STRUCTURAL PHASE-TRANSITIONS IN RBFEF4 .2. RAMAN-SCATTERING STUDY

For pt.I see ibid., vol.2, p.8269 (1990). The layer compound RbFeF 4, which undergoes structural phase transitions at 923 K, 416 K (second order) and 380 K (first order), has been studied by polarized Raman scattering (under the microscope) in the temperature range 473 K to 143 K. The Raman spectra were analysed with the help of a calculation of the phonon spectrum in the higher symmetry phase (phase I, P4/mmm) and the determination of symmetry compatibility with subgroups. The spectra collected in phase II (923-416 K) are consistent with the P4/mbm, Z=2, symmetry resulting from the condensation of the M3 mode. Very little changes are observed in the phase III Raman spectra, which can be explained with the Pmmn, Z=4 space group induced by the additional condensation of X3 modes Below 380 K (phase IV) the Raman spectra exhibit drastic changes which are consistent with the Pmab, Z=4, symmetry. The transition has to be imputed to the condensation of one component of the twice degenerate soft mode M9, maintaining the condensation of one of the components of the already condensed X3 mode, while M3 is no longer condensed. Pmab is not a Pmmn subgroup, which corroborates the first-order character of the III-IV transition. The sequence of phases in RbFeF4 can be schematized as a 0a0c0 to a0a0c + to ap+bp+c+ to ap+bp-c0. It is also shown that the FeF6 sheets can be characterize by several internal mode frequencies.