Structural studies of Cs2AgBiBr6 double perovskite

Abstract

The structure of Cs₂AgBiBr₆ between 80 and 350 K was determined by Rietveld refinement of high-resolution synchrotron X-ray diffraction data. At room temperature, the material adopts a cubic Fm $\bar{3}$m structure with rock-salt-like ordering of Ag⁺ and Bi³⁺ cations, doubling the perovskite unit cell. Despite the lower valence Ag⁺ being larger than Bi³⁺ (1.15 vs 1.03 Å), the Ag–Br bond is slightly shorter than the Bi–Br bond (2.8103(8) vs 2.8229(8) Å), with nearly equal MO₆ octahedral volumes, suggesting greater covalency and distortion in AgBr₆ units. The Cs⁺ cations are underbonded in the cubic phase and large anisotropic Br displacements perpendicular to Ag–Br–Bi chains, attributed to dynamic rotations of corner-sharing octahedra, partially offsetting this underbonding. Below 120 K, the Br displacements freeze, inducing a continuous transition to a tetragonal I4/m structure. An unusual expansion in the volume of the BiBr₆ polyhedara occurs in the low temperature tetragonal phase. The structural changes with temperature reflect the balance between ionic size, bonding character, and dynamic lattice effects.