Stretched Exponential relaxation in pure Se glass.

A universal feature of glasses is the stretched exponential relaxation, f(t) $=$ exp[-t/$\tau $]$^{\mathrm{\beta }}$. The model of diffusion of excitations to randomly distributed traps in a glass by Phillips$^{\mathrm{1}}$ yields the stretched exponent $\beta =$ d[d$+$2] where d, the effective dimensionality. We have measured the enthalpy of relaxation $\Delta $H$_{\mathrm{nr}}$(t$_{\mathrm{w}})$ at T_{\mathrm{g}}$ of Se glass in modulated DSC experiments as glasses age at 300K and find $\beta\quad=$ 0.43(2) for t$_{\mathrm{w}}$ in the 0 \textless t$_{\mathrm{w}}$ \textless 8 months range. The observed $\beta $ is in harmony with the trap model. The result is consistent with the growth of interchain structural correlations mediated by both long range (van der Waals forces) and short-range (covalent) interactions. A striking consequence of this relaxation is a narrowing of the glass transition width from 7.1\textdegree C to 1.4\textdegree C, and the $\Delta $H$_{\mathrm{nr}}$ term increasing from 0.21 cal/gm to 0.92 cal/gm. In bulk Ge$_{\mathrm{x}}$Se$_{\mathrm{100-x}}$ glasses as x increases to 20{\%}, the length of the polymeric Se$_{\mathrm{n}}$ chains between the Ge-crosslinks decreases to n $=$ 2. and the striking relaxation effects nearly vanish.