Solid metal-hydrogen solutions with a symmetric miscibility gap

Abstract

It is shown that the T-x projection of the miscibility gap in solid interstitial MeH x solutions will be symmetric and the pressure vs . temperature dependence of the corresponding isomorphic phase transformation will be determined by the reaction Me + ( z /2)H 2 = MeH z with a fixed z value attained at T → 0 K, if the mixing Gibbs energy of the solutions is a symmetric two-well function of the H content with a maximum at z /2. Based on these findings and using the literature data for the pure metal Me and the hydride MeH z , it is explained why the experimental dependences of the standard Gibbs energy for the isomorphic transformation between two solid phases of variable composition in the Pd–H and Pd-D systems ( z = 0.63) and in the Ni–H and Ni-D systems ( z = 1) are close to linear in the entire investigated temperature range. • Model of solid solutions Pd–H with a symmetric two-well mixing energy is considered. • The model explains the linear dependence ln P (1/ T ) for formation of palladium hydride. • The formation pressure is determined by the reaction Pd + ( z /2)H 2 = PdH z at z = 0.63 • Deviation of ln P (1/ T ) from linearity in this approximation is 1% at 194.5 ≤ T ≤ 565 K. • Similar results are obtained for the systems Pd-D, Ni–H, and Ni-D.