Cubic to tetragonal phase transitions in(NH4)2PtBr6andRb2PtI6

Abstract

Nuclear-quadrupole-resonance data for the bromine and iodine nuclci in ${(\mathrm{N}{\mathrm{H}}_{4})}_{2}$Pt${\mathrm{Br}}_{6}$ and ${\mathrm{Rb}}_{2}$Pt${\mathrm{I}}_{6}$ have revealed a hitherto unreported type of cubic to tetragonal phase transition in ${R}_{2}M{X}_{6}$ compounds. These transitions are characterized by a tetragonal distortion of the lattice but no static rotation of the $M{X}_{6}$ octahedra. Several models for the specific physical nature of these transitions are discussed including both static and dynamic possibilities. The final identification must await another kind of experiment.