Neutron diffraction studies of Zr-containing intermetallic hydrides with ordered hydrogen sublattice, III.: Orthorhombic Zr3FeDx (x=1.3, 2.5 and 5.0) with partially filled Re3B-type structure

Lower deuterides of Zr3FeDx (x=1.3, 2.5 and 5.0) were obtained by desorption of deuterium from Zr3FeD6.7 and were studied by means of powder X-ray and neutron diffraction. Their metal sublattices are of the Re3B-type (space group Cmcm; a=3.3261(4)-3.4524(1); b=11.2333(5)-11.317(1); c=8.999(1)-9.4746(4) Angstrom for x=1.3-5.0), and are hence mainly unchanged with respect to the initial intermetallic Zr3Fe and the "highest" deuteride Zr3FeD6.7. A gradual expansion of the orthorhombic unit cells accompanies the increase in D content. There appears to exist a broad solid solution phase, and the volume increase is in the range 4.8(x=1.3)-14.2%(x=5.0) relative to Zr3Fe. The expansion is anisotropic, for low x mainly located in the be plane, for larger x in the ac plane. Rietveld refinements of the high resolution powder neutron diffraction data show that the initially occupied four types of interstices in Zr3FeD6.7, i.e. Zr3Fe2, Zr3Fe and two non-equivalent Zr-4 sites, are depopulated in a step-wise manner on desorption at increasing temperatures. On decreasing the D content, deuterium is first removed from interstices having iron atoms in their surroundings (Zr3Fe2 and Zr3Fe sites), whereas the occupancy of the Zr-4 tetrahedra remains complete. In the "lower" deuterides, Zr3FeD2.5 and Zr3FeD1.3, these tetrahedra become differently occupied. The reduced stability of one of these sites correlates with having an unfavourable smaller size. On going from Zr3FeD6.7 to Zr3FeD5.0 the originally completely ordered hydrogen sublattice with all D-D distances exceeding 2.0 Angstrom becomes partially disordered. The partially filled Zr3Fe interstices (50% occupancy) become closer on average (in Zr3FeD5.0: 1.54 Angstrom), which must be understood in terms of significant short range order. The metal-deuterium distances decrease gradually with decreasing D/Zr3Fe ratio, being shorter than those of Zr3FeD6.7: Zr-D=2.058(5)-2.204(7) Angstrom; Fe-D=1.713(4)-1.7914(3) Angstrom (at 293 K). There are no indications for either D ordering or for magnetic long range order in the powder neutron diffraction data of Zr3FeD5.0 at 7 K. (C) 1999 Elsevier Science S.A. All rights reserved.