L Journal of Alloys and Compounds 330–332 (2002) 169–174 www.elsevier.com / locate / jallcom Structure and magnetic properties of TbNiAl-based deuterides a a, a b * ˚ H.W. Brinks , V.A. Yartys , B.C. Hauback , H. Fjellvag a Institute for Energy Technology, P .O. Box 40, N-2007 Kjeller, Norway b Department of Chemistry, University of Oslo, P .O. Box 1033 Blindern, N-0315 Oslo, Norway Abstract This work concerns the study of the crystal and magnetic structure of TbNiAlD compounds ( x50.54, 1.23 and 1.33) by neutron x diffraction. TbNiAlD is isostructural with TbNiAl. It undergoes two magnetic transitions at T 516 K and T 536 K, with magnetic 0.54 1 N structure similar to that of TbNiAl below T 523 and T 545 K. TbNiAlD is in crystallography and magnetism comparable to 1 N 1.23 TbNiAlD . When the Tb NiAl positions are occupied, an orthorhombic distortion is induced. This completely changes the magnetic 1.1 2 ordering and could be explained by the easy magnetisation directions being perpendicular to the shortest Tb–Tb distances.  2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetic structure; Terbium nickel aluminium deuteride; Crystal structure; Metal hydride; Neutron diffraction ´ 1. Introduction determined below the Neel temperature T 545 K as well N as below the magnetic order–order transition at T 523 K. 1 On hydrogenation of intermetallic compounds, the crys- At 5 K, neutron diffraction experiments prove that 2 / 3 of tal structure is typically principally retained for the metal the Tb atoms are ordered with propagation vector k5 1 1 11 ] ] ]] sublattice, even though the unit-cell volume may increase ( ,0, ) and 1/3 with propagation vector k95( , ,0) or 2 2 22 111 ]]] by some 10–25%. In some systems several phases are k05( 2 , , ), with equally large moments ( |7.9 m ) along B 222 observed, distinguished by symmetry changes due to c [4]. Above 10 K, the moments of the latter sublattice hex distortions or changes in filling interstices. Introduction of decrease quickly (relative to the former) and are |1 m in B hydrogen into intermetallic compounds perturbs the crystal size above T [5]. Between T and T both orbits have 1 1 N structure of the metal sublattice, and the hydrogen atoms propagation vector k with moments along c [4]. are normally considered to donate electrons to the elec- The magnetic ordering temperature tends to decrease tronic band structure. This influences the electronic and with increasing deuteration (TbNiAl, 45 K; TbNiAlD , 0.3 magnetic properties. Often a weakening of the magnetic 42 K; TbNiAlD , 11 K; TbNiAlD , 14.5 K) [1,4,6,7]. 1.10 1.40 ordering takes place, e.g. for RNiAl (R5rare earth) [1]. Powder neutron diffraction (PND) studies at 7 and 28 K RNiAl for most R takes the ZrNiAl-type structure [2]. have shown that TbNiAlD takes the high-temperature 0.3 Several of these form hydrides with 1.2–1.6 hydrogen per magnetic structure of TbNiAl [6]. In TbNiAlD , studied 0.8 formula unit at ambient temperature and pressure [3]. as a two-phase mixture, reflections of magnetic origin are TbNiAlD is saturated at x51.4, but intermediate phases observed in the PND pattern below 20 K, and they can x with x51.1 and x50.3 have been synthesised [3]. At low partially be indexed on a unit cell with doubling of the deuterium content the symmetry is hexagonal like TbNiAl, orthorhombic a and b axes [8]. For TbNiAlD , the 1.1 11 ]] but higher deuteration gives an orthorhombic distortion complete magnetic structure is determined: k5(0, , ) with 22 (Amm2): a 5c , b 52a 1b , c 5b (unit-cell dimen- the magnetic moments in the bc plane; i.e. perpendicular to o h o h h o h sions: |c , œ3a , a ). that of the hexagonal structures [7]. For TbNiAlD , h h h 1.28 The antiferromagnetic ordering of Tb in TbNiAl is magnetic reflections, which are in correspondance with the magnetic structure of TbNiAlD , are present in the PND 1.10 pattern below 16 K [8]. The present study describes the crystal structure and magnetism of TbNiAlD , TbNiAlD and 0.54 1.23 TbNiAlD , and the extended knowledge is summarised *Corresponding author. Tel.: 147-63-806-453; fax: 147-63-812-905. 1.33 E-mail address: Volodymyr.Yartys@ife.no (V.A. Yartys). in an overall presentation for the TbNiAlD system. x 0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388(01)01644-9