Statistical mechanics models for PdH[subscript x] and PdD[subscript x]

Abstract

In recent years we have developed simple empirical mean-field statistical mechanics models for PdHx and PdDx that include both O-site and T-site occupation. A brief account of this work is given here. At low loading it is impossible to fit solubility data over a wide range of temperature assuming only O-site occupation; however, with T-site occupation a good fit is obtained and an O-site to T-site excitation energy near 105 meV is found. Results from neutron diffraction studies at modest loading, and resistance and solubility measurements at high pressure and high loading, are used to infer O-site to T-site excitation energies and develop a smooth fit. The resulting statistical mechanics models have been optimized to provide systematic results at high loading for PdH and PdD, and also to fit the isotherms of the phase diagram of PdH. Estimates for the O-site energy as a function of loading and temperature have been developed in the latter case. Keywords: Mean field model; Palladium hydride; Phase diagram; Solubility; Tetrahedral occupation