Phase equilibria of the VS-V₅S₈ system from 923 to 1323 K

Abstract

Sulfur partial pressure-isotherms at temperatures from 923 to 1323 K were obtained over a wide range of compositions of VS-V₅S₈ by combining results from the phase equilibria of the VS-V₃S₄ system in the present study and those from previously reported ones for the V₃S₄-V₅S₈ system at temperatures from 923 to 1173 K. The phase equilibria of VS-V₃S₄ were determined for the composition range from VS_1.08 to VS_1.333 by the quenching method under controlled partial pressure of sulfur in the range from 10¹⁴ to 10⁻³ atm (1 atm = 101.3 kPa) at temperatures from 1023 to 1323 K. Three vanadium sulfides were observed for this composition range in agreement with results reported in the literature,i.e. orthorhombic VS, hexagonal VS, and V₃S₄. The quenched V₃S₄ phase with nonstoichiometric composition extended over both metal-rich and sulfur-rich regions of the plateau, supposedly representing a two phase region, observed on each Ps₂-composition isotherm. This anomalous phase relation was discussed along with the profiles of the isotherms. The partial molar enthalpies of sulfur in the nonstoichiometric hexagonal VS and V₃S₄ regions were calculated from the isotherms. Their values ranged from −46 to −56 kcal/g-at. (from −190 to −240 kJ/g-at.) and decreased with increasing deviation from stoichiometry.