### ?

## Solid metal-hydrogen solutions with a symmetric miscibility gap

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 = 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.