Structural Phase Transition in Glasses GexAsySe100-x-y

Optical properties (refractive index n, optical bandgap Ego, slope of fundamental absorption edge $W$) and density of GexAsySe100-x-y ($0 \leq \mathrm{x} \leq 30; 10 \leq \mathrm{y}\leq 40$) chalcogenide glasses were studied at room temperature. The molar refraction of glasses was calculated using the Lorentz-Lorenz formula, and the ionicity of possible chemical bonds was estimated. The results were analyzed depending on the mean coordination number $\langle (m\rangle (\langle m\rangle = 2 + 0.01 (2\mathrm{x}+\mathrm{y})$ for GexAsySe100-x-y glasses). A different character of the dependences of all investigated physical properties on $\langle m\rangle$ was established in the ranges $2.1\leq\langle m\rangle\leq 2.78$ and $2.78 \leq \langle m\rangle \leq 3$, which may indicate a qualitatively different structure of the corresponding glasses. Glasses of the first group can be classified as molecular solids, which are characterized by a significant van der Waals interaction, which is carried out mainly by lone-pair electrons of As and Se atoms. At $\langle m\rangle\approx 2.78$, a structural phase transition to a continuous three-dimensional glassy network was observed, accompanied by a qualitative rearrangement of the band structure of the alloys.