Journal of Alloys and Compounds 384 (2004) 115–120 Crystal structure and thermal desorption properties of HoNiAlD 1.2 M. Stange a , A.V. Kolomiets b , V.A. Yartys a,∗ , R.G. Delaplane c , H. Rundlöf c a Institute for Energy Technology, P.O. Box 40, Kjeller N-2027, Norway b Department of Electronic Structures, Charles University, 5 Ke Karlovu, 12116 Prague 2, Czech Republic c The Studsvik Neutron Research Laboratory, Uppsala University, S-611 82 Nyköping, Sweden Received 17 March 2004; accepted 19 March 2004 Abstract The intermetallic compound HoNiAl with the hexagonal ZrNiAl-type structure absorbs deuterium up to HoNiAlD 1.3±0.1 under a D 2 pressure of 3–5 bar at room temperature. HoNiAlD 1.20(1) has been studied by powder neutron diffraction (PND). The unit cell of the hydride is orthorhombically distorted (space group Amm2) with unit cell dimensions a(∼c hex ) = 3.7016(2) Å, b(∼a hex √ 3) = 12.379(1) Å and c(∼a hex ) = 7.6288(4) Å (b/c = 0.937 √ 3) as determined from a Rietveld refinement of high-resolution powder neutron diffraction data. The deuterium atoms fill completely the trigonal bipyramid Ho 3 Ni 2 and the most expanded of six chemically equivalent and crystallographically different Ho 2 NiAl tetrahedra (r = 0.49 Å; occupancy 81±1%). The degree of orthorhombic deformation in RENiAl-hydrides is highly dependent on the occupation of the largest RE 2 NiAl tetrahedron. The metal-deuterium bond distances are: Ho-D = 2.16–2.44 Å, Ni-D = 1.61–1.85 Å and Al-D = 1.91 Å. The shortest deuterium-deuterium interatomic distances are quite long (D-D ≥ 2.42 Å). © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogen storage materials; Intermetallics; Neutron diffraction; Crystal structure 1. Introduction Equiatomic RENiAl compounds (RE = rare-earth metal) with the hexagonal ZrNiAl-type structure (P ¯ 62m) are ex- tensively studied as hydride-forming materials [1]. Hydro- gen absorption can take place at room temperature under H 2 (D 2 ) pressure of about 1 bar [2–6]. Previous studies of RENiAl have shown that both magnetic and structural prop- erties are modified by hydrogenation [4,7–9]. In most cases, the maximum hydrogen (deuterium) uptake reaches 1.0–1.4 atoms per formula unit and leads to an orthorhombic distor- tion of the original hexagonal structure (space group Amm2; a ∼ c hex ; b ∼ a hex √ 3; c ∼ a hex ) accompanied by a rela- tive increase of cell volume, V/V ≤ 7% (RE = Y, Gd, Tb, Dy, Ho and Er) [4]. For the orthorhombic hydrides, up to three interstitial sites are occupied by hydrogen (deu- terium); one RE 3 Ni 2 trigonal bipyramidal position and two RE 2 NiAl tetrahedral positions as observed for TbNiAlD 1.04 and TbNiAlD 1.1 [5,6,8]. Changes of both lattice symmetry and magnetic properties are believed to be closely related to ∗ Corresponding author. Tel.: +47-63-806-453; fax: +47-63-812-905. E-mail address: volodymyr.yartys@ife.no (V.A. Yartys). the modifications in the electronic structure on hydrogena- tion [4]. Previous studies of RENiAl deuterides have shown that the orthorhombic distortion (illustrated by the deviation of b/c from √ 3) increases with deuterium content. It is also dependent on the relative filling of the tetrahedral RE 2 NiAl sites. For example, detailed studies of TbNiAl deuterides [5,6,8] show that TbNiAlD 1.04 and TbNiAlD 1.1 being quite similar in D content have different filling of two tetrahedral Tb 2 NiAl deuterium sites and a significant difference in orthorhombic splitting. In TbNiAlD 1.04 the trigonal bipyramidal Tb 3 Ni 2 site is 88% occupied, the two tetrahedral Tb 2 NiAl sites are 50% and 18% occupied and b/c = 0.986 √ 3. For TbNiAlD 1.1 the trigonal bipyramidal Tb 3 Ni 2 site is 88% occupied, the two tetrahedral Tb 2 NiAl sites are 67 and 10% occupied, respectively, and b/c = 0.941 √ 3. The least occupied sites are crystallographically different in TbNiAlD 1.04 and TbNiAlD 1.1 while the first two, with higher H occupation, are common for both crystal structures. Earlier work on the HoNiAl–H 2 (D 2 ) system [7] sug- gested two hydride phases with different hydrogen content; HoNiAlD 1.0 and HoNiAlH 2.0 . The latter hydride has a much higher hydrogen content compared to the chemically similar 0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.03.121