L Journal of Alloys and Compounds 274 (1998) 29–37 Hydrogen induced phase transitions in YMn 2 a a b, c c b * H. Figiel , J. Przewoznik ,V. Paul-Boncour , A. Lindbaum , E. Gratz , M. Latroche , b b a ´ M. Escorne , A. Percheron-Guegan , P. Mietniowski a Department of Solid State Physics, University of Mining and Metallurgy, Al. Mickiewicza 30, 30-059 Krakow, Poland b LCMTR, GLVT, CNRS,2 –8 rue H. Dunant, 94320 Thiais, France c Institute for Experimental Physics, Technical University, Wiedner Haupstrasse 8 –10, A-1040 Wien, Austria Received 4 February 1998 Abstract X-ray and magnetic investigations of the YMn H hydrides at temperatures ranging from room temperature down to 4.2 K for the 2 x concentration range x#1.2 are presented and analyzed. It is shown, that in the concentration range 0.5,x,0.8 the paramagnetic cubic phase of the hydrides transforms at 245 K into a tetragonally distorted ( c / a ,1) and magnetically ordered phase. This transition temperature is more than two times higher than for pure YMn . Below |100 K a second transition into a phase also tetragonally distorted 2 and magnetically ordered is observed, but with c / a.1. The observed phase transitions are interpreted as of martensitic type. In the border ranges for x,0.5 and x.0.8 a complex behaviour with several two phase ranges as a function of temperature is observed. For x,0.35 a spinodal decomposition is detected in the temperature range 100–130 K. For x$1.2 no structural distortion is visible when crossing the temperature of the magnetic transition. On the basis of the presented data a phase diagram is proposed. The observed phase transitions are discussed and interpreted in terms of Mn–Mn magnetic interactions in dependence of the lattice expansion caused by hydrogen, taking into account the possible ordering of the hydrogen atoms.  1998 Elsevier Science S.A. Keywords: Intermetallic compounds; Hydrides; Laves phases; Phase diagram; Magnetic ordering 1. Introduction first order magnetic transition to a helicoidal magnetic ˚ structure with a period of about 400 A. This magnetic The Rare Earth and Yttrium compounds with transition transition is accompanied by a large increase of the unit metals readily absorb hydrogen which induces strong cell volume (about 5%) with a small tetragonal distortion 23 modifications of their structural and physical properties. ( ´5(12c / a)53310 ) [4]. The magnetism of YMn is 2 Until now the observed phenomena are not completely very sensitive to external pressure: about 3.7 kbar suppress understood and need further research. The manganese the onset of magnetic order [5]. The same effect can be compound with the non magnetic yttrium atoms is the obtained by substituting only 3% Scandium for Yttrium most suitable among the RMn Laves phase compounds to [1]. On the contrary, replacing Mn by Al atoms stabilises 2 study the influence of hydrogen of the magnetism on the the magnetic state of the Mn atoms due to volume manganese sublattice. expansion [6]. The sensitivity of the magnetism of the The YMn Laves phase compound has already been manganese sublattice to the Mn–Mn distance has been 2 widely studied [1]. At room temperature it crystallises in confirmed by a theoretical calculation for RMn com- 2 ¯ the cubic C15 type structure ( Fd3m space group) and is an pounds [7]. Similarly the hydrogen in YMn expands the 2 enhanced Pauli paramagnet with giant spin fluctuations. lattice and consequently the Mn–Mn distance, it can The Mn atoms have magnetic moments of about 2.8 m therefore be regarded as a negative chemical pressure. A B with localised character in the ordered state [2]. They change of the electronic structure due to hydrogen absorp- decrease in magnitude to about 1.6 m and possess more tion has been shown on the basis of full potential LMTO B itinerant character above the magnetic ordering tempera- calculations [8]. | ture ( T 100 K) [3]. At T the compound undergoes a Previous works on the YMn H hydrides [9] have 5 N N 2 x shown that at room temperature a solid solution of * Corresponding author. hydrogen in the C15 structure is obtained up to x53.5 0925-8388 / 98 / $19.00  1998 Elsevier Science S.A. All rights reserved. PII: S0925-8388(98)00566-0