitting Sr[Li3AlO4]:Eu2+ and at λem = 554 nm (fwhm equals 49 nm, 1589 cm–1, 0.20 eV) for green-emitting Sr[Li3GaO4]:Eu2+. Sr[Li3AlO4]:Eu2+ has excellent thermal quenching resistance with a photoluminescence emission intensity of >93% at T = 423 K relative to the room temperature value, making this inorganic phosphor a potential candidate for solid-state lighting applications.


INTRODUCTION
Over the past decade, the substance class of alkaline earth oxo(nitrido)lithoaluminates has increasingly attracted the interest of researchers in the field of inorganic phosphors, when Eu 2 + -activated Ca[LiAl 3 N 4 ], 1 Sr[LiAl 3 N 4 ], 2 Ca 18.75 Li 10.5 [Al 39 N 55 ], 3 Sr 4 [LiAl 11 N 14 ], 4 and Sr[Li 2 Al 2 O 2 N 2 ] 5 were discovered.The interconfigurational transitions from the excited 4f 6 5d 1 states to the 4f 7 ground state of the Eu 2+ activator ion in the above-mentioned compounds lead to narrow-band emissions in the red spectral region of the visible spectrum, making some of them promising candidates for applications in phosphor-converted light-emitting diodes (pc-LEDs). 6−11 Except for Ca 18.75 Li 10.5 [Al 39 N 55 ] and Sr 4 [LiAl 11 N 14 ], the crystal structures of the described oxo(nitrido)lithoaluminates are ordered variants of the U[Cr 4 C 4 ] aristotype. 12The rediscovery of the U[Cr 4 C 4 ]-type structure or closely related structures was driven by the observation of (ultra-) narrow-band emissions emanating from Eu 2+ -activated alkaline-rich lithosilicates 13−17 and alkaline earth-rich lithoaluminates. 1,2,5The nitridolithoaluminate Sr-[LiAl 3 N 4 ] crystallizes in the ordered variant, which was first described for the isotypic crystal structure of Cs[Na 3 PbO 4 ]. 18he complete replacement of nitrogen by oxygen and the partial exchange of aluminum by lithium in Sr[LiAl 3 N 4 ] leads to Sr[Li 3 AlO 4 ].The nitridolithoaluminate and the oxolithoaluminate exhibit isotypic crystal structures, 19−22 which differ not only in the anionic ligands but also in the distribution of the tetrahedrally coordinated cations in the highly condensed rigid framework. 2,22A detailed description of the solid-state synthesis, single-crystal X-ray diffraction data and the luminescence properties of Eu 2+ -activated Sr [Li 3  AlO 4 ] is presented in the literature, 22 but no database-accessible X-ray diffraction data have been published to date.The first experimental single-crystal X-ray diffraction data of crystals with chemical compositions close to that of Sr [Li 3  AlO 4 ] were presented by A. Ooishi and colleagues. 23They demonstrated on different crystallites that disordered average structures can be observed in Sr x [Li 2+x Al 2−x O 4 ], which are closely related to the U[Cr 4 C 4 ]type structure, but require the description of commensurate and incommensurate modulated superstructures.Once a phosphor material has been structurally elucidated and its luminescence properties are known, tuning the emission in the meaning of changing the spectral position or spectral bandwidth is a common way to improve its luminescence performance for future applications, or, conversely, to prove its unsuitability for these.Frequently used strategies to achieve the described objectives are based on the selection of the activator, the variation of its concentration, and the complete replacement or partial substitution of the cation(s) or anion(s) in the host structure. 24As already shown in the introduction, numerous representatives of quaternary alkaline earth oxo(nitrido)lithoaluminates have been discovered and structurally elucidated.Also, several crystallographic studies have been performed in the corresponding quaternary lithogallates 25,26 and in the field of nitridogallates, e.g., (Ca,Sr) 3 Ga 2 N 4 27 and (Sr,Ba) 3 Ga 3 N 5 27,28   GaO 4 ] can be described as isomorphous in the sense of the definition given by P. H. Groth. 