Glass transition line of As$_2$S$_3$ on the $P, T$-phase diagram up to the metallization pressures of the melt

An inelastic behavior is observed in the form of diffuse structural transformations with complex logarithmic kinetics in most chalcogenide glasses compressed above pressures of 1–1.5 GPa. The corresponding melts also exhibit a change in the structure of the intermediate- and short-range orders usually in the pressure range from 1 to 10 GPa. At the same time, the glass transition temperature $T_g$ for chalcogenide systems at pressures above 1 GPa has not yet been studied. In this work, the glass transition in the “classical” glass-forming compound As$_2$S$_3$ has been studied for the first time at a high hydrostatic pressure up to 5 GPa, i.e., in the range where diffuse transformations and growth of chemical disorder in glass and in melt are observed. At higher pressures, smooth metallization and chemical disproportion of the melt begins, and glasses are not formed during cooling. The initial slope $dT_g/dP$ of the glass transition line agrees well with the estimate from the Prigogine–Defay thermodynamic relations. The glass transition line has a large curvature, sharply flattens under compression up to a pressure of 3 GPa, and then becomes almost straight. The ratio of the glass transition temperature to the melting point $T_g/T_m$ for As$_2$S$_3$ decreases from 0.8 to 0.6 with increasing pressure to 5 GPa, which indicates a drastic decrease in the tendency to glass formation under compression. A possible relationship between the features of the glass transition line and transformations in melt and glass at the corresponding pressures has been discussed.