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

Highlights: • Congruent melting of natural realgar α-As₄S₄ and orpiment As₂S₃ minerals. • Incongruent double-peak melting of synthetic powdered β-As₄S₄ polymorphs. • Non-equilibrium melting due to crystalline-amorphous heterogeneity in synthetic β-As₄S₄. • Re-amorphization and vitrification in synthetic β-As₄S₄ caused by high-energy milling. • Dual nature of appeared amorphous phase with low and high glass transition mid-points. -- Abstract: Thermal-alteration interphase transformations in natural (realgar α-As₄S₄ of two mineral origins) and synthetic (commercial powdered high-temperature β-As₄S₄ modification synthesized from elemental constituents and subjected to high-energy mechanical ball milling) arsenic monosulfide polymorphs are studied exploring temperature-modulated DSC TOPEM® method. 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.