High-temperature structural phase transitions of GexS1-x alloys studied byRaman spectroscopy - art. no. 125210

A high-temperature Raman study of GexS1-x alloys is reported up to a temper ature close to the melting point, including both Ge-rich (x=0.35) and S-ric h (x=0.20,0.30) glasses, as well as the compound glass (g-GeS; x=1/3). The variation in the Raman spectra indicates that above certain temperatures g- GeS2 gradually crystallizes, first to the three-dimensional (3D) phase and then to the layered two-dimensional (2D) phase, with the latter being maint ained up to melting point and upon subsequent cooling to room temperature. There is evidence that the controversial Al companion band of g-GeS2 evolve s to a counterpart band of the 2D crystalline phase, implying that this ban d is due to symmetric stretch vibrations of S atoms in bridges of edge-shar ing Ge(S-1/2)(4) tetrahedra, in agreement with a previous prediction. Simil ar two step irreversible crystallization to the 3D and 2D phases of GeS2 ha ve been observed above T-g for the moderately rich in Ge (x=0.35) or in S ( x=0.30) GexS1-x glasses, but at lower thresholds of crystallization tempera ture. In the strongly enriched in S (x=0.20) glass, though, crystallization takes place only to the 3D phase of GeS2, a process which is reversible af ter cooling the alloy to room temperature, i.e., the material returns to it s initial amorphous phase. This reversible crystallization is explained in terms of the three-dimensional network of S-rich GexS1-x glasses which evol ves only to the respective 3D crystalline phase lattice at high temperature s. It is pointed out that all GexS1-x glasses studied undergo a first-step transition to the 3D crystalline phase, which shows that the network of the se glasses is, by large, three dimensional.