Electronic structure and stability of new FCC magnesium hydrides Mg 7 MH 16 and Mg 6 MH 16 (M [ Ti, V, Nb): An ab initio study M.G. Shelyapina a, *, D. Fruchart b , P. Wolfers b a V.A. Fock Institute of Physics, St. Petersburg State University, Ulyanovskaya 1, Petrodvorets, St. Petersburg 198504, Russia b Institut Ne ´el, CNRS/UJF, 25 avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France article info Article history: Received 7 August 2009 Received in revised form 21 December 2009 Accepted 25 December 2009 Available online 21 January 2010 Keywords: Hydrogen-storage materials Magnesium hydrides Ab initio calculations abstract MgH 2 is one of the most promising materials for hydrogen storage. However, its rather slow hydrogen absorption and desorption kinetics and high dissociation temperature essentially limit its application in this field. Nevertheless mixing Mg or MgH 2 with small amount of transition metals or their oxides remarkably accelerates the hydrogen kinetics. Recently a series of new hydrides Mg 7 TiH x , Mg 6.5 NbH x and Mg 6 VH x of Ca 7 Ge type structure has been synthesized. The hydrogen desorption properties have been found to be better than for pure MgH 2 . Here, we report on the results of our theoretical study of the electronic structure of these new hydrides carried out within the framework of the full-potential, self- consistent linearized augmented plane-wave method. We use these results, along with calculations of the heat of formation and relative stability, to discuss the bonding of these materials and their hydrogen-storage properties. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. 1. Introduction Magnesium is one of the most attractive materials for hydrogen-storage application due to its outstanding hydrogen capacity up to 7.6 wt.% for MgH 2 . However, its rather slow hydrogen absorption and desorption kinetics as well as high dissociation temperature, which is above 673 K, essentially limit its application for hydrogen storage. Numerous attempts have been made in order to improve magnesium hydrogen absorbing–desorbing characteristics. It has been reported experimentally that mixing magnesium or magnesium hydride with small amount of transition metals (TM) [1–6] or their oxides [7–9] accelerates the hydrogen kinetics. However, up to now basic physical aspects of these phenomena are not completely understood and a comprehensive insight in metal–hydrogen bonding, that plays a major role in the stability of MgH 2 , is needed in finding further modifications of magnesium hydride with better characteristics. Theoretical researchers could provide such insight and predict which additives affect better the thermodynamics and hydrogen kinetics. A number of theoretical investigations of pure MgH 2 have been reported [10–13]. It has been established that bonding in MgH 2 is a complex mixture of ionic and covalent contributions [10,11]. To understand the intrinsic mechanism of alloying effects on magnesium–hydrogen bonding several theoretical studies have been carried out. A number of simulations of binary MgH 2 –TM [5,14–16] and ternary (Mg, Fe, Ni)H 2 [17] hydrides based on the supercell approach have shown that the stability of the alloyed hydride is reduced by the alloying transition elements. Analysis of the density of states and charge distribution in MgH 2 –Ti [15] show that the bonding * Corresponding author. Tel.: þ7 812 428 44 69; fax: þ7 812 428 72 40. E-mail address: marinashelyapina@mail.ru (M.G. Shelyapina). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 35 (2010) 2025–2032 0360-3199/$ – see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.12.171