Torsion vapor pressures and sublimation enthalpies of arsenic triselenide and tritelluride

Total vapor pressures of As2Se3 and As2Te3 were measured by the torsion-effusion method, and their temperature dependences were found to fit the following equations: As2Se3(s), log(p/kPa) = (12.20 +/- 0.20) to (9170 +/- 130)/(T/K) and As2Te3(s), log(p/kPa) = (10.45 +/- 0.20) to (8185 +/- 150)/(T/K). Considering the complex mode of vaporizing of As2Se3, the mean second-law enthalpy and entropy values associated to the sublimation of 1 mol of the vapor mixture at the mid-point temperature, Delta H degrees(587 K) = (175 +/- 3) kJ center dot mol(-1) and Delta S degrees(587 K) = (233 +/- 4) J center dot K-1 center dot mol(-1), were calculated from the temperature dependence of its vapor pressure. As2Te3 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 As2Te3, 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, As2Te3(s) = 0.5 As-4(g) + 3 Te(s), Delta H degrees(298 K) = 81 +/- 2 kJ center dot mol(-1), was determined.