Model phonon spectra of Cu7SiS5I and Ag7SiS5I crystals.

In the concept of superspatial symmetry the crystal structure of Cu7SiS5I and Ag7SiS5I superionic conductors has been analyzed. To calculate the phonon spectra, the model of FCC superlattice (8а, 8а, 0; 8а, 0, 8а; 0, 8а, 8а) in the metric of protocrystal (а, а, 0; а, 0, а; 0, а, а) has been developed. For the developed model, the general (3+3)- dimensional basis, the array of modulation vectors and mass modulation functions have been presented. The model calculations of phonon spectra dispersion for Cu7SiS5I and Ag7SiS5I crystals in schemes with various partial occupation of crystallographic orbits by Cu(Ag) atoms have been performed. The dispersion dependences of phonon spectra for Cu7SiS5I and Ag7SiS5I crystals in the high-symmetric directions of Brillouin zone have been presented. The genesis of phonon branches of vibrational spectra has been analyzed. [ABSTRACT FROM AUTHOR]