29The claim of isomorphism is supported by the observation of Vegard behavior 30,31 and the emission tunability in the substitution series of Eu 2+ -activated Sr[Li 3 (Al 1−x Ga x )O 4 ].Finally, the thermal quenching behaviors of the aluminate and gallate representatives were investigated to evaluate their use as inorganic phosphors in solid-state lighting (SSL) applications.] with the nominal gallium mole fraction x being equal to 0.1, 0.2, 0.4, 0.6, 0.8, and 1 were prepared according to the previous approach using Ga 2 O 3 (>99.9%,Vollmer) or GaN (99.99%, abcr) as the gallium source.The stoichiometric ratios of the starting materials used are listed in Table S1.Li 2 B 4 O 7 (0.6 g, 0.0035 mol) was added to each reaction mixture as a mineralizing agent and additional source of lithium.The starting materials were mixed in an agate mortar in a glovebox filled with inert gas (H 2 O <1 ppm, O 2 <1 ppm, MBraun, Garching, Germany) and crushed in closed plastic containers for 12 h using ZrO 2 grinding bowls.The samples were transferred into nickel crucibles, which were closed with lids and placed in a chamber furnace.After heating the samples in a forming gas atmosphere to 1073 K with 4.2 K•min −1 , the temperature was kept for 4 h, and then cooled to room temperature by turning off the furnace.Single crystals of Sr[Li 3 GaO 4 ] were obtained from the reaction between SrO (4.15 g, 0.04 mol), Li 2 CO 3 (1.48g, 0.02 mol, ≥99.0%,Merck), Ga 2 O 3 (3.75g, 0.02 mol, 99.998%, Strem Chemicals), and lithium metal (1.11 g, 0.16 mol, 99%, Merck).The starting materials for the oxolithogallate were mixed in an agate mortar and filled into tantalum ampules.The tantalum ampules were sealed in an inert gas atmosphere (Linde Gas, Stadl-Paura, Austria) using a tungsten welding system (Fronius International, Pettenbach, Austria) and transferred into a silica tube filled with 400 mbar argon.The silica tube was placed in a self-assembled horizontal tube furnace (Controller 3216, Eurotherm, Durrington, United Kingdom), heated to 1123 K with 3.0 K•min −1 , and the temperature was kept for 8 h.After cooling the sample to 973 K with 0.1 K•min −1 , the tube furnace was turned off.The Eu 2+ -activated compounds were prepared using Eu 2 O 3 (≥99.99%,Strem Chemicals) as an activating agent at nominal concentrations of approximately 0.5 mol % (Sr formally substituted by Eu).The reduction of Eu 3+ to Eu 2+ was most likely achieved by the reducing forming gas atmosphere in the nickel crucibles and the lithium metal in the sealed tantalum ampules, respectively.Crystals and powders of the products appeared slightly green to yellow in daylight, showing luminescence with a green-toyellow color impression when irradiated by UV to blue light.Figure 1 shows the powder products in nickel crucibles that were irradiated with a UV lamp (λ ≈ 254 nm, CAMAG, Muttenz, Switzerland).The powder products were handled and stored under atmospheric conditions or, in the case of single crystals, in perfluorinated oil.Powder X-ray diffraction measurements have confirmed the presence of Sr[Li 3 (Al 1−x Ga x )O 4 ] even after months of storage under ambient conditions.

Powder X-Ray Diffraction. Powders of Sr
with the nominal gallium mole fraction x being equal to 0, 0.1, 0.2, 0.4, 0.6, 0.8, and 1 were investigated on an Empyrean powder diffractometer (PANalytical Xcelerator detector, Malvern Panalytical, Kassel, Germany) in the Bragg−Brentano geometry using Ge(111)-monochromatized Cu K-L 3 radiation (λ = 1.54056Å).The measurements were carried out in 2θ range from 10 to 80°with a step size of 0.0084°a nd 15 s•step −1 .Data collection and processing were performed in HIGHSCOREPLUS (v.4.9) 42 including refinement of the unit-cell parameters and the phase compositions based on theoretical diffraction patterns of the respective compound in the triclinic space group P1̅ (no.2).
