High volumetric hydrogen density phases of magnesium borohydride at high-pressure: A first-principles study

The previously proposed theoretical and experimental structures, bond characterization, and compressibility of Mg(BH4)(2) in a pressure range from 0 to 10 GPa are studied by ab initio density-functional calculations. It is found that the ambient pressure phases of meta-stable I4(1)/amd and unstable P-3m1 proposed recently are extra stable and cannot decompose under high pressure. Enthalpy calculation indicates that the ground state of F222 structure proposed by Zhou et al. [2009 Phys. Rev. B79 212102] will transfer to I4(1)/amd at 0.7 GPa, and then to a P-3m1 structure at 6.3 GPa. The experimental P6(1)22 structure (alpha-phase) transfers to I4(1)/amd at 1.2 GPa. Furthermore, both I4(1) = a m d and P-3m1 can exist as high volumetric hydrogen density phases at low pressure. Their theoretical volumetric hydrogen densities reach 146.351 g H-2/L and 134.028 g H-2/L at ambient pressure, respectively. The calculated phonon dispersion curve shows that the I4(1)/amd phase is dynamically stable in a pressure range from 0 to 4 GPa and the P-3m1 phase is stable at pressures higher than 1 GPa. So the I4(1) = a m d phase may be synthesized under high pressure and retained to ambient pressure. Energy band structures show that they are both always ionic crystalline and insulating with a band-gap of about 5 eV in this pressure range. In addition, they each have an anisotropic compressibility. The c axis of these structures is easy to compress. Especially, the c axis and volume of P-3m1 phase are extraordinarily compressible, showing that compression along the c axis can increase the volumetric hydrogen content for both I4(1) = a m d and P-3m1 structures.