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 Pnma  ⟷563KPnma+Pm3¯m  ⟷602KPm3¯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.