Dual Chemical Role of Ag as an Additive in Chalcogenide Glasses

Ternary $({\mathrm{Ge}}_x{\mathrm{Se}}_{1-x}{)}_{1-y}{\mathrm{Ag}}_y$ bulk glasses in the Se-rich region $(x<\mathrm{}\frac{1}{3})$ are shown to be intrinsically phase separated into an ${\mathrm{Ag}}_2\mathrm{Se}$-rich glass and a residual ${\mathrm{Ge}}_t{\mathrm{Se}}_{1-t}$ backbone ( $t>x$ at $y\ne 0$) with Ag acting as a network modifier. The existence of an ${\mathrm{Ag}}_2\mathrm{Se}$ glass with a $T_g=230\mathrm{}°\mathrm{C}$ is explained quantitatively in terms of constraint counting algorithms. In contrast, Ge-rich glasses $(x\ge \frac{2}{5})$ are homogeneous, wherein Ag acts as a network former, replacing available Ge sites of the backbone to be 3-fold coordinated to Se.