Phase separation in amorphous metal hydrides: A Stoner-type criterion

The existence of dissociation plateaus in the pressure-composition isotherms for hydrogen absorption in amorphous metals is shown to depend on the degree of configurational disorder in the host lattice. Within a mean-field approximation for the attractive elastic hydrogen-hydrogen interaction, the Stoner-type condition -an (E₀) 1 is derived for the existence of a positive critical temperature T₂ a is a measure of the strength of the effective hydrogen-hydrogen interaction and n (E₀) is the maximum of the symmetric density-of-sites function n (E) which describes the spread in the energy of the sites which can be occupied by hydrogen atoms. Analytical expressions for T₂ are derived for a square and a doublesquare density-of-sites function. These simple models for n (E) are used to reproduce experimental results on crystalline and amorphous nickel-zirconium hydrides.