NEUTRON-DIFFRACTION STUDIES OF ZR-CONTAINING INTERMETALLIC HYDRIDES WITH ORDERED HYDROGEN SUBLATTICE - I - CRYSTAL-STRUCTURE OF ZR2FED5

The deuteration of intermetallic Zr2Fe with CuAl2-type structure was s tudied by Thermal Desorption Spectroscopy, powder X-ray and neutron di ffraction. The tetragonal crystal structure of the saturated (1 bar D- 2) Zr2FeD5 deuteride (space group P4/ncc (No.130); 298 K: a=6.93566(8) , c=5.62061(8) Angstrom; 4.2 K: a=6.92112(7), c=5.62045(7) Angstrom) h as a completely ordered hydrogen sublattice both at 4.2 and 298 K. All interatomic D-D distances exceed 2.08 Angstrom. The crystal structure was determined by Rietveld analysis of high resolution powder neutron diffraction data. The presence of small amounts of three (four) impur ity phases was included in the refinements. The D-sublattice is built from distorted tetragonal antiprisms of ZrD8, and can be described in terms of layers altering along [001], Deuterium atoms occupy two types of tetrahedral Zr-4 and Zr3Fe interstices. Intratomic bond distances are in the range: Zr-D 2.054-2.136 Angstrom (298 K), 2.052-2.134 Angst rom (4.2 K); Fe-D 1.662 Angstrom (298 K), 1.658 Angstrom (4.2 K). The crystal structure is isotypic with Zr2CoD5. The unit cell expansion on deuteration and on heating from 4.2 to 298 K is highly anisotropic. T he c-axis remains unchanged and preferable expansion in the a-b plane is discussed as connected to characteristics of metal-metal bonding an d deuterium-deuterium repulsive interactions. The hydrogenation increa ses the stability of the CuAl2-type metal matrix of Zr2Fe. In the pres ence of interstitial hydrogen, the phase is stable far below the tempe rature of peritectoid decomposition of pure Zr2Fe. (C) 1998 Elsevier S cience S.A.