2.4.Photoluminescence.The photoluminescence measurements described below were carried out on the Eu 2+ -activated compounds.The emission spectra of powder plaques of Sr[Li 3 (Al 1−x Ga x )O 4 ] with the nominal gallium mole fraction x being equal to 0, 0.1, 0.2, 0.4, 0.6, 0.8, and 1 were recorded at room temperature on a Fluoromax 4 spectrophotometer (Horiba, Kyoto, Japan) in diffuse reflection mode.A 150 W xenon arc lamp was used as the excitation source (λ exc = 460 nm) and the data were processed in FLUORESSENCE (V.3.5.1.20). 43ingle crystals of Sr[Li 3 AlO 4 ] and Sr[Li 3 GaO 4 ] were excited on a microscope slide at ambient conditions using a pigtailed InGaN-based laser diode (λ exc = 448 nm, Thorlabs, Ostfildern, Germany).The emission intensities were detected with a luminescence spectrometer (AvaSpec-ULS2048, Avantes, Apeldoorn, Netherlands) and the data were processed with OCEANVIEW (v.2.0.4). 44The relative emission intensities of Sr[Li 3 AlO 4 ] and Sr[Li 3 GaO 4 ] were measured in the temperature range from 298 to 498 K with a 25 K step size (λ exc = 460 nm) to study their thermal quenching behavior.Therefore, the powders were placed on a heating plate in the Fluoromax 4 spectrophotometer in form of a thin layer and the temperature was controlled using a thermocouple.Powder samples of Sr[Li 3 (Al 1−x Ga x )O 4 ] with the nominal gallium mole fraction x being equal to 0, 0.1, 0.2, and 0.6 were embedded in a silicon matrix.The emission intensities were recorded with a Quantaurus-QY spectrometer (Hamamatsu Photonics, Hamamatsu, Japan) using a 150 W xenon arc lamp (λ exc = 450 nm) as the excitation source to determine the quantum efficiency Φ (integrating sphere diameter ∼8.4 cm).Data processing was performed with PLQY U6039-05 (v.4.0.1). 45The luminescence decay curve of Sr[Li 3 AlO 4 ] was measured at T = 298 K using a FLSP920 spectrometer (Edinburgh Instruments, Livingston, Scotland) equipped with a nF900 hydrogen flash lamp.For data acquisition with F980 (v.1.4.5), 46the sample was excited at λ exc = 440 nm and the emission was detected at λ em = 570 nm.

RESULTS AND DISCUSSION
3.1.Crystal Structure.Detailed information on the crystallographic data of Sr[Li 3 AlO 4 ] and Sr[Li 3 GaO 4 ] are summarized in Table 1.The atomic coordinates, displacement parameters, and interatomic distances are listed in Tables S2− S6.Due to the low activator concentrations of approximately 0.5 mol % used in the syntheses, the presence of europium was neglected in both structure refinements as no contribution to the X-ray scattering density was expected.Each atom is assigned to the Wyckoff position 2i and the structure refinements were checked for occupational and positional disorder with respect to the above-mentioned disordered average structures of the crystallites with chemical compositions close to that of Sr[Li 3 AlO 4 ]. 23The displacement parameters, interatomic distances, and site occupancy factors showed no indications for the occurrence of disordered strontium sites and no satellite reflections or irregular extinction conditions were observed in the reciprocal space to necessitate the use of modulated structure models.Sr[Li 3 AlO 4 ] crystallizes in the triclinic space group P1̅ (no.2) with the unit cell parameters a = 5.7515(2), b = 7.3268(2), c = 9.7196(3) Å, α = 83.978(1),β = 76.647(1),γ = 79.625(1)°, and a volume V of 391.14(2)Å 3 measured at T = 296(2) K (R int = 0.0288, R σ = 0.0151).Our findings in the crystallographic investigation of Sr[Li 3 AlO 4 ] are consistent with the data available in literature 22 and the structural relationship to the well-known Sr[LiAl 3 N 4 ] 2 is confirmed.The graphical representation of the crystal structure is shown in Figure 2a and the structural section of a vierer 47 ring channel is depicted in   53,54 The 18 crystallographically distinct sites in Sr[Li 3 GaO 4 ] are occupied analogously to those described for Sr[Li 3 AlO 4 ]. 