METAL HYDRIDES: STRUCTURE AND BAND STRUCTURE.

Previous calculations for Group IIIB transition metal and rare earth compounds with hydrogen showed that for the dihydrides a new band is added below the Fermi energy. This bond corresponds to the antibonding combination of the two hydrogen 1s-orbitals in the unit cell. The original transition metal s-band hybridizes with the bonding combination of the two hydrogens, the implications being that for compounds with only one hydrogen per unit cell no new band is formed and the additional electron goes into host metal states. This result has now been verified by calculations for YH and PdH. Previous conclusions about the relative stability of the trivalent trihydrides have been extended to Groups IVB, VB, and VIB dihydrides. The relative position of the added hydrogen bands below the Fermi energy determines the relative stability of a structure. This position depends primarily on interatomic distance. Thus the trihydride forms only for yttrium and the rare earths and not for scandium. The dihydride only forms for transition metals and rare earths for which the metal ion radius (which determines the metal spacing but not the structure) is greater than 1.25 A. The monohydride forms only when the dihydride is relatively unstable and the density of states in the d-band is high, e.g. VH, NbH, TaH, NiH, and PdH. Quantitative substantiation of these ideas from band structure calculations will be presented.