Neutron-powder-diffraction profiles are reported for the antifluorite K2PtBr6 at temperatures of 296, 160, 141, and 120 K. Bromine nuclear-quadrupole-resonance measurements have shown that each of these temperatures corresponds to a different crystal structure and have provided information on the most probable structures. A variety of structural models with symmetries obtained from a group theoretical classification of structural phase transitions are fitted to the diffraction data. A comparison of goodness-of-fit parameters for the models considered at each temperature allows the space group of each phase to be identified. The results provide (i) a definitive test of the validity of a classification scheme proposed for nuclear-quadrupole-resonance spectral characteristics, and (ii) support for the proposition that the allowable, continuous distortions of a crystal are given by the nondisruptive criterion rather than the more restrictive group–subgroup criterion.
