Crystal structure of the quaternary lithogallate Sr

: The new quaternary lithogallate Sr 2 LiGaO 4 was prepared by conventional solid-state synthesis in a welded tantalum ampoule at T = 850 ° C. Single-crystal X-ray di ﬀ raction 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 re ﬁ nement based on X-ray di ﬀ raction data. The crystal structure is composed of layers of corner-sharing LiO 4 and GaO 4 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 SrO 8 units with common faces. The crystal structure of Sr 2 LiGaO 4 is isotypic to that of the lithoaluminate derivate Sr 1.85 Ba 0.15 LiAlO 4 .


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 Ce 3+ -or Eu 2+ -activated layered compound Sr 2 LiAlO 4 was published in 2018 along with theoretical calculations and reports on its photoluminescence performance [15].The crystal structure of monoclinic Sr 2 LiAlO 4 (space group P2 1 /m) derived from density-functional theory calculations and powder X-ray diffraction data was confirmed by singlecrystal X-ray diffraction in 2019 [15,16].Polymorphism and cationic or anionic substitution of the lithoaluminate yielded the isostructural compounds Sr 2 LiAlO 4 :Eu 2+ (space group Cmcm), Sr 1.85 Ba 0.15 LiAlO 4 :Eu 2+ (space group Pnma), and Sr 2 Li 0.9 Al 1.1 O 3.8 N 0.2 :Eu 2+ (space group Cmcm) showing greento-red emissions upon blue-light excitation due to the electric dipole allowed 4f 6 5d 1 ↔ 4f 7 transitions of the Eu 2+ ion [16,17].The effect of activator concentration on the emission intensity and the temperature-dependent luminescence of Sr 2 LiAlO 4 :Ce 3+ /Eu 2+ were investigated, including coactivation 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 Eu 2+ over two crystallographically distinct Sr sites in the monoclinic polymorph (space group P2 1 /m) of Sr 2 LiAlO 4 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 Eu 2+ occupying the thermally less stable SrO 8 polyhedron with shorter average interatomic distances, which is supported by the deconvolution spectrum of Sr 2 LiAlO 4 :Eu 2+ , showing an intensity decrease of the longer wavelength emission at room temperature [15].The lower-energy emission is associated with Eu 2+ occupying the thermally less stable Sr2 site in the P2 1 /m polymorph of Sr 2 LiAlO 4 as was also observed for the barium-containing lithoaluminate derivate [16].Recently, spectral tailoring was assigned to partial cationic substitution in Sr 2 Li(Al 1−x Ga x ) O 4 :Eu 2+ leading to a narrow-band emission peaking at 512 nm that is caused by the compression of the Sr2O 8 polyhedron and forces the Eu 2+ ion to occupy exclusively the Sr1 site [24].The existence of the gallium-containing derivates of Sr 2 Li(Al O (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 rodshaped transparent crystals of Sr 2 LiGaO 4 were obtained from the tubefurnace approach.

Single-crystal X-ray diffraction
Single crystals of Sr 2 LiGaO 4 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].

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].

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.

Crystal structure
Isostructural phases of monoclinic (space group P2 1 /m) and orthorhombic (space group Cmcm) polymorphs of Sr 2 LiAlO 4 , including the lithoaluminate derivate Sr 1.85 Ba 0.15 LiAlO 4 (space group Pnma), were previously discussed in detail [16].The aforementioned alkaline earth lithoaluminates are  [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 Sr 2 LiGaO 4 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.
In accordance with the findings for the Sr1 site in the substitution series of Sr 2 Li(Al 1-x Ga x )O 4 (x = 0.1-0.4) and the above-mentioned polymorphs of Sr 2 LiAlO 4 , the average interatomic Sr1-O distances remain approximately unchanged when either polymorphism occurs, and the Al1-site is partially substituted, or Al 3+ is completely replaced by Ga 3+ [16,24].Only the partial substitution of Sr 2+ by Ba 2+ leads to a lengthening of the interatomic Sr1/Ba1-O distances compared to orthorhombic Sr 2 LiGaO 4 and monoclinic Sr 2 LiAlO 4 [16].The lengthening is about twice as large for the barium-containing lithoaluminate derivate as for the new lithogallate Sr 2 LiGaO 4 with respect to the P2 1 /m polymorph of Sr 2 LiAlO 4 .On the other hand, the trend of shortening of the average interatomic Sr2-O distances in the galliumcontaining derivates of Sr 2 Li(Al 1-x Ga x )O 4 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 Sr 2 LiGaO 4 , which show only minor differences in the average Sr2-O distances [24].Starting from the crystal structure of monoclinic Sr 2 LiAlO 4 , 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 Sr 2 LiGaO 4 is shown in Figure 3, where each LiO 4 tetrahedron is connected to four GaO 4 tetrahedra via shared corners and vice versa.
The interatomic Ga-O distances of Sr 2 LiGaO 4 are elongated compared to the average Al-O distances of Sr 1.85 Ba 0.15 LiAlO 4 and monoclinic Sr 2 LiAlO 4 .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 LiO 4 tetrahedra compared to the monoclinic lithoaluminate and the barium-containing lithoaluminate derivate.The crystal structure of Sr 2 LiGaO 4 compensates for the described elongation and compression through the evasion of the two capping oxygen atoms of the SrO 8 polyhedra of both Sr sites, which exhibit longer interatomic Sr-O distances Table : Crystal structure refinement of Sr  LiGaO  derived from singlecrystal X-ray diffraction data.

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      (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.

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 Sr 2 LiGaO 4 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.
The lattice energy of a hypothetical compound with the formula Sr 2 LiGaO 4 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.

Conclusions
The complete replacement of aluminum by its heavier homologue gallium in Sr 2 LiAlO 4 was successfully achieved, leading to the isostructural and hitherto unknown phase Sr 2 LiGaO 4 .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 Sr 2 LiAlO 4 , 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 Sr 2 LiGaO 4 were not successful.

Figure 4 :
Figure 4: Coordination spheres of the three distinct O sites (blue spheres) in Sr 2 LiGaO 4 with the Sr sites (red spheres), Li sites (orange spheres), and the Ga sites (green spheres).

Figure 5 :
Figure 5: 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 Sr 2 LiGaO 4 and the by-products.The difference curve (grey) is plotted along with the theoretical reflection positions of Sr 2 LiGaO 4 (blue), SrO (violet), Sr(OH) 2 (green), and Sr 3 Ga 2 OH 12 (orange).
[16]Ga x )O 4 and the associated effects on the luminescence properties of the Eu 2+ -activated compounds motivated the studies on the complete replacement of Al 3+ by Ga 3+ in Sr 2 LiAlO 4 .The new lithogallate Sr 2 LiGaO 4 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 Sr 1.85 Ba 0.15 LiAlO 4 is at the heart of discussion in this contribution[16].

Table  :
[16]off positions, atomic coordinates, and isotropic displacement parameters U iso (/Å  ) for Sr  LiGaO  .compared to the corresponding lithoaluminates.As depicted in Figure4, there are three distinct O sites present in the crystal structure of Sr 2 LiGaO 4 , as is also found for the coordination of the oxygen atoms in the lithoaluminate derivate Sr 1.85 Ba 0.15 LiAlO 4[16].