Thermal-alteration interphase transformations in natural and synthetic arsenic sulfide polymorphs

Specific heat capacity and non-reversing heat flow variations in realgar α-As4S4 demonstrate two endothermic events, these being ascribed to interphase α → β transformation at ∼(540-550) K, and melting of this newly-formed high-temperature β-As4S4 phase at 581-582 K. This polymorph originated from thermal alteration of mineral realgar possesses congruent melting in contrast to synthetic β-As4S4 polymorph, which shows non-equilibrium melting due to accompanied generation of compositionally-authentic amorphous phase. Calorimetric studies on synthetic β-As4S4 in powdered coarse-grained and milled states demonstrate complicated non-equilibrium melting in principally different crystalline-amorphous environments along with crystal-to-glass transformation. Structural-chemical heterogeneity of β-As4S4 crystallites results in incongruent double-peak melting through two endothermic events at ∼578 K and ∼588 K. The amorphous phase formed under high-energy milling of synthetic β-As4S4 possesses a dual nature due to stabilization of As-rich glassy substances with low- and high-temperature glass transition mid-points. This process in the powdered synthetic β-As4S4, identified as re-amorphization of initial amorphous phase and direct vitrification from β-As4S4 crystallites, was parameterized as compared to calorimetric thermal-alteration events in orpiment As2S3 mineral.