Lattice gas with random-site energies and theory of novel amorphous metal hydride phase

Abstract

A lattice gas with random-site energies is investigated as a model for hydrogen in amorphous metals. The author's recent theory for calculating the chemical potential in a system with many competing interactions is modified to include the random-site energies. Results are qualitatively different from those recently presented by Griessen using simple mean-field (MF) theory. Whereas MF theory predicts no phase separation above a critical value of the site energy width ..delta.., the present model gives a finite critical temperature T/sub c/..cap alpha..1/..delta.. for large ..delta... It also predicts the critical concentration to decrease proportionally to 1/..delta../sup 2/ and yields a closed-loop, retrograde-solubility phase diagram. Thus, analogous to binary liquids with orientation-dependent interactions, there is a maximum concentration above which no phase separation occurs. For interactions and spread in site energy expected for Pd-based amorphous hydrides, the critical temperature is predicted to be approximately 200--250 K, which may be detectable by heat-capacity or spectroscopic techniques even if it is too low for pressure-versus-composition studies.