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

Pressure-composition isotherms of unstable intermetallic hydrides of some L aves phases (TiCr-2, TiCrMn) were measured over a wide pressure range up to 1000 atm H-2. These measurements enabled the evaluation of the critical te mperatures, T-c, of the respective systems as well as the derivation of the ir thermodynamic characteristics above T-c. For this one-phase high-tempera ture range, simplified statistical-thermodynamics models can be adapted to calculate analytical forms of the corresponding isotherms. A comparison bet ween the model-derived and the experimental isotherms then yields the avera ge pairwise nearest neighbor H-H interaction parameter, eta, and its temper ature dependence. In the present study, a rigid-metal sublattice model was utilized and solved employing the conventional Bragg-Williams (BW) and Quas i-Chemical(QC) approximations. In fact, both approximations resulted in sim ilar eta(T) values, as well as close estimates of T-c. For the TiCr-2-H-2 s ystem the above analysis indicated that eta changes from attractive (i.e., negative) to repulsive (i.e., positive) with increasing isotherm temperatur es. This trend was qualitatively interpreted as resulting from the net inte rplay of two energy terms, the elastic strain contribution, which induces a n effective attractive interaction, and the electrostatic contribution whic h adds a repulsive term. For the TiCrMn-H-2 system, it turned out that the partial substitution of chromium by manganese had only a minor effect on th e 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 com pound. (C) 1999 Elsevier Science S.A. All rights reserved.