First-Principles Investions Of The Pressure-Induced Structrual Transitions In Alkali-Earth Metal Hydrides Of Ca(Bh4)2

New types of clean energy sources have attracted intense attention, due to the limited supply of fossil and the environmental issues. Hydrogen is one of the clean and sustainable energy carriers which will replace fossil fuel. Because of their high volumetric and gravimetric hydrogen density, the complex metal hydrides are promising compounds for hydrogen storage applications. Among them, the alkali earth metal borohydride Ca(BH4)2has recently received a considerable amount of attention due to its high theoretical hydrogen storage capacity(11.6wt.%).Recently, the structure and physical properties of Ca(BH4)2have been explored both theoretically and experimentally. In our knowledge, investigations of Ca(BH4)2under pressure is lacking in