On the theory of the interactions and phase transitions of hydrogen in the transition metals

Abstract

Analytical methods based on the cluster approximation described previously are proposed for calculating the thermodynamic properties and phase diagrams of hydrogen in transition metals (Me). In contrast with the mean field approaches usually adopted, these methods allow quantitatively for the strong short-range correlation effects, including blocking and competitive interaction, which are characteristic of Me-H systems. Detailed calculations of the stress-induced and screened Coulomb H-H interactions for the NbH x and PdH x systems have also been performed. They reveal that the blocking effects in b.c.c. Me-H systems are apparently due to the anharmonicity of the stress-induced interactions at small H-H distances. Applications of the methods developed to the description of the incoherent α-α′ phase transitions in NbH x and PdH x and α⊂α)-β phase transitions in NbH x have shown that the methods have a high accuracy and are well suited to the theory of the Me-H systems. In particular, results have been obtained which suggest that many-body interactions of hydrogen atoms have a noticeable effect on the phase diagrams of b.c.c. MeH x alloys.