High-temperature thermodynamics of the Ti–H2 and Ti–D2 systems

The high-temperature thermodynamic properties of the Ti+H2 and Ti+D2 solid systems have been studied by a combined calorimetric and equilibrium method at 706–721 K (both systems) and 971 K (Ti+H2 only). The limiting enthalpies of solution of hydrogen in close-packed hexagonal α-Ti is −10.6 kcal mole−1(H) at the lower temperature, and about −10.9 kcal mole−1(H) at the higher temperature. The corresponding value of ΔH̄D at 707 K is −10.1 kcal mole−1(D). From the hydrogen and deuterium data at 707–721 K we estimate ν̃H in α–Ti to be about 800 cm−1 and ν̃D about 565 cm−1. The equilibrium pressures and the partial enthalpies of hydrogen and deuterium allow an evaluation of the partial entropies and excess entropies, S̄EH and S̄ED. The limiting value of S̄EH at NH≈0 and 706–721 K is 6.5 cal K−1⋅mole−1(H), that of S̄ED at the same temperature is 8.9 cal K−1⋅mole−1(D). These values are larger than corresponding values calculated from ν̃H and ν̃D. This discrepancy suggests either much lower vibrational frequencies (than calculated from ΔH̄H and ΔH̄D), or perhaps a very significant communal entropy term.