A high-temperature Raman study of ${\mathrm{Ge}}_x{\mathrm{S}}_{1-x}$ alloys is reported up to a temperature close to the melting point, including both Ge-rich $\mathrm{}(x=0.35\mathrm{})\mathrm{}$ and S-rich $\mathrm{}(x=0.20,0.30\mathrm{})\mathrm{}$ glasses, as well as the compound glass $(g-{\mathrm{GeS}}_2;$ $x=\frac{1}{3}).\mathrm{}$ The variation in the Raman spectra indicates that above certain temperatures $g-{\mathrm{GeS}}_2$ 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 $g-{\mathrm{GeS}}_2$ 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 $\mathrm{}(x=0.35\mathrm{})\mathrm{}$ or in S $\mathrm{}(x=0.30\mathrm{})\mathrm{}$ ${\mathrm{Ge}}_x{\mathrm{S}}_{1-x}$ glasses, but at lower thresholds of crystallization temperature. In the strongly enriched in S $\mathrm{}(x=0.20\mathrm{})\mathrm{}$ 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 ${\mathrm{Ge}}_x{\mathrm{S}}_{1-x}$ glasses which evolves only to the respective 3D crystalline phase lattice at high temperatures. It is pointed out that all ${\mathrm{Ge}}_x{\mathrm{S}}_{1-x}$ 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.

• Received 27 December 2000

DOI:https://doi.org/10.1103/PhysRevB.64.125210