22The Ga 3+ ions (c.n.= 4: r ion = 0.47 Å) replace the Al 3+ ions (c.n.= 4: r ion = 0.39 Å), 48    .Checking the presence of Vegard behavior is one way to demonstrate that a solid-solutions series with unlimited miscibility exists, which allows us to attribute isomorphism to the title compounds.After deviations in the linear relationship between the unit-cell parameters and the concentrations of the solid-solution constituents were observed in various systems, the so-called Vegard's law was recapitulated and proposed as an approximation valid for ideal solid solutions when the difference in the unit-cell parameters of the pure compounds is less than 5%. 55In Figure 3b 4a and details on the spectral data are summarized in Table 2.The tabulated color point coordinates are depicted in the CIE 1931 color space 56 in Figure 4b.The phosphors show green-to-yellow photoluminescence and can be effectively excited with UV to blue light, as desired for the development of pc-LEDs. 57−22 The spectral position of the emission band can be tuned in Eu 2+ -activated Sr[Li 3 (Al 1−x Ga x )-O 4 ] by shifting the emission maximum to higher energies with increasing gallium mole fraction x, reaching a peak wavelength at λ em = 554 nm (fwhm equals 49 nm, 1589 cm −1 , 0.20 eV) for green-emitting Sr[Li 3 GaO 4 ]:Eu 2+ .The tunability of the spectral position of the emission band supports the claim of isomorphism by indicating the successful incorporation of gallium into the crystal structure in the solid-solution series of Eu 2+ -activated Sr[Li 3 (Al 1−x Ga x )O 4 ].As reviewed by G. Li and colleagues for various inorganic host structures, the influence of cationic substitution on the emission shift of the 4f 6 5d 1 → 4f 7 transitions of rare-earth ions includes different parameters that effect the  nature of the activator's environment. 24As a result of the cationic substitution of aluminum by gallium, the peak wavelength is shifted to higher energies, which is mainly caused by the weakening of the crystal field strength due to the less pronounced Eu-ligand interaction.Based on the single-crystal Xray diffraction data of the boundary components of Sr-[Li 3 (Al 1−x Ga x )O 4 ] with x = 0 and 1, the slightly larger shortest Sr/Eu−O distances in combination with a reduced polyhedron distortion, which means less structure relaxation around the activator in its excited state, the reduced crystal field splitting of the 5d-orbitals causes the shift of the emission maximum to higher energies.The observed emission bands of Eu 2+ -activated Sr[Li 3 (Al 1−x Ga x )O 4 ] are located at higher energies compared to the isotypic compound Sr[LiAl 3 N 4 ]:Eu 2+ with a peak wavelength at approximately λ em = 654 nm (fwhm equals 50 nm, 1180 cm −1 , 0.146 eV). 2 The new phosphors show emissions at even higher energies as described for Sr[Li 2 Al 2 O 2 N 2 ]:Eu 2+ (λ em = 614 nm, fwhm equals 48 nm, 1286 cm −1 , 0.159 eV), 5  We assume that Eu 2+ is distributed on two crystallographically distinct sites in Sr [Li 3 (Al/Ga)O 4 ].A comparable situation exisits for Sr[LiAl 3 N 4 ]:Eu 2+ , where time-resolved and lowtemperature luminescence spectroscopy at T = 10 K were used to decompose the virbronic structure of the emission transitions, revealing that Eu 2+ occupies two crystallographically distinct strontium sites. 