X-ray structure determination of divanadium hydride, β1-V2H, and divanadium deuteride, β-V2D

Abstract

fl l-V2H: M r = 102.89, tetragonal, I4~/amd, a = 6.0349 (4), c = 6.8716 (3)/k, V = 250.3 A 3, D x = 5 . 4 6 M g m -3, Z = 8 , 2 (MoKa)=0 .7107 /k , /~= 13.65 mm -1, F(000)= 376.0, room temperature, final R-0 .031 for 154 independent reflections; fl-V2D: M r = 103.9, monoclinic, Cm, a = 4 . 4 5 5 4 ( 7 ) , b = 3.0090 (3), c = 4 . 4 7 1 9 (7)/k, t = 95.30 (1) °, V = 59.70 (1) A a, D x = 5.78 Mg m -a, Z ---2, 2(Mo Kct) = 0.7107/k, ~t = 14.31 mm -l, F(000) = 94.0, room temperature, final R = 0.024 got 598 independent reflections. The crystal structures of fll-VEH and fl-VzD have been studied using a four-circle diffractometer with Mo Kt~ radiation, and refined by the full-matrix leastsquares method. The specimens examined were grown under tensile stresses. The positional and thermal parameters of V atoms in fl~-V2H and fl-V2D are found to be hydrogen-isotope dependent and reflect directly the respective ordered arrangements of the H and D atoms which are located in the different types of octahedral interstices of the metal lattices. Introduction. The vanadium-hydrogen(deuterium) system has been the subject of extensive studies because of its significant isotope effects on equilibrium phase diagrams (Asano & Hirabayashi, 1973, 1977; Schober & Wenzel, 1978; Moss, 1983), diffusion constant (V61kl & Alefeld, 1978; Bowman, Attalla & Craft, 1983) and crystal structures (Somenkov & Shil'shtein, 1980; Asano & Hirabayashi, 1979, 1981). 0108-2701/85/111566-06501.50 The atomic arrangements of hydrogen in ill-V2 H and of deuterium in fl-V2D have been studied by several workers by X-ray, neutron and electron diffraction. The structural models proposed so far for these two compounds are summarized in Table 1. In all the models, the H(D) atoms occupy orderly specific octahedral interstices in the fundamental body-centered lattice of the V atoms. The metal lattice proposed is either monoclinic or tetragonal with the axial ratio co/ao~_ 1.1. For the monoclinic model, the space groups C2, Cm and C2/m are assumed for the hydride, and Cm is chosen for the deuteride. The tetragonal model of space group/4 Jamd is presented alternatively for the deuteride. The monoclinic and tetragonal models are illustrated in Fig. 1: in the monoclinic model (a), the rows of the H(D) atoms in the base-centered positions of the metal lattice are parallel to those in the edge-centered positions, while they are normal to each other in the tetragonal model (b). On the other hand, some fraction of H(D) atoms in fll-V2H (Wanagel, Sass & Batterman, 1972) and in fl-V2D (Westlake, Mueller & Knott, 1973) were suggested to occupy tetrahedral sites surrounding the H(D) atom at an octahedral site. A recent study by single-crystal neutron diffraction (Kajitani & Hirabayashi, 1985) indicates that the crystal structure offl~-V2H differs from that offl-VED in the arrangement of H(D) atoms, and corresponds to the model of I4Jamd. In the neutron diffraction study, however, vanadium atoms cannot be seen since its © 1985 International Union of Crystallography Y. NODA, T. KAJITANI, M. HIRABAYASHI AND S. SATO 1567 coherent scattering cross section for thermal neutrons is small as compared with H and D atoms. The present study is aimed to refine the host V structure of flcV2H and fl-V2D by X-ray diffraction. Because of the small atomic scattering factor of hydrogen (deuterium), it cannot be easily detected by X-ray or electron diffraction (Wanagel, Sass & Batterman, 1972). However, the diffraction intensities are affected by the V-atom displacements induced by the ordering of H(D) atoms (Metzger, Jo, Moss & Westlake, 1978; Metzger, Jo & Moss, 1979; Jo, Moss & Westlake, 1980; Moss, 1983). Since the static displacement reflects directly the interaction between V and H(D) atoms, its evaluation is important for elucidating the stability of metal hydrides (deuterides). Experimental. (1) Specimen preparation and X-ray intensity measurement. Single crystals of fll-V2H and fl-VzD were prepared by the reaction of vanadium single crystals with hydrogen or deuterium gas using a Sieverts-type apparatus. The vanadium single crystals, 8 mm in diameter and 40 mm in length, were initially grown by electron-beam zone melting and then machined to have a narrow neck, 1 mm in diameter, at one end of the specimen. Under deformation at the neck by application of tensile stress, the hydrogen (deuterium) Table 1. Crystal structures proposed for flI-V2H and