Torsion Vapor Pressures and Sublimation Enthalpies of Arsenic Triselenide and Tritelluride

Total vapor pressures of As 2 Se 3 and As 2 Te 3 were measured by the torsion-effusion method, and their temperature dependences were found to fit the following equations: As 2 Se 3 (s), log( p /kPa) = (12.20 ± 0.20) to (9170 ± 130)/( T /K) and As 2 Te 3 (s), log( p /kPa) = (10.45 ± 0.20) to (8185 ± 150)/( T /K). Considering the complex mode of vaporizing of As 2 Se 3, the mean second-law enthalpy and entropy values associated to the sublimation of 1 mol of the vapor mixture at the mid-point temperature, Δ H °(587 K) = (175 ± 3) kJ·mol -1 and Δ S °(587 K) = (233 ± 4) J·K -1 ·mol -1, were calculated from the temperature dependence of its vapor pressure. As 2 Te 3 dissociates upon sublimation yielding As 4 (g) and Te(s). Both As 4 (g) and Te 2 (g) are present in the vapor over Te-saturated As 2 Te 3, where the Te 2 (g) partial pressures are considered equal to the vapor pressure of the pure element. On this basis, treating the pressure data by second- and third-law methods, the standard sublimation enthalpy associated to the sublimation reaction, As 2 Te 3 (s) = 0.5 As 4 (g) + 3 Te(s), Δ H °(298 K) = 81 ± 2 kJ·mol -1, was determined.