8,9The luminescence of the single crystals used for the structure determinations of the title compounds were investigated and the normalized emission spectra of the Eu 2+ -activated compounds are shown in Figure S3.Peak wavelengths were observed at λ em = 568 nm with a fwhm equals 46 nm for Sr[Li 3 AlO 4 ]:Eu 2+ and at λ em = 552 nm with a fwhm equals 48 nm for Sr[Li 3 GaO 4 ]:Eu 2+ .The single-crystal emission data agree very well with the emission data of the bulk samples.We conclude that the emissions of the Eu 2+ -activated powder samples are represented by the single-crystal emissions of Sr [Li 3 (Al/Ga)O 4 ]:Eu 2+ .These spectra measured on single crystals are the best representatives for the emission of the Eu 2+ -activated title compounds, as their spectra are not subject to reabsorption effects.These reabsorption effects are also most likely the cause for the minor deviations from the powder sample data.In order to assign the experimental emission spectra to the 4f 6 5d 1 → 4f 7 transitions of Eu 2+ , the luminescence decay curve of Sr [Li 3 AlO 4 ]:xEu 2+ (x = 0.5 mol % nominal concentration) was measured at room temperature, which is shown in Figure S6.The luminescence decay is single exponential with a radiative decay time of τ = (0.916 ± 0.005) μs.As observed for other inorganic phosphors with the U[Cr 4 C 4 ]-type structure such as Sr[Li 2 Al 2 O 2 N 2 ]:Eu 2+ (τ = 0.86 ± 0.01 μs at T = 298 K), 11 the value of τ is within the range expected for 4f 6 5d 1 ↔ 4f 7 transitions of Eu 2+ .Although no 4f → 4f emission transitions attributable to the spin−orbit levels of the Eu 3+ ion and the associated crystal-field sublevels were observed in the emission spectra, the presence of Eu 3+ ions that were not reduced to Eu 2+ under the applied synthesis conditions cannot be ruled out.Regarding the thermal quenching behavior, the requirement for an (inorganic) phosphor is that the photoluminescence emission intensity decreases only slightly up to an LED operating temperature of approximately 423 K relative to the emission intensity at room temperature. 58In Figure 5a  Sr [Li 3 GaO 4 ]:Eu 2+ exhibits a poor thermal quenching resistance with an integrated photoluminescence emission intensity of 5% at T = 398 K relative to the room temperature value.The temperature-dependent photoluminescence emission spectra are presented in Figures S4 and S5.The emission band of Sr[Li 3 AlO 4 ]:Eu 2+ is shifted to higher energies and its shape becomes more asymmetric on the high-energy side of the spectrum with increasing temperature.This temperature behavior of the emission spectrum is already known from structurally related Sr[Li 2 Al 2 O 2 N 2 ]:Eu 2+ . 5he UV-irradiated powder samples of Eu 2+ -activated Sr-[Li 3 (Al 1−x Ga x )O 4 ] in Figure 1 show that the replacement or partial substitution of aluminum by gallium lead to a decrease in the photoluminescence emission intensity with increasing gallium mole fraction, which is already perceptible to the human eye.The qualitative trend is experimentally confirmed by the measured photoluminescence quantum efficiency Φ of four selected powder samples under blue-light excitation, as shown in Figure 5b.Relative quantum efficiencies of Sr[Li 3 (Al 1−x Ga x )-O 4 ]:Eu 2+ were obtained for x = 0 (Φ = 83%), x = 0.1 (Φ = 76%) and x = 0.2 (Φ = 62%).To confirm the decrease of the quantum efficiency for a gallium-rich solid solution of Sr[Li 3 (Al 1−x Ga x )-O 4 ]:Eu 2+ (x > 0.5), a measurement was performed that yielded a quantum efficiency of Φ = 22% for the nominal gallium mole fraction x = 0.6.The missing values of Φ for the members of the solid-solution series are due to the presence of probably absorbing or scattering impurities and secondary phases caused by the material of the crucibles and/or the starting materials used in the syntheses.

