High-temperature dynamic octahedral tilting in the ionic conductor Sr11Mo4O23

Abstract

This work presents the crystal structure evolution of a novel ionic conductor Sr11Mo4O23 at high temperature. The formula of this phase can be rewritten as Sr1.75□0.25SrMoO5.75, highlighting the relationship with double perovskites A2B′B″O6. The crystal network contains oxygen-anion and strontium-cation vacancies. The structure is complex; Sr, Mo and O atoms are distributed in four, two and six distinct Wyckoff sites, respectively. It was refined from neutron powder diffraction data collected at 473, 673, 873 and 1073 K. The thermal evolution of crystallographic parameters supports the known reversible process of removal/uptake of O-atom content in the 673-873 K temperature range. Above 873 K, from difference Fourier maps, it was found that the structure exhibits an oxygen delocalization around one of the Mo sites. This delocalization was understood as a dynamical octahedral tilt of the MoO6 octahedron, yielding an increase in the ionic conductivity at high temperature.