A study of the structural phase transitions in AlF₃: X-ray powder diffraction, differential scanning calorimetry (DSC) and Raman scattering investigations of the lattice dynamics and phonon spectrum

The cubic-rhombohedral phase transition at 450 degrees C of AlF₃ is studied by DSC, X-ray powder diffraction and Raman scattering. It is demonstrated that the transition is of first order with a hysteresis of about 6 degrees. It is established by X-ray powder diffraction patterns that the room temperature space group is R3c. A temperature study of the Raman scattering spectra (that confirms the above conclusion) evidences the presence of two soft modes. It is shown from group theory that the transition can be imputed to the condensation of the R₅ mode of the cubic Brillouin zone and the attribution of the Raman lines is deduced on the basis of the compatibility diagram between the cubic and rhombohedral symmetries. The frequencies of the Raman lines are used to adjust the parameters of a rigid ion model and to calculate the phonon spectrum in the cubic phase. The calculated phonon density of states appears to be strongly dependent on the soft phonon frequency.