NEUTRON AND X-RAY-SCATTERING STUDIES OF STRUCTURAL PHASE-TRANSITIONS AND SOFT MODES IN RB2ZNBR4

The low-temperature phase transitions of rubidium tetrabromozineate ha ve been investigated with the use of neutron and x-ray scattering tech niques to clarify the mechanism of the transitions and to determine th e space groups of the low-temperature phases. The low-frequency excita tions which soften at the phase transition at T-3 = 112 K have been me asured by the inelastic neutron scattering technique. On the other han d, no soft phonon mode which makes a contribution to the phase IV-phas e V transition at T-4 = 76 K has been observed. For both phase IV and phase V, superlattice reflections appear at the position h/2 k/3 l/3, where the Miller indices h, k, and l are referred to the lattice const ants of phase I. In phase V, new Bragg reflections emerge at the posit ion hk0: h + k = 2n + 1 and increase in intensity continuously with de creasing temperature. The space groups are considered to be P11b with Z = 24 for phase IV and Clcl with Z = 48 for phase V on the basis of g eneral group theoretical considerations. Furthermore, structural model s for phase IV are proposed, to explain the extra extinction rule, the antiferroelectricity, and other observed characteristics of the cryst al.