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 I4₁/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. B 79 212102 will transfer to I4₁/amd at 0.7 GPa, and then to P-3m1 structure at 6.3 GPa. And the experimental P6₁22 structure (α-phase) transfers to I4₁/amd at 1.2 GPa. Furthermore, both I4₁/amd and P-3m1 can exist as high volumetric hydrogen density phases at low pressure. Their theoretical volumetric hydrogen densities reach 146.351 g H₂/L and 134.028 g H₂/L at ambient pressure respectively. The calculated phonon dispersion curve shows that the I4₁/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 I4₁/amd phase may be synthesized under high pressure and retained to ambient pressure. Energy band structures show that both of them are 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 compressing alone c axis can increase the volumetric hydrogen content for both I4₁/amd and P-3m1 structures.