High-temperature structural phase transitions ofGexS1−xalloys studied by Raman spectroscopy

Abstract

A high-temperature Raman study of ${\mathrm{Ge}}_{x}{\mathrm{S}}_{1\ensuremath{-}x}$ alloys is reported up to a temperature close to the melting point, including both Ge-rich $(x=0.35)$ and S-rich $(x=0.20,0.30)$ glasses, as well as the compound glass $(g\ensuremath{-}{\mathrm{GeS}}_{2};$ $x=\frac{1}{3}).$ The variation in the Raman spectra indicates that above certain temperatures $g\ensuremath{-}{\mathrm{GeS}}_{2}$ gradually crystallizes, first to the three-dimensional (3D) phase and then to the layered two-dimensional (2D) phase, with the latter being maintained up to melting point and upon subsequent cooling to room temperature. There is evidence that the controversial ${A}_{1}^{c}$ companion band of $g\ensuremath{-}{\mathrm{GeS}}_{2}$ evolves to a counterpart band of the 2D crystalline phase, implying that this band is due to symmetric stretch vibrations of S atoms in bridges of edge-sharing ${Ge(S}_{1/2}{)}_{4}$ tetrahedra, in agreement with a previous prediction. Similar two step irreversible crystallization to the 3D and 2D phases of ${\mathrm{GeS}}_{2}$ have been observed above ${T}_{g}$ for the moderately rich in Ge $(x=0.35)$ or in S $(x=0.30)$ ${\mathrm{Ge}}_{x}{\mathrm{S}}_{1\ensuremath{-}x}$ glasses, but at lower thresholds of crystallization temperature. In the strongly enriched in S $(x=0.20)$ glass, though, crystallization takes place only to the 3D phase of ${\mathrm{GeS}}_{2},$ a process which is reversible after cooling the alloy to room temperature, i.e., the material returns to its initial amorphous phase. This reversible crystallization is explained in terms of the three-dimensional network of S-rich ${\mathrm{Ge}}_{x}{\mathrm{S}}_{1\ensuremath{-}x}$ glasses which evolves only to the respective 3D crystalline phase lattice at high temperatures. It is pointed out that all ${\mathrm{Ge}}_{x}{\mathrm{S}}_{1\ensuremath{-}x}$ glasses studied undergo a first-step transition to the 3D crystalline phase, which shows that the network of these glasses is, by large, three dimensional.