Cu₆P(S_1-xSe_x)₅Br_1-yI_y mixed crystals, known as superionic conductors and ferroelastic materials, were obtained by chemical transport. Raman spectra were measured in a 90^o geometry using a LOMO DFS-24 double grating monochromator and a Не-Ne (λ=632.8 nm) laser.

Here we report on the studies of the influence of temperature-related, structural (dynamic and static) and compositional disordering on the low-frequency range (ω<100 cm^-1) of phonon  spectra of Cu₆P(S_1-xSe_x)₅Br_1-yI_y superionic conductors. In this frequency range the modes belonging to the diffusive-type vibrations of Cu atoms and Cu–Br(I) bond vibrations are observed.

The monotonous broadening of all phonon bands at the temperature increase from 77 K to the temperature of the transition to the superionic state is due to the temperature-related disordering caused by thermal vibrations of the lattice. Near the superionic phase transition, anomalies in the temperature dependences of the band frequencies are observed as well as their broadening by 10-20 cm^-1, explained by dynamic structural disordering of Cu sublattice at the transition to the superionic state.

Raman studies of Cu_6+δРS₅Br crystals where δ is the deviation from stoichiometry, were performed. With the increase of δ, a high-frequency shift of the bands is observed, as well as an increase of intensity and halfwidth of the lower-frequency band, corresponding to Cu atom vibrations. Meanwhile, the intensity and halfwidth of the higher-frequency band, corresponding to Cu–Br bond vibrations, decrease.

For the phonon spectra of Cu₆PS₅Br_1-yI_y mixed crystals the one-mode type of compositional transformation is shown, contrary to the two-mode type in Cu₆P(S_1-xSe_x)₅I system. The most essential changes are revealed in the low-frequency range due to the cationic sublattice disordering and the motion of mobile Cu⁺ ions in the superionic phase. The I→Br substitution results in a nonlinear decrease of the low-frequency band frequency and increase of its halfwidth.

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1. Phonon spectra of nanometric powders and composites of Cu₆PS₅Br superionic conductors