Theoretical investigation of Pd-H phase equilibria by the cluster variation method

By combining the tetrahedron-octahedron approximation of the cluster variation method with a phenomenological expression for the atomic interaction energies, the free energy of the Pd-H system is constructed for both the disordered and chalcopyrite phases. The atomic interaction energies are most conveniently described as the sum of configuration-dependent and -independent contributions. The former originates from chemical atomic interactions while the latter is due to the elastic energy associated with the lattice expansion induced by the dissolution of hydrogen. In the high temperature region, the experimentally observed miscibility-gap type phase diagram is reproduced. The phase separation is caused by the configuration-independent elastic energy contribution. The temperature dependences of the grand potentials of disordered and chalcopyrite phases indicates that an order-disorder transition takes place at 216 K at similar to50 at% with weakly second order. The stabilization of the ordered phase is due to the configuration-dependent energies. The Site Occupancies of hydrogen in the chalcopyrite phase, which consists of two sublattices, alpha and beta, are investigated. It is confirmed that only one of the two sublattices is predominantly occupied by hydrogen at low temperature. Four kinds of independent nearest neighbor pair correlations are calculated and the higher order correlations, including distant pair and multibody correlations, are obtained. (C) 2002 Elsevier Science B.V. All rights reserved.