The strongly defective double perovskite Sr11Mo4O23: crystal structure in relation to ionic conductivity

Abstract

The crystal structure and ionic conductivity properties of a novel microcrystalline Sr11Mo4O23ceramic material are presented. This material has been prepared by thermal treatment up to 1473 K, in air, of previously decomposed citrate precursors. The complex crystal structure was refined from combined X-ray powder diffraction and neutron powder diffraction data. The formula of this phase can be rewritten as Sr1.75□0.25SrMoO5.75, highlighting the relationship with double perovskitesA2B′B′′O6. At room temperature, the crystal structure is tetragonal in space groupI41/a, witha= 11.6107 (6) Å,c= 16.422 (1) Å andV = 2213.8 (2) Å3. The crystal network contains O anion and Sr cation vacancies. The structure is complex, with Sr, Mo and O atoms distributed over four, two and six distinct Wyckoff sites, respectively. Only one of the Sr sites (SrO6) corresponds to the octahedral network; one of the two MoO6types of octahedra is axially distorted. The three other Sr positions occupy theAsite with higher coordination. There is an occupational deficit of O atoms of 22 (4)%. This defective framework material presents an interesting ionic mobility, enhanced above 773 K owing to a further reduction in the oxygen content.