The Observation of a Displacive Phase Transition in (NH4)2PtBr6

Measurements of the 79Br nuclear quadrupole resonance frequency and spin–lattice relaxation time in a polycrystalline sample of (NH4)2PtBr6 from 4 to 450 K are reported. The frequency data indicate the occurrence of a structural phase transition at 58 K. The spin–lattice relaxation time data exhibit a distinct minimum, also at 58 K. The data are discussed in terms of a model previously introduced to account for phase transitions from cubic to tetragonal symmetry in R2MX6 compounds. A point charge calculation provides an explanation of the observed frequency splitting in terms of the alteration in the neighboring ion contribution to the electric field gradient at the 79Br nuclear sites. This alteration results from a small distortion of the cages defined by the NH4 ions and an accompanying rotation of the PtBr6 octahedra situated within these cages. The temperature dependences of two different average rotary lattice mode frequencies are deduced; in each case the observed behavior supports the hypothesis that it is the rotary lattice mode which softens thereby bringing about the change in structure.