High-pressure studies of Laves phase intermetallic hydrides - adaptation of statistical thermodynamic models

Pressure-composition isotherms of unstable intermetallic hydrides of some Laves phases (TiCr_{approximately 2}, TiCrMn) were measured over a wide pressure range up to 1000 atm H₂. These measurements enabled the evaluation of the critical temperatures, T_c, of the respective systems as well as the derivation of their thermodynamic characteristics above T_c. For this one-phase high-temperature range, simplified statistical-thermodynamics models can be adapted to calculate analytical forms of the corresponding isotherms. A comparison between the model-derived and the experimental isotherms then yields the average pairwise nearest neighbor H-H interaction parameter, η, and its temperature dependence. In the present study, a rigid-metal sublattice model was utilized and solved employing the conventional Bragg-Williams (BW) and Quasi-Chemical (QC) approximations. In fact, both approximations resulted in similar η(T) values, as well as close estimates of T_c. For the TiCr_{approximately 2}-H₂ system the above analysis indicated that η changes from attractive (i.e., negative) to repulsive (i.e., positive) with increasing isotherm temperatures. This trend was qualitatively interpreted as resulting from the net interplay of two energy terms, the elastic strain contribution, which induces an effective attractive interaction, and the electrostatic contribution which adds a repulsive term. For the TiCrMn-H₂ system, it turned out that the partial substitution of chromium by manganese had only a minor effect on the stability of the hydride, however, it pronouncedly increased the critical temperature (T_c) of the system. This observation can be accounted for by the simultaneous electronic and structural effects of manganese in this compound.