Vibrational spectra and phase transitions of Cu8MX6 (M=Si, Ge; X=S, Se) and Cu4GeS4

The infrared transmission spectra of the high- and low-temperature phases of Cu8MX6 (M=Si, Ge; X=S, Se), which belong to the argyrodite family of compounds, and Cu4GeS4 were studied near the phase transition temperature. These compounds undergo phase transitions in the temperature range from 320 to 360 K. The observed M–X stretching bands vary discontinuously at the transition temperature. The Raman spectrum of Cu8GeSe6 in the low-temperature phase was also measured. The infrared- or Raman-active lattice vibrational frequencies and the relative intensities of the infrared absorption bands were calculated based on simple lattice dynamical models.