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Neutron and x-ray scattering studies of structural phase transitions and soft modes in

The low-temperature phase transitions of rubidium tetrabromozincate have been investigated with the use of neutron and x-ray scattering techniques to clarify the mechanism of the transitions and to determine the space groups of the low-temperature phases. The low-frequency excitations which soften at the phase transition at have been measured by the inelastic neutron scattering technique. On the other hand, no soft phonon mode which makes a contribution to the phase IV-phase V transition at has been observed. For both phase IV and phase V, superlattice reflections appear at the position , where the Miller indices h, k, and l are referred to the lattice constants of phase I. In phase V, new Bragg reflections emerge at the position hk0: h + k = 2n + 1 and increase in intensity continuously with decreasing temperature. The space groups are considered to be P11b with Z = 24 for phase IV and C1c1 with Z = 48 for phase V on the basis of general group theoretical considerations. Furthermore, structural models for phase IV are proposed, to explain the extra extinction rule, the antiferroelectricity, and other observed characteristics of the crystal.