On the Molecular Structure of Ge$_{x}$Sb$_{x}$Se$_{1-2x}$ glasses

The Ge$_{x}$Sb$_{x}$Se$_{100-2x }$ternary is isovalent to the phase-change material, Ge$_{x}$Sb$_{x}$Te$_{100-2x }$, except the Selenides can be prepared as bulk alloy glasses while the Tellurides exist only as amorphous thin-films. Here we report on the Selenides synthesized over a wide composition range, 0 $<$ x $<$ 25{\%}, and examined in modulated-DSC, Raman scattering and molar volume experiments. The enthalpy of relaxation at T$_{g}$ shows the opening of a reversibility window or Intermediate Phase (IP) in the 13{\%} $<$ x $<$ 18{\%} range, or 2.40 $<$ \textbf{\textit{r }}$<$ 2.54 mean coordination number range, where \textbf{\textit{r }}= 2 + 3x. FT- Raman studies reveal frequency of the CS mode of GeSe$_{4}$ tetrahedra to steadily blue-shift with increasing x as networks stiffen. New vibrational modes are observed near 150 cm$^{-1}$ and near 220 cm$^{-1}$ at x $>$ 18.18{\%}, the chemical threshold, and are thought to result from homopolar bonds. Ab-initio cluster calculations place pyramidal SbSe$_{3}$ units and ethylene-like Sb$_{2}$Se$_{2}$ units to reveal Raman activity near 215 cm$^{-1}$ and 228 cm$^{-1}$ respectively. Evolution of glass structure with composition x will be discussed.