Evidence for isoelectronic Sn for Ge substitution in crystalline and glassy GeSe2

Ternary alloy glasses ${\mathrm{Ge}}_{1-x}{\mathrm{Sn}}_x{\mathrm{Se}}_2$ in the low-Sn-concentration range $0.002<x<\mathrm{}0.010\mathrm{}$ have been prepared and crystallized by heating to the crystallization temperature $T_{\mathrm{cryst}}$. Mössbauer spectroscopy is used to follow changes in bonding chemistry of the Sn dopant on crystallization of the glasses. Polymorphous crystallization of these Sn-poor glasses leads to the formation of tetrahedral Sn as in $\mathrm{Sn}{\left({\mathrm{Se}}_{\frac{1}{2}}\right)}_4$ local unit and octahedral Sn as in a $c-\mathrm{S}\mathrm{n}\mathrm{S}\mathrm{e}$ phase but not as in a $c-\mathrm{S}\mathrm{n}{\mathrm{Se}}_2$ phase. The latter crystalline phase is found when sputtered amorphous Sn${\mathrm{Se}}_2$ films are crystallized. The tetrahedral Sn site observed in the glasses, as in the corresponding crystals, can therefore not come from an amorphous Sn-rich tetrahedral Sn${\mathrm{Se}}_2$ phase but must come from Sn replacing Ge sites in a tetrahedral Ge${\mathrm{Se}}_2$ phase. A lower limit to the solubility of Sn at Ge in $c-\mathrm{G}\mathrm{e}{\mathrm{Se}}_2$ is placed at 0.2 at.% while in the glasses it is substantially larger and is conservatively placed at 3 at.%. These results demonstrate that the two sites provide evidence for broken chemical order in $g-\mathrm{Ge}{\mathrm{Se}}_2$.