Solid metal-hydrogen solutions with a symmetric miscibility gap

International Journal of Hydrogen Energy. 2022. Vol. 47. No. 34. P. 15198–15208.

Antonov V., Muzalevsky V., Orlov N.

It is shown that the T-x projection of the miscibility gap in solid interstitial MeHx 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)H2 = MeHz
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 MeHz, 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.

Keywords: Phase diagrams, Palladium-hydrogen system, Nickel-hydrogen system, Thermodynamics of solid solutions, Система палладий-водород, Система никель-водород, Фазовые диаграммы, Термодинамика твердых растворов