CONCLUSION
The new quaternary compound Sr[Li 3 GaO 4 ] was synthesized at moderate temperature, demonstrating the successful replacement of aluminum by gallium in Sr[Li 3 AlO 4 ].Single-crystal and powder X-ray diffraction analysis revealed the presence of a solid-solution series in Sr[Li 3 (Al 1−x Ga x )O 4 ] and the isomorphic crystallization of the oxolithoaluminate and the oxolithogallate was confirmed.In a first approximation and under the assumption that Eu 2+ occupies the two crystallographically distinct strontium sites in Sr [Li 3 (Al/Ga)O 4 ], the position of the emission band is presumable related to the bonding situation within the cation polyhedra.Although the average Sr−O distances of the [SrO 8 ] polyhedra are almost equal in Sr[Li 3 AlO 4 ] and Sr[Li 3 GaO 4 ], the shortest Sr−O distances are slightly longer in the oxolithogallate compared to the oxolithoaluminate.In addition, the polyhedra distortion is lower in the oxolithogallate as in the oxolithoaluminate.Both, the reduced crystal-field splitting and the lower polyhedra distortion in the oxolithogallate would cause a slight shift of the emission band to higher energies (lower wavelenghts).Sr-[Li 3 AlO 4 ]:Eu 2+ exhibits potential as an inorganic phosphor for solid-state lighting applications due to its high quantum efficiency and excellent thermal quenching behavior, while both properties deteriorate significantly for the Eu 2+ -activated gallium-containing derivates and the oxolithogallate, respectively.A possible explanation for the reduction in quantum yield with increasing gallium content could be the different thermal behavior.The thermal quenching data for Sr [Li 3 AlO 4 ]:Eu 2+ suggest only minimal thermal quenching at room temperature.The significantly worse thermal behavior for Sr [Li 3 GaO 4 ]:Eu 2+ , however, could hint at significant thermal losses already at room temperature.We thus hypothesize, that the reduction in quantum yield with increasing gallium mole fraction x in The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.chemmater.4c01382.

Figure 3 .
Figure 3. (a) Experimental X-ray diffraction patterns of seven powder samples of Sr[Li 3 (Al 1−x Ga x )O 4 ] with the nominal gallium mole fraction x being equal to 0 (black), 0.1 (red), 0.2 (blue), 0.4 (green), 0.6 (purple), 0.8 (brown), and 1 (turquoise).The simulated X-ray diffraction patterns of Sr[Li 3 AlO 4 ] (gray) and Sr[Li 3 GaO 4 ] (pink) were calculated from the single-crystal data.(b) Unit-cell volumes (black squares) as a function of the nominal gallium mole fraction x in Sr[Li 3 (Al 1−x Ga x )O 4 ].V is derived from the Rietveld refined unit-cell parameters obtained from powder X-ray diffraction data (Cu K-L 3 radiation, λ = 1.54056Å) and the straight line (red) represents the linear regression of the variables shown.
, the unit-cell volumes are plotted against the nominal gallium mole fraction x in Sr[Li 3 (Al 1−x Ga x )O 4 ].As expected from the obtained unit-cell parameters of Sr[Li 3 AlO 4 ] and Sr[Li 3 GaO 4 ], which do not differ by more than 3%, we observe an ideal Vegard behavior in the solid-solution series of Sr[Li 3 (Al 1−x Ga x )O 4 ] and therefore the crystal structures can be described as isomorphous.3.3.Photoluminescence.The normalized emission spectra of the Eu 2+ -activated powder samples of Sr[Li 3 (Al 1−x Ga x )O 4 ] are shown in Figure
which is also an ordered variant of the U[Cr 4 C 4 ]-type structure with comparable Sr−N/O distances as in Sr[Li 3 (Al/Ga)O 4 ] and Sr[LiAl 3 N 4 ].The shift of spectral position of the emission band to higher energies can be explained by the reduced nephelauxetic effect regarding oxygen ligands compared to nitrogen ligands in the activators' coordination sphere, since the 4f 6 5d 1 ↔ 4f 7 transitions of Eu 2+ are highly sensitive to the local chemical environment.In the crystal structure of Sr[LiAl 3 N 4 ]:Eu 2+ , only Eu−N bonds are present and the covalency reduces for the Eu−N/O bonds in Sr[Li 2 Al 2 O 2 N 2 ]:Eu 2+ to exclusively Eu−O bonds in Sr-[Li 3 (Al 1−x Ga x )O 4 ]:Eu 2+ .The value of the full width at halfmaximum (fwhm) increases on the energy proportional scales at room temperature.Sr[LiAl 3 N 4 ]:Eu 2+ (1180 cm −1 , 0.146 eV) exhibits the smallest value, which increases from Sr-[Li 2 Al 2 O 2 N 2 ]:Eu 2+ (1286 cm −1 , 0.159 eV) to Sr[Li 3 AlO 4 ]:Eu 2+ (1446 cm −1 , 0.18 eV) with the highest one found for Sr[Li 3 GaO 4 ]:Eu 2+ (1589 cm −1 , 0.20 eV).
, the temperature-dependent emission intensities of the powder samples of Sr[Li 3 AlO 4 ]:Eu 2+ and Sr[Li 3 GaO 4 ]:Eu 2+ are shown in the temperature range from 298 to 498 K.The integrated photoluminescence intensity of the emission spectrum of Sr[Li 3 AlO 4 ]:Eu 2+ decreases to 83% within the measured temperature range, revealing excellent thermal quenching resistance with an emission intensity of >93% at T = 423 K relative to the value at room temperature.The thermal quenching resistance of Sr[Li 3 AlO 4 ]:Eu 2+ is slightly weaker than that for Sr[Li 2 Al 2 O 2N 2]:Eu 2+ with a relative photoluminescence emission intensity of >96% at T = 420 K.5 Even superior thermal quenching resistance was reported for isotypically crystallizing Sr[LiAl 3 N 4 ]:Eu 2+ , retaining >95% of the relative photoluminescence emission intensity at T = 500 K.2

Sr[Li 3 (
Al 1−x Ga x )O 4 ]:Eu 2+ is mainly caused by increased thermal quenching at room temperature.Due to the structural relationship of the presented oxolitho(alumo)gallates with already known and intensively studied U[Cr 4 C 4 ]-type phosphors, future investigations involving time-resolved and lowtemperature luminescence spectroscopy in combination with first-principles calculations could be of interest to develop a deeper understanding of the Eu(II) luminescence as a function of the composition of the host structure.■ ASSOCIATED CONTENT * sı Supporting Information ) representations of the refined unit-cell parameters including the Rietveld refinements (Figure S2) of Sr[Li 3 (Al 1−x Ga x )O 4 ] derived from powder X-ray diffraction; normalized emission spectra of Eu 2+ -activated Sr[Li 3 (Al/Ga)O 4 ] single crystals (Figure S3); temperature-dependent photoluminescence spectra of Eu 2+activated powder samples of Sr[Li 3 (Al/Ga)O 4 ] (Figures S4 and S5); room temperature luminescence decay curve of Sr[Li 3 AlO 4 ]:Eu 2+ (Figure S6) (PDF) ■ AUTHOR INFORMATION Corresponding Author Hubert Huppertz − Department of General, Inorganic and Theoretical Chemistry, University of Innsbruck, Innsbruck AT-6020, Austria; orcid.org/0000-0002-2098-6087;Email: Hubert.Huppertz@uibk.ac.at presented by the groups of F. J. DiSalvo and Clarke and W. Schnick et al., respectively.Based on solid-state synthesis at moderate temperature, aluminum was completely replaced by gallium in Sr[Li 3 AlO 4 ], which yields the hitherto unknown quaternary oxolithogallate Sr[Li 3 GaO 4 ].Powder Xray diffraction experiments in the cationic substitution series of Sr[Li 3 (Al 1−x Ga x )O 4 ] demonstrate that a solid-solution series is present, and therefore the crystal structures of Sr[Li 3 AlO 4 ] and Sr[Li 3 4

Table 2 .
Spectral Data of the Eu 2+ -Activated Powder Samples of Sr[Li 3 (Al 1−x Ga x )O 4 ] Showing the Nominal Gallium Mole Fraction x, Peak Wavelength λ em (nm), Dominant Wavelength λ dom (nm), and the CIE-xy Values in the CIE 1931 Color Space