A correlation between the interstitial hole sizes in intermetallic compounds and the thermodynamic properties of the hydrides formed from those compounds

A correlation has been established between the stabilities of metal hydrides (measured in terms of the free energies of formation of the hydride phase per mole H2) and the radii of tetrahedral holes in both hexagonal AB5 (D2sub(d)) and cubic AB (B2) intermetallic compounds. The correlation demonstrates that as the tetrahedral hole size increases, the stability increases. The results of this correlation show conclusively that hole size can be employed effectively in determining the stabilities of intermetallic compound-hydrogen phases. A model, which depends solely upon lattice parameters, was developed to compute the radii of tetrahedral holes in hexagonal AB5 intermetallic compounds. A similar model, which requires a ratio of metallic radii in addition to the lattice parameter, has been employed to compute the radii of tetrahedral holes in the cubic AB intermetallic compounds. The thermodynamic and structural properties of hexagonal AB5-hydrogen systems and cubic AB-hydrogen systems have been compiled and are presented. The quantitative correlations are excellent. (Auth.)