Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

Thermal-alteration interphase transformations in natural (realgar a-As4S4 of two mineral origins) and synthetic (commercial powdered high-temperature b-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 a-As4S4 demonstrate two endothermic events, these being ascribed to interphase a? b transformation at (540 550) K, and melting of this newly-formed high-temperature b-As4S4 phase at 581 582 K. This polymorph originated from thermal alteration of mineral realgar possesses congruent melting in contrast to synthetic b-As4S4 polymorph, which shows non-equilibrium melting due to accompanied generation of compositionallyauthentic amorphous phase. Calorimetric studies on synthetic b-As4S4 in powdered coarse-grained and milled states demonstrate complicated non-equilibrium melting in principally different crystallineamorphous environments along with crystal-to-glass transformation. Structural-chemical heterogeneity of b-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 b-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 b-As4S4, identified as re-amorphization of initial amorphous phase and direct vitrification from b-As4S4 crystallites, was parameterized as compared to calorimetric thermal-alteration events in orpiment As2S3 mineral.