First-Principles Investigations On The Pressure-Induced Structural Transitions Of Ternary Metal Hydride Mg（BH₄）₂

The metal borohydrides have recently attracted much attentions due to their high capacity for hydrogen storage. They have been suitable effort made to explore sustainable for the fuel cells and the heat storage materials as they can be absorbed and desorbed hydrogen under certain conditions. The metal borohydrides are considered to be the most one of the promising hydrogen storage materials, because of the ability of lower price, lightweight and other advantages. The Mg(BH4)2has hydrogen quality density14.8wt%. In view of the fact, Mg(BH4)2is always being attention by the scientific workers.The theoretical and experimental study of the structure of Mg(BH4)2have been widely confirmed under the ambient pressure. But many aspects of phase transition of Mg(BH4)2are still unknown. In this work, we carried out a first principles calculation on the properties of the Mg(BH4)2under high pressures. The high pressure behaviors of the Mg(BH4)2have been uncovered. To the bonding interactions, the Mulliken population are investigated.The research shows a discontinuity first order phase transformation from the α-Mg(BH4)2to the β-Mg(BH4)2at0.64GPa. The phase of β-Mg(BH4)2is stable as the dynamically stable. The analysis results of electron density show, the structures of a-Mg(BH4)2and β-Mg(BH4)2exhibit a common insulating feature with a finite energy gap. The overlap between B and H is similar to that seen in the group of [BH4]. The results of Mulliken population analysis show that there are a ionic interaction between the Mg2+and the subunit [BH4]". The bond B-H has no change along with the crease of pressure. But the atom B s and p states and the H s state have stronger hybridization. It results that the value of the BOPSB-H at β-Mg(BH4)2is larger than the ambient pressure. Furthermore, Mulliken population analyses show that the activation energy of β-Mg(BH4)2is higher than a-Mg(BH4)2. The study of the optical properties show that the properties change are not obvious of a-Mg(BH4)2on the pressure increased. Comparing with the ambient pressure phase, the high-pressure phase β-Mg(BH4)2has more strong absorption peak. All of the above can provide an important theory in experimental study of Mg(BH4)2under high pressures.