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 intermetalli c 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 (ne w structure type of intermetallic hydrides) were derived at 7 K and 29 3 K from Rietveld refinements of high-resolution powder neutron diffra ction data. The orthorhombic symmetry of Zr3Fe (space group Cmcm) is r etained during hydrogen absorption; however, there is an anisotropic e xpansion 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 f illing of four different types of interstices, one trigonal bipyramida l Zr3Fe2 interstice and three types of tetrahedral interstices of Zr3F e (one) and Zr-4 (two). Metal-deuterium bond distances are in the rang e 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 su rround zirconium atoms, consisting of deformed cubes with an additiona l 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 diffra ction data at 7 K. A multistaged deuterium desorption starts just abov e room temperature for Zr3FeD6.7. At 448 K the less saturated deuterid e Zr3FeDsimilar to 5, with a reduced unit cell volume, is obtained. De uteration, when performed at 873 K and 1 bar D-2, results in dispropor tionation of Zr3Fe into ZrD2 and ZrFe2. Both for Zr3FeD6.7 and the dis proportionated phase mixture, deuterium desorption under secondary vac uum conditions is completed below 973 K and results in a full recovery of the Zr3Fe intermetallic. Zr3Fe presents a new example of successfu l application of the nation-Disproportionation-Desorption-Recombinatio n process to intermetallic phases of zirconium. (C) 1998 Elsevier Scie nce S.A. All rights reserved.