Tetrahedral displacive disorder in the scheelite-type oxide RbReO4

Abstract

Oxides exhibiting the scheelite-type structure are an important class of functional materials with notable applications in photocatalysis, luminescence and ionic conductivity. Like all materials, understanding their atomic structure is fundamental to engineering their physical properties. This study outlines a detailed structural investigation of scheelite-type oxide RbReO4, which exhibits a rare long-range phase transition from I41/a to I41/amd upon heating. Additionally, in the long-range I41/a model, the Re-O tetrahedral distance undergoes significant contraction upon warming. Recent studies of other scheelite oxides have attributed this apparent contraction to incoherent local scale tetrahedral rotations. In this study we use X-ray pair distribution function analysis to show that RbReO4 undergoes a unique symmetry lowering process on the local scale, which involves incoherent tetrahedral displacements. The rare I41/a to I41/amd long-range phase transition was found to occur via a change from static to dynamic disorder on the local scale, which is due to the combination of the size of the A-site cation and lattice expansion. This demonstrates how careful manipulation of the ionic radius of the A-site in the scheelite structure can be used to induce local scale disorder, which has valuable implications for tailoring the physical properties of related materials.