Raman scattering study of the structural phase transitions in RbAlF4

A Raman scattering study of the non-ferroic and of the improper ferroelastic structural phase transitions (SPT) in the perovskite-type layer compound RbAlF 4 is reported. It is shown that the SPT occurring at T cl =551K (D 4h 1 from or to D 4h 5 ) and at T c2 =282K (D 4h 5 from or to D 2h 13 ) are related to displacive mechanisms owing to the occurrence of one and two soft phonons below T c1 and T c2 respectively. The results are consistent with a first-order character for the SPT at T c1 and a second-order character for that at T c2 . Moreover, in the tetragonal D 4h 1 and D 4h 5 phases, the Raman spectra exhibit a number of features which cannot be explained by the selection rules at q=0. In particular, one of these additional lines exhibits a soft-mode behaviour above the T c2 transition, while the condensing phonon is a zone-boundary mode. Such features are imputed to the loss of translational symmetry caused by the existence of intrinsic structural disorder.