Structural evolution of Ge–Ga–Se glass upon compositional change across the stoichiometric GeSe₂–Ga₂Se₃ tie-line

A compositional series of Ge_xGa₁₀Se_90−x (at.%; x = 20, 23.3, 25, 27.5, and 30), set to cross both the stoichiometric GeSe₂–Ga₂Se₃ tie-line and the pseudo-stoichiometric GeSe₂–GaSe₂ tie-line compositions, is employed in this study to refine the local structural environments of Se atoms. Our structural analysis reconfirms that the amorphous network of Ge₂₅Ga₁₀Se₆₅ glass consists mainly of [Ge(Ga)Se₄] tetrahedral units connected through either corner-sharing or edge-sharing, in addition to involvement of threefold-coordinated Se atoms. Interestingly, however, upon increase of Se content up to Ge_23.3Ga₁₀Se_66.7, which belongs to the pseudo-stoichiometric GeSe₂–GaSe₂ tie-line, the excess Se atoms turn out to form homopolar –Se–Se– chain fragments rather than to transform the heteropolar threefold-coordinated Se atoms into the typical heteropolar twofold-coordinated Se atoms bridging two tetrahedra. Coexistence of the homopolar twofold-coordinated and heteropolar threefold-coordinated Se atoms is considered a unique structural characteristic of Ge–Ga–Se glass, which needs to be taken into account in the compositional optimization of its macroscopic properties.