Cubic MgH 2 stabilized by alloying with transition metals: A density functional theory study B.R. Pauw a , W.P. Kalisvaart a, * , S.X. Tao a , M.T.M. Koper b , A.P.J. Jansen a , P.H.L. Notten a,c a Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands b Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands c Philips Research Laboratories, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands Received 26 September 2007; received in revised form 7 February 2008; accepted 21 February 2008 Available online 21 March 2008 Abstract The stability of two crystallographic modifications of Mg-transition metal (Sc, Ti, Zr, Hf) dihydrides was studied using density func- tional theory. Beyond a certain transition metal content, the rutile structure characteristic of pure MgH 2 is no longer stable, and the hydride transforms into a fluorite-type structure, similar to that of the transition metal dihydride. The transition point for both Mg– Sc and Mg–Ti hydrides is estimated to be at 20 at.% Sc/Ti, which is in very good agreement with previous experimental studies. For Mg–Zr and Mg–Hf, no experimental data are available for comparison, so the calculations have a predictive value for these systems. For Zr and Hf, the transition point is predicted to be at lower transition metal content than for Sc and Ti, at 13 at.%. This means that the Mg–Zr hydride is also of practical importance, because a fluorite structured hydride is predicted with a hydrogen content in excess of 6 wt.%. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Magnesium alloys; Hydrogen storage; Density functional theory 1. Introduction As fossil fuels are slowly running out, alternative ways have to be found to meet the energy demands of the future. The development of a hydrogen-driven economy may be a viable solution to future energy shortages [1]. However, some major technical challenges will have to be overcome, one of which is finding an efficient and safe hydrogen stor- age medium with a small volume and low weight. Revers- ible metal hydrides have the advantage of moderate operating temperatures and pressures compared with pres- surized or liquefied hydrogen, which makes them a serious option for applications in nickel–metal hydride (Ni–MH) batteries or fuel cells in portable electronics or hybrid elec- tric vehicles (HEV) [2]. Mg has a very high reversible storage capacity of 7.6 wt.% hydrogen, which in principle makes it a promising candidate as a storage medium in mobile applications. However, a major disadvantage is the high stability of MgH 2 , which has an enthalpy of formation of 77 kJ mol 1 H 2 , resulting in an equilibrium pressure of 1 bar only at 300 °C [3]. This is far outside the working range of a fuel cell and, therefore, MgH 2 must be destabi- lized, for example by alloying with other elements. First-principles investigations by Song et al. [4,5] into ways of destabilizing MgH 2 demonstrated the influence of different alloying elements on the stability of rutile-struc- tured MgH 2 . It was found that Al, Ti, Fe, Ni, Cu and Nb destabilized the rutile structure to a large extent, ranging from 22.9 kJ mol 1 H 2 for Ti to 69.1 kJ mol 1 H 2 for Cu, for 20 at.% substitution of Mg by the alloying element. Chen et al. [6] also investigated a large number of possible alloying elements and found only very small variations in 1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.02.028 * Corresponding author. Tel.: +31 40 2746247; fax: +31 40 2743352. E-mail address: w.p.kalisvaart@tue.nl (W.P. Kalisvaart). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 56 (2008) 2948–2954