High-pressure studies of the TiCr_1.8-H₂ system Statistical thermodynamics above the critical temperature

Pressure-composition isotherms of the TiCr_1.8-H₂ system were measured within the temperature range 298-433°K and over a wide pressure range up to 1000 atm H₂. The above temperature range is well above the critical temperature, T_C, of the system. Hence, partial molal enthalpies and entropies of formation were evaluated as a function of hydrogen composition. Both thermodynamic quantities obeyed a linear decrease (i.e. becoming more negative) with increasing H/M atomic composition ratio (with M=Ti+Cr atomic content). The experimental isotherms were compared to calculated expressions derived by a rigid-metal sublattice statistical thermodynamics model. Two approximations applied in solving the model, the Bragg-Williams (B.W.) and the Quasi-Chemical (Q.C.) were compared, respectively. The pairwise nearest neighbors H-H interaction parameter, η, was evaluated for each isotherm. For both approximations a similar temperature dependence of η(T) was obtained, with η changing from attractive (i.e. negative) to repulsive (i.e. positive) with increasing isotherms temperatures. A good agreement was obtained between the calculated T_C values (derived from the η(T) parameters) and the experimental observations.