Hydride Phase Composition and Crystal Structure in Zr sub 2 PdH/sub X/

摘要

The ordered intermetallic Zr sub 2 Pd forms several ternary hydride phases. The crystal structures of Zr sub 2 PdH/sub x/ samples with various stoichiometries have been determined by powder x-ray diffraction methods. For samples with compositions between 1.68 less than or equal to x less than or equal to 2.2, the ternary hydrides possess the tetragonal MoSi sub 2 -type structure of the original Zr sub 2 Pd intermetallic but with expanded lattice parameters. However, the hydrides with greater than or equal to 2.9 have distorted MoSi sub 2 structures. Differences in proton occupancies among the various interstitial sites of the Zr sub 2 Pd lattice are believed to be responsible for these variations in structure. This association is strongly supported by comparisons of the experimental second moments for the proton nuclear magnetic resonance (NMR) lineshapes with dipolar moments calculated for model tetragonal structures when samples have hydrogen compositions in the range 1.68 less than or equal to x less than or equal to 3.40. The NMR results clearly show that protons preferentially occupy tetrahedral interstitials when x less than or equal to 2.0 or the preparation temperature exceeds 700 sup 0 K. Additionally, we report the existence of the ternary hydride Zr sub 2 PdH/sub 4.8/ at 298 sup 0 K and 36 atm H sub 2 . 15 refs., 3 figs., 3 tabs. (ERA citation 11:036424)