Pressure Raman experiments on Ge$_{x}$As$_{x}$Se$_{1-2x}$ glasses$^{\ast }$

It is known$^{1}$ that variations in the non-reversing enthalpy associated with glass transitions, $\Delta $H$_{nr}$(x), display a global minimum ($\sim $0) in the 0.09 $<$ x $<$ 0.16 range and the term increases at x $>$ 0.16 and at x $<$ 0.09 in the titled glasses. In this reversibility window, glasses are thought to in the Intermediate phase and form stress-free networks. Since the size of As, Ge and Se are nearly the same, Raman pressure experiments using a DAC provide a useful way to check the stress-free nature of glasses in the window$^{2}$. Preliminary results at x = 0.11, and 0.14, compositions in the reversibility window, reveal Raman frequency of the symmetric stretch of Ge(Se$_{1/2})_{4}$ tetrahedra to blue-shift linearly with external pressure (P) once P$>$0. At x = 0.18, a composition in the stressed-rigid phase, a blue-shift of the mode is also observed but only once P exceeds a threshold (P$_{c})$ value of 14 kbar. The present finding of a finite value of P$_{c}$ at x = 0.18, but its vanishing at x = 0.11 and 0.14, is quite similar to a previous one in binary Ge$_{x}$Se$_{1-x}$ glasses$^{2}$. We are now examining other glass compositions in the present ternary. $^{\ast }$ Supported by NSF grant DMR 04-56472 $^{1}$ T. Qu et al. Mater. Res. Soc. Symp. Proc. \textbf{754}, 111 (2003). $^{2}$ F. Wang et al. Phys. Rev. B, \textbf{71}, 174201 (2005).