Neutron diffraction study of the structural changes in the antifluorite crystals Rb2PtI6, (NH4)2PtBr6, and (NH4)2PtI6

Abstract

A neutron diffraction study of the structural phase transitions in Rb2PtI6, (NH4)2PtBr6, and (NH4)2PtI6 is presented. Cubic antifluorites studied to date exhibit transitions involving ferro- or antiferro-rotations of the halide octahedra with little or no tetragonal distortions. In this study we show that tetragonal distortions also occur in the antifluorites with or without accompanying rotations. Profile analysis reveals that below 65 K the Rb2PtI6 structure and below 50 K the (NH4)2PtBr6 structure become tetragonal with no evidence for rotation of the octahedra. Further, the 258 K transition in (NH4)2PtI6 is antiferro-rotative, with octahedra in successive (001) planes rotated in opposite senses, accompanied by a tetragonal distortion; the transition at 100 K involves a further ferro-rotation.