Decomposition pressures and enthalpies of formation of some transition metal diarsenides and diselenides

The phase relations for the systems Pt/As, Os/As, Os/Se, and Ru/Se have been examined in the range 850–1 250 °K. Using the torsion effusion method, the decomposition pressures of PtAs2, OsAs2, OsSe2, and RuSe2 have been measured as a function of temperature. The apparatus and procedures were verified by measuring the vapor pressure of liquid silver. Equilibrium pressures were obtained for PtAs2 and RuSe2. OsAs2 exhibited an orifice area dependent nonequilibrium behavior and OsSe2 exhibited a time dependent nonequilibrium behavior. Equilibrium pressures were estimated in these latter two cases. Total equilibrium decomposition pressures (Pt), i.e. the pressure of arsenic or selenium vapors in equilibrium with each phase, are best described by the equations: [Formula: see text]Literature data on the equilibria between the various molecular species in selenium and arsenic vapor indicate that at these pressures and temperatures the vapor in equilibrium with the selenides is essentially pure Se2, but the vapor over the arsenides is a mixture of As2 and As4.The heats and entropies of decomposition of PtAs2, OsAs2, OsSe2, and RuSe2 have been calculated from the decomposition pressure data. The standard heats of formation at 298 °K are −42.0 ± 3.5, −18.5 ± 2.0, −32.5 ± 5.0, and −42.9 ± 3.5 kcal/mole, respectively. Heats of formation of MnSe2, CoSe2, NiSe2, and RhSe2 have been estimated using the heats of formation of OsSe2 and RuSe2, the Born–Mayer equation, and the Born–Haber cycle. These are −96 ± 8, −70 ± 8, −41 ± 8, and −12 ± 12 kcal/mole, respectively.