Journal of Alloys and Compounds 373 (2004) 161–166 Hydrogen absorption–desorption characteristics of the LaNi 5 Sn intermetallic compound Masashi Sato, Volodymyr A. Yartys ∗ Institute for Energy Technology, Instituttveien 18, P.O. Box 40, Kjeller NO-2027, Norway Received 29 September 2003; received in revised form 16 October 2003; accepted 16 October 2003 Abstract The pressure–composition–temperature (P–C–T) relations in the LaNi 5 Sn–H system were measured volumetrically at temperatures between 258 and 423 K. Two hydride phases, -LaNi 5 SnH 2 and -LaNi 5 SnH 3 are formed in this system in addition to the -solid solution with a limiting H content of 0.3 H atoms per formula unit. The calculated work loss due to hysteresis is rather small, 112 J (mol H) -1 at 298 K. The partial molar enthalpy and entropy calculated for the formation of the -LaNi 5 SnH 2 are -18.5 ± 0.8 kJ (mol H) -1 and -53.7 ± 2.3 J (K mol H) -1 , respectively. An advanced van der Waals lattice gas model was applied to fit the isotherms. The critical temperature of the LaNi 5 SnH 2 equals T C = 421 ± 17 K. © 2003 Elsevier B.V. All rights reserved. Keywords: Intermetallic compound; Rare earth compound; Metal hydride; Gas–solid reaction; Thermodynamic properties 1. Introduction Substitution of Sn for Ni in LaNi 5 alloys reduces hystere- sis and leads to remarkable improvements in cyclic stability, kinetics of hydrogen absorption and desorption and corro- sion resistance making these alloys promising materials for Ni-metal hydride batteries [1–6]. Studies of the chemically related LaNiSnD 2 showed that Sn is not a typical p-element and does not block occupation of the specific Sn-surrounded interstitial sites [7]. From these works, it is evident that Sn substitutions can be an attractive step towards advanced metal hydrides. Further studies are required to understand better the ben- eficial influence of Sn on the hydrogenation properties. This work was devoted to studies of the hydrogen interaction with the ternary intermetallic compound LaNi 5 Sn. Its stoi- chiometry is close to the solid solution range LaNi 5-x Sn x (x< 0.4), so in accordance with the phase diagram of the ternary system La–Ni–Sn, LaNi 5 Sn is in equilibrium with LaNi 5-x Sn x [8]. The LaNi 5 Sn intermetallic compound adopts the hexago- nal CeNi 5 Sn type of structure (space group P6 3 /mmc), which ∗ Corresponding author. Tel.: +47-63-80-64-53; fax: +47-63-81-29-05. E-mail address: volodymyr.yartys@ife.no (V.A. Yartys). is related to the CaCu 5 type and is characterised by a rela- tively large unit cell (a LaNi 5 Sn ∼ a LaNi 5 ; c LaNi 5 Sn ∼ 5c LaNi 5 ). In CeNi 5 Sn three different sites are collectively occupied by Ni and Sn and two pairs of sites are exclusively occupied by either Ce or Ni [9]. No studies of hydrogen interaction with LaNi 5 Sn intermetallic compound have been reported so far. The aim of this work was to study the phase equilib- ria in the system LaNi 5 Sn-H by measuring the pressure– composition–temperature (P–C–T) relations. The thermo- dynamic parameters were calculated and the critical temperature for the -LaNi 5 SnH 2 hydride was evaluated on the basis of these diagrams. 2. Experimental 2.1. Sample preparation The LaNi 5 Sn sample was prepared by arc melting in an Ar atmosphere using high purity elements with grade bet- ter than 99.9%. The alloy was sealed under vacuum in a quartz tube and annealed at 773 K for 4 weeks. Finally it was quenched into a mixture of ice and water. The formation of the LaNi 5 Sn intermetallic compound crystallising with the CeNi 5 Sn type was confirmed by powder X-ray diffrac- tion with a Siemens D5000 diffractometer using Cu K 1 0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.10.027