Nature of electrical conductivity threshold in bulk (Ag$_{2}$Se)$_{x}$(GeSe$_{4}$)$_{1-x}$ glasses

Bulk glasses were synthesized over the 0 $<$ x $<$ 25{\%} range, and examined in FT-Raman, m-DSC, Electric Force Microscopy (EFM) and complex impedance experiments. Ag$_{2}$Se as an additive to GeSe$_{4}$ base glass leads to macroscopic phase separation as revealed by bimodal T$_{g}$s ( base glass-T$_{g}$ =168\r{ }C, additive glass T$_{g}$ = 230\r{ }C at low x $<$ 15{\%} . In addition, at higher x ($>$ 16{\%}), a third T$_{g}$ near 211\r{ }C is observed, and its strength increases with increasing x. EFM confirms the heterogeneous character of the glasses displaying a conducting phase that is segregated at low x ($<$12{\%}), and which percolates at higher x ($>$16{\%}). Electrical conductivity results show a step-like jump of nearly 2 orders of magnitude in the 16{\%} $<$ x $<$ 20{\%} range. Frequency of the Raman active corner-sharing mode of GeSe$_{4 }$units (200 cm$^{-1})$ steadily decreases with increasing x, suggesting that the third phase (T$_{g}$=211 C) most likely is a conducting GeSe$_{4}$-Ag$_{2}$Se phase. These data are consistent with a volume percolation$^{2}$ of solid electrolyte phases near x$\sim $ 16{\%} contributing to the step-like jump in conductivity of glasses. $^{1}$ V.Balan et al. J.Optoelectronics Adv. Mater. 8, 2112(2006). $^{2 }$H. Scher and R.Zallen, J. Chem. Phys.53,3759(1970). * Supported by NSF grant DMR 04-56472