High-temperature structural evolution of caesium and rubidium triiodoplumbates

CsPbI₃ and RbPbI₃ were investigated by in situ powder diffraction within temperature ranges of 298-687 K and 298-714 K, respectively. Both compounds crystallize in orthorhombic Pnma symmetry and expand isotropically upon a heating, revealing almost the same relative change of the lattice parameters. A pronounced difference in the structural evolution close to 600 K was observed, namely, CsPbI₃ undergoes first-order reversible phase transformation PnmaPnma+Pm3¯mPm3¯m, whereas no transitions (except of the sample's melting) in RbPbI₃ were detected. An attempt to clarify the relation between the existence/absence of a phase transition and bulging out of the iodine environment around alkaline ions was undertaken.