Raman Scattering Spectra of Proustite and Pyrargyrite Crystals in Low-Temperature Phases

Abstract

Crystals of proustite (Ag₃AsS₃) and pyrargyrite (Ag₃SbS₃) are of considerable interest as new nonlinear materials. The structure of both crystals at room temperature belongs to the trigonal space group C ⁶_3v (3 m) with two molecules per unit cell and differences in lattice parameters within a few per cent (1). Low-temperature phase transitions were found in proustite at 56 K (second order) and at 24 – 28°K (first order) on the basis of NQR spectra (2), electrical constants (3) and optical absorption (4) behaviour. For pyrargyrite a second-order phase transition was found at 9.7°K (5). The Raman and infrared absorption spectra of the crystals were first measured by Byer et al. (6) at 65°K and found to correspond well to the trigonal structure. New data on the Raman spectra of proustite above 60°K displaying some more lines were interpreted in (7) on the basis of monoclinic C₂(m) structure and an additional phase transition C_3v-C_s at 210°K was assumed. We present the Raman scattering spectra of prousitite and pyrargyrite crystals at low temperatures and discuss their structures and phase transitions taking into account ordering in the cation sublattice.