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

Abstract

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 H 2) and the radii of tetrahedral holes in both hexagonal AB 5 (D2 d) and cubic AB (B 2) 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 AB 5 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 AB 5-hydrogen systems and cubic AB-hydrogen systems have been compiled and are presented. The quantitative correlations are excellent. A change in valence of the A atom in the hexagonal AB 5 intermetallic compounds may have a significant effect on the stabilities of the hydride phases formed. In those cases where the B atoms were partially substituted in the hexagonal AB 6 and the cubic AB intermetallic compounds, a good correlation of stability with hole size was found. Gross deviations of the Ce-base AB 5 hydride data from the correlation line established by all the other AB 5 hydrides were observed. These were rationalized on the basis of well-documented stress-induced lattice contractions of Ce intermetallic compounds. The resulting contracted hole sizes of the CeNi 5 and CeCo 5 compounds modified the stability of their hydrides. These compound hydrides demonstrate a displaced but similar correlation to the other AB 5 hydrides, i.e. the smaller the hole size, the less stable is the system.