Tetrahedra Rotational and Displacive Disorder in the Scheelite-Type Oxide CsReO₄

Scheelite-type metal oxides are a notable class of functional materials, with applications including ionic conductivity, photocatalysis, and the safe storage of radioactive waste. To further engineer these materials for specific applications, a detailed understanding of how their properties can change under different conditions is required─not just in the long-range average structure but also in the short-range local structure. This paper outlines a detailed investigation of the metal oxide CsReO₄, which exhibits an uncommon orthorhombic Pnma pseudo-scheelite-type structure at room temperature. Using synchrotron X-ray diffraction, the average structure of CsReO₄ is found to undergo a transformation from the orthorhombic Pnma pseudo-scheelite-type structure to the tetragonal I4₁/a scheelite-type structure at ∼440 K. In the X-ray pair distribution function analysis, lattice strain and rotations of the ReO₄ tetrahedra are apparent above 440 K despite the increase in long-range average symmetry, revealing a disconnect between the structural models at different length scales. This study demonstrates how the bonding requirements and ionic radii of the A-site cation can induce disorder that is detectable at different length scales, affecting the physical properties of the material.