Raman scattering in K1-xRbxAlF4: evidence of a new phase

Abstract

A Raman scattering study of the quasi-reversible structural phase transitions observed below room temperature in the perovskite-type layer compounds K1-xRbxAlF4 with x=0.06, 0.09 and 0.15 is reported. The Raman spectra were analysed with the help of the result of a calculation of the phonon spectrum in the ideal (aristotypic) phase (phase I, P4/mmm symmetry). For all the studied compounds, the spectra collected at room temperature are consistent with the P4/mbm symmetry (phase II), resulting from the condensation of the M3 mode. The phase IIIa Raman spectra for x=0.06 and x=0.09 are explained with the P21/m space group and the phase II-phase IIIa transition can be imputed to the condensation of two components of the degenerate X13 mode plus one of the components of the degenerate M19 mode. The Raman spectra of the K0.85Rb0.15AlF4 compound, on lowering the temperature, reveal the existence of a new phase (IIIb) between phase II and phase IIIa An assignment to vibrational modes, in this phase IIIb, is possible in the Pmmn space group and so the phase IIIa-phase IIIb transition could be attributed to the condensation of the X13 mode like in pure RbAlF4. The non-reversible martensitic phase transition of the pure compound KAlF4 is no longer observed for x=0.15.