X-ray diffraction and nuclear magnetic resonance studies of the relationship between electronic structure and the tetragonal distortion in zirconium hydride (ZrHx)

Abstract

Low-temperature (LT) X-ray diffraction (XRD) studies of 13 ZrH/sub x/ samples in the range 1.50 less than or equal to x less than or equal to 2.00 were made by using a Guinier-Simon (GS) focusing camera and other methods. The ZrH/sub x/ phase diagram was made more complete for the fcc (delta-phase) and fct (epsilon-phase) regions from ambient to low temperature (i.e., to about 110 K). No new low-temperature phases were observed by XRD. Careful determinations of the composition and temperature dependences of the following parameters were made: c/a ratio, V (unit-cell volume), and ..delta..l/(l..delta..T). When the XRD results are combined with the solid-state proton NMR parameters that reflect the density of electron states at the Fermi level, mutual support is obtained for the Jahn-Teller splitting of the Zr d bands to produce the tetragonal distortion of the ideal fcc lattice when the stoichiometry exceeds x = 1.66. The lattice constants from XRD confirm that the maximum in the values of the proton NMR parameters that are observed when 1.80 less than or equal to x less than or equal to 1.85 is due primarily to the Fermi level falling at the maximum of the lower split d band andmore » is not merely a lattice-temperature effect.« less