First-Principles Investigation of the Structural, Electronic and Magnetic Properties of α-, β- and γ-Mg(AlH4)2 Complex Hydride

Abstract

The structural, electronic and magnetic properties of Mg(AlH4)2 in its pressure-induced α-, β- and γ-phases are extensively studied by employing first-principles method within the framework of density functional theory (DFT). This system undergoes a transition from α- to β- and then β- to γ-phase at 0.67 and 10.28 GPa respectively. A slight structural distortion is detected across α- to β-transition due to a small tilting of AlH4 tetrahedron along the c-axis. In the β- to γ-transition, the crystal structure of Mg(AlH4)2 changes markedly. The coordination number of H with Al and Mg increases significantly, which is responsible for the instability of the AlH4 tetrahedron. The drastic change in the structure would originate from the higher coordination number and much more tilting of AlH4 tetrahedron. Alternative long-short/short-long Al-Mg double chains are observed in the c-direction for α- and β-Mg(AlH4)2 but no such double chain is observed in the γ-phase of Mg(AlH4)2. The electronic properties of Mg(AlH4)2 in all phases are predominated by the Al-2p and H-1s orbitals. The ground state properties remain almost similar for both α- and β-phases. The hybridizations of H-1s orbitals with Al-2p and 3s orbitals invoke highly ionic covalent bonding of H and Al within the AlH4− anion complex that is responsible for the non-magnetic nature of α-, β- and γ-Mg(AlH4)2. The volume reduction in the β- and γ-Mg(AlH4)2 phases causes broadening of p bandwidths, which is crucial for the decrease in the band gaps.