Crystal structure of Ba9La2W4O24 and Sr9Gd2W4O24: A new B-site vacancy ordered 4×4×4 cubic perovskite

Sr9La2(WO6)4 containing [WO6] octa-hedra

A polycrystalline sample of Sr9La2(WO6)4, nona-strontium dilanthanum tetra-kis-[orthotungstate(VI)], was prepared by heating a compacted powder mixture of SrCO3, WO3, and La2O3 with an Sr:La:W molar ratio of 9:2:4 at 1473 K. X-ray crystal structure analysis was performed for a Sr9La2(WO6)4 single-crystal grain grown by reheating the sample at 1673 K. Sr9La2(WO6)4 crystallizes with four formula units in the tetra-gonal space group I41/a and is isotypic with Sr11(ReO6)4. Two W sites with site symmetries of are located at the center of isolated [WO6] octa-hedra, and four mixed (Sr/La) sites are surrounded by eight to twelve O atoms of the [WO6] octa-hedra. The structure of Sr9La2(WO6)4 can be described on the basis of the double-perovskite structure with [WO6] and [(Sr/La)O x ] polyhedra alternately placed, and a vacancy (□).

Crystal structure of Ba2(La0.727Ba0.182M0.091)MO6 (M = Nb, Sb, Bi): symmetry nuance identified in photoluminescence and IR spectroscopy studies

A one-third lanthanum deficiency was created in Ba₂LaM⁵⁺O₆ compounds (LaM compounds) to form Ba₂La_2/3M⁵⁺O_5.5 compounds (La2/3M compounds) for M = Nb, Sb, and Bi. The compounds were prepared by a gel-combustion method using citric acid as a fuel. All the compounds were characterized by powder X-ray diffraction (XRD). The XRD analysis showed that the space group of the La2/3M compounds remains the same for the Bi and Sb samples when compared to the reported LaM compounds, except for the Nb sample. La2/3Nb and La2/3Sb adopt a rhombohedral structure with the space group R3[combining macron], whereas La2/3Bi adopts a monoclinic structure with the space group I2/m. As the positron annihilation spectroscopy (PALS) technique is sensitive to cation deficiency, it was used to detect the presence of cation vacancies in the samples, which are formed due to the decrease in the lanthanum concentration. The PALS analyses indicated that the absence of cation deficiency in the La2/3M compounds is similar to that observed in the LaM compound. Thus, the crystal structure of the La2/3M compound was modeled, such that the cation deficiency at the La site is filled by Ba²⁺ and M⁵⁺ ions, and the crystal structure formula is given as Ba₂(La_0.727Ba_0.182M_0.091)MO₆. This model was confirmed by Rietveld refinement of the XRD data. The emission spectra of Eu³⁺ showed a strong dependence on its local site symmetry in the host material, in which it is being doped and this can be used as a spectroscopic probe for detecting any differences in the symmetry. Comparison of the local symmetry around La³⁺ cation was studied using photoluminescence (PL) by doping 2 atom% Eu³⁺ in LaM and La2/3M compounds. Infrared spectroscopy (IRS) analyses were also carried out for LaM and La2/3M compounds. There was complete agreement between the PL and IRS results and they were also in concordance with the predicted crystal structure model. Interestingly in these La2/3M compounds, the equilibrium structure prefers large Ba²⁺ ion to occupy the octahedral B-site rather than forming an octahedral vacancy at that site, making these perovskite compounds rare and novel in their class.