Novel perovskite-related barium tungstate Ba11W4O23

Ba11W4O23 was synthesized at 1300 °C, followed by quenching with liquid nitrogen. The crystal structure, which was known to be cryolite-related but has remained unclear, was initially determined by single-crystal X-ray diffraction for the isostructural Ru-substituted compound Ba11(W3.1Ru0.9)O22.5, which was discovered during exploratory synthesis in the Ba–Ru–O system. The structure of Ba11W4O23 was refined by a combined powder X-ray and neutron Rietveld method (Fd-3m, a=17.1823(1) Å, Z=8, Rp=3.09%, Rwp=4.25%, χ2=2.8, 23 °C). The structure is an example of A-site vacancy-ordered 4×4×4 superstructure of a simple perovskite ABO3, and it may be written as (Ba1.75□0.25)BaWO5.75□0.25, emphasizing vacancies on both metal and anion sites. The local structure of one of two asymmetric tungsten ions is the WO6 octahedron, typical of perovskite. The other tungsten, however, is surrounded by oxygen and anionic vacancies statistically distributed over three divided sites to form 18 partially occupied oxygen atoms (∼30% on average), represented as WO18/3. The A-site cation-vacancies are ordered at the 8a (image, image, image) site in between adjoining WO18/3 polyhedra which form 1-D arrangements along [110] and equivalent directions. In situ high-temperature XRD data have shown that the quenched Ba11W4O23 at room temperature is isostructural to the high-temperature phase at 1100 °C.