Abstract
A Raman scattering study of the non-ferroic and of the improper ferroelastic structural phase transitions (SPT) in the perovskite-type layer compound RbAlF4 is reported. It is shown that the SPT occurring at Tcl=551K (D4h1 from or to D4h5) and at Tc2=282K (D4h5 from or to D2h13) are related to displacive mechanisms owing to the occurrence of one and two soft phonons below Tc1 and Tc2 respectively. The results are consistent with a first-order character for the SPT at Tc1 and a second-order character for that at Tc2. Moreover, in the tetragonal D4h1 and D4h5 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 Tc2 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.
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