RAMAN-SPECTROSCOPIC STUDIES OF SILVER-TANTALUM SULFIDE, SILVER-NIOBIUM SULFIDE AND SILVER-TANTALUM SELENIDE WITH ARGYRODITE STRUCTURES

Raman spectra of the superionic conductors Ag7TaS6, Ag7NbS6 and Ag7TaSe6, which crystallize in the argyrodite structures, have been studied in the temperature range from 300 to 80K for the first time. In these compounds, there exist the nearly isolated tetrahedral MX(4) (M=Ta, Nb; X=S, Se) units, and the peaks due to the M-X stretching modes of these units have been identified. As for Ag7NbS6, the observed Nb-S stretching peaks have shown the shifts or the splittings, which suggest the phase transitions, with decreasing temperature. In the low frequency region below 80 cm(-1), the peaks due to the vibration modes of the mobile silver ions have been observed at 80K, and the results have been discussed as compared with those of Ag2S and Ag2Se.