Contents lists available at ScienceDirect Calphad journal homepage: www.elsevier.com/locate/calphad Density functional study of the thermodynamic properties and phase diagram of the magnesium hydride Hasan S. AlMatrouk a , Viorel Chihaia b, ⁎ , Valentin Alexiev c a Kuwait Institute for Scientific Research, PO Box 24885, Safat 13109, Kuwait b Institute of Physical Chemistry Ilie Murgulescu, Romanian Academy, Splaiul Independentei 202, Bucharest, Romania c Institute of Catalysis, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bldg. 11, Sofia 1113, Bulgaria ARTICLE INFO Keywords: Magnesium hydride polymorphs Density functional theory Thermodynamic calculations Phase diagram P-T ABSTRACT This paper focuses primarily on the P-T phase diagram determination by considering six polymorphs of mag- nesium hydride (α - rutile TiO 2 , P4 2 /mnm, β - cubic modified CaF 2 , Pa 3, γ - orthorhombic PbO 2 , Pbcn, δ' - orthorhombic, Pbca and cubic - Fm 3 m). The Gibbs free energy and other thermodynamic properties were evaluated by DFT-based thermodynamic calculations, within the frame of the quasi-harmonic approximation, for the pressure range 0–10 GPa and temperatures between 0 and 1200 K. Furthermore, the structural, energetic, and electronic properties of the investigated structures are conversed. 1. Introduction The metal hydrides are promising candidate materials for the hy- drogen storage as the cleanest solution for electric vehicles and energy storages. In particular, the magnesium hydride is an attractive system for the hydrogen storage, as it is one of the systems with a high hy- drogen weight content (7.6 wt%). However, magnesium has poor ki- netics and thermodynamic properties, requiring high ab- and deso- rption temperatures. In order to improve the reaction speed at lower temperatures, the magnesium hydride is mixed with other metals. It is important to know the dependence of the magnesium-based hydrate stability and phase transitions on pressure and temperature, in order to find the proper paths for the formation and decomposition of magne- sium hydrides phases. Several polymorphs of magnesium hydride were identified as ther- modynamically stable forms of MgH 2 for different pressure and tem- perature domains by experimental and theoretical investigations. The importance of knowledge of the various MgH 2 phase stability has led to an increase in research regarding the pressure-temperature phase dia- gram for the magnesium hydride. Under ambient conditions the mag- nesium hydride crystallizes as α-MgH 2 phase, with a rutile-type struc- ture (space group P4 2 /mnm) [1,2]. Bastide et al. found that under high pressure and temperature conditions the α-MgH 2 structure is trans- formed into β-MgH 2 (with a modified fluorite-type CaF 2 structure, space group Pa 3) and γ-MgH 2 (with an orthorhombic structure of α- PbO 2 -type, space group Pbcn); by decreasing the pressure the β-MgH 2 is transformed into γ-MgH 2 [3]. Bortz et al. detected at high pressure the γ-MgH 2 phase by the X-ray and neutron diffraction measurements [4]. Vajeeston et al. [5,6] identified a phase transition from α-MgH 2 to γ- MgH 2 at 0.39 GPa by Density Functional Theory (DFT) calculations. Moriwaki et al. detected by the X-ray diffraction (XRD) measurements at room temperature the transition α → γ, but at much higher pressure of 9 GPa [7]. Moses et al. built the P-T phase diagram by DFT ther- modynamic calculations and investigated the isotopic effects on the α, β and γ phases of the magnesium hydrides and identified the decom- position conditions [8]. Several other magnesium hydride phases are identified at high pressure by theoretical and experimental studies: Pbc2 1 [6,7], Pbca [6,7], Pnnm [5,9], Fm 3 m [5,10]. The successive phase transitions α(P4 2 /mnm) → γ(Pbcn) → β(Pa 3) → δ(Pbc2 1 ) → ε(Pnma) were predicted by several DFT studies [5,6,10]. Cui et al. es- tablished a similar sequence of transitions by DFT-based enthalpy cal- culations, but found that δ is unstable [11]. A metastable phase of MgH 2 (I4 1 /amd), which meets all the mechanical stability criteria for a tet- ragonal crystal, has been suggested by DFT calculations [12]. A new pressure-induced transition from the α-MgH 2 phase to an orthorhombic CaCl 2 -type (Pnnm) phase has been predicted by ab initio phonon cal- culations [9]. Durandurdu identified by ab initio Molecular Dynamics simulations the transition of α-MgH 2 to CaCl 2 -type phase around 5 GPa, which is stable up to 10 GPa. For values above 10 GPa a new phase with distorted-CaF 2 (Pbcm) structure is identified [13]. Lonie et al. predicted by DFT-based thermodynamics calculations several stable magnesium polyhydrides (MgH n , n > 2) for T = 0 K and much higher pressures [14]. The cubic structure of fluorite-type, with the space-group Fm 3 m was theoretically considered as a possible structure of the magnesium https://doi.org/10.1016/j.calphad.2017.11.001 Received 30 June 2017; Received in revised form 1 November 2017; Accepted 2 November 2017 ⁎ Corresponding author. E-mail addresses: hmatrouk@kisr.edu.kw (H.S. AlMatrouk), vchihaia@icf.ro (V. Chihaia), valexiev@ic.bas.bg (V. Alexiev). Calphad 60 (2018) 7–15 0364-5916/ © 2017 Elsevier Ltd. All rights reserved. MARK