RAMAN-SCATTERING STUDY OF KFEF4

The layered compound KFeF4 that exhibits a structural phase transition at T(c) congruent-to 380 K from Bmmb symmetry (phase I) to Pcmn symmetry (phase 11) is studied as a function of temperature by polarized Raman scattering. All the Raman lines are attributed on the basis of (i) symmetry analysis of the normal modes of vibration in the aristotype phase (phase I) and the compatibilities relations with the symmetries in phase II; (ii) comparison with RbFeF4 in which the phonon spectrum can be calculated. The phase transition is clearly associated with a soft mode but a residual Raman signal is still observed above T(c); this is attributed to the existence of a disordered phase. Two additional modes exhibit a soft behaviour connected with a (virtual) martensitic transformation.