Phonon spectra of Cu_{6}PS_{5}Br superionic ferroelastic: experimental and theoretical studies

The anomalous temperature behavior of the Raman scattering bands at superionic phase transition of is revealed. Using density functional perturbation theory within local density approximation, ab initio long-wavelength phonons have been obtained for high-symmetry cubic structure. In the calculated phonon spectrum two unstable modes indicate the experimentally observed sequence of ferroelastic and superionic phase transitions