Lattice dynamics and structural phase transitions in RbAlF4: group theory, inelastic neutron scattering results and the calculation of the phonon spectrum

The low-frequency phonon spectrum of RbAlF 4 is investigated by inelastic neutron scattering at 400 degrees C. The structural phase transitions at T cl =280 degrees C and T c2 9 degrees C are shown to arise from the condensation of the zone boundary M 3 and XZ 3 soft modes, respectively, and they are preceded by the appearance of a central component. The symmetry properties of the vibrations in the ideal high-temperature phase are calculated and the compatibility relations of the symmetries between high- and low-temperature phases are given. A rigid ion model involving ten parameters is proposed to describe the phonon spectrum. Seventeen experimental data are used for the refinement; the discrepancy between experimental and calculated values is less than 10%. The main features of the spectrum are discussed, the Raman scattering anomalies are explained, and an attribution of infrared active modes is proposed. The Al-F short-range interaction parameters are given for the first time and the interionic distance dependence of the fluorine-fluorine short-range interaction parameters is deduced.