Neutron diffraction studies of Zr-containing intermetallic hydrides with ordered hydrogen sublattice.: II.: Orthorhombic Zr3FeD6.7 with filled Re3B-type structure

Zr3FeD6.7 was obtained by deuteration of the Zr3Fe binary intermetallic compound with Re3B-type structure at temperatures near 273 K and D-2-pressures below 0.25 bar, and was characterised by Thermal Desorption Spectroscopy and powder X-ray diffraction. Crystal structure data (new structure type of intermetallic hydrides) were derived at 7 K and 293 K from Rietveld refinements of high-resolution powder neutron diffraction data. The orthorhombic symmetry of Zr3Fe (space group Cmcm) is retained during hydrogen absorption; however, there is an anisotropic expansion in the unit cell dimensions. At 293 K a=3.5803(3); b=11.059(1); c=9.6486(8) Angstrom (Delta a/a=7.7%, Delta b/b=0.8%, Delta c/c=9.4%). The deuterium atoms take an ordered structure with near complete filling of four different types of interstices, one trigonal bipyramidal Zr3Fe2 interstice and three types of tetrahedral interstices of Zr3Fe (one) and Zr-4 (two). Metal-deuterium bond distances are in the range Zr-D=2.058-2.204 Angstrom, Fe-D=1.713-1.791 Angstrom (293 K). All D-D distances in the completely ordered hydrogen sublattice exceed 2.01 Angstrom. The sublattice is built from two types of polyhedra which surround zirconium atoms, consisting of deformed cubes with an additional ninth vertex [Zr1D(9)] and deformed cubes [Zr2D(8)]. These polyhedra form a spatial framework by sharing vertexes and edges. There are no indications for magnetic long range order in the powder neutron diffraction data at 7 K. A multistaged deuterium desorption starts just above room temperature for Zr3FeD6.7. At 448 K the less saturated deuteride Zr3FeDsimilar to 5, with a reduced unit cell volume, is obtained. Deuteration, when performed at 873 K and 1 bar D-2, results in disproportionation of Zr3Fe into ZrD2 and ZrFe2. Both for Zr3FeD6.7 and the disproportionated phase mixture, deuterium desorption under secondary vacuum conditions is completed below 973 K and results in a full recovery of the Zr3Fe intermetallic. Zr3Fe presents a new example of successful application of the Hydrogenation-Disproportionation-Desorption-Recombination process to intermetallic phases of zirconium. (C) 1998 Elsevier Science S.A. All rights reserved.