Molecular structure of Si$_x$Ge$_x$Te$_{100-2x}$ glasses

Bulk glasses of the titled ternary have been synthesized in the 8\% $<$ x $<$ 16\% range by melt quenching. Glass transition temperatures, T$_g$(x), increase linearly from 140$^{\circ}$C at x = 8\% to 200$^{\circ}$C at x = 12\%, then decrease steadily to 160$^{\circ}$C thereafter. The non-reversing enthalpy of relaxation at T$_g$ shows a broad minimum near 9\% but a maximum near 12\%. Glasses appear to be fully polymerized at x $<$ 12\%, but segregate as x $>$ 12\%. The broad minimum near 9\%, most likely, represents the opening of an Intermediate Phase. Raman scattering, excited using 785 nm radiation of a glass at x = 10\%, shows two modes, a broad one near 160 cm$^{-1}$ and a narrow one near 127 cm$^{-1}$. A mode near 157 cm$^{-1}$ has been previously\footnote{M.Brodsky, Phys. Stat. Solidi(b) 52, 609 (1972)} identified with polymeric Te$_n$ chains of a-Te. Tentatively, we assign the 127 cm$^{-1}$ mode with face-sharing\footnote{M. Malyj et al. Phys. Rev. B 31, 3672 (1985)} GeTe$_4$, and a mode near 170 cm$^{-1}$ with edge-sharing SiTe$_4$ tetrahedra in these glasses. The nature of glass structure evolution with composition will be elucidated.