Phase transition of the orthorhombic fluorite-related compounds Ln3IrO7 (Ln = Pr, Nd, Sm, Eu)

Rare earth iridium oxides Ln3IrO7 (Ln = Pr, Nd, Sm, and Eu) were prepared and their structures were determined by X-ray diffraction measurements. At room temperature, Pr3IrO7 crystallized in an orthorhombic superstructure of cubic fluorite with space group Cmcm. The differential thermal analysis (DTA) and specific heat measurements for Ln3IrO7 (Ln = Pr, Nd, Sm, and Eu) showed a phase transition at 262, 342, 420, and 485 K, respectively. At low temperatures, Ln3IrO7 crystallized in a monoclinic structure with the space group P21/n. The transition temperatures increased with decreasing the ionic radius of rare earths, which indicates that the transition is stress-induced and occurs with the lattice contraction on cooling. These results for Ln3IrO7 were compared with the phase transitions observed for Ln3MoO7, Ln3RuO7, Ln3ReO7, and Ln3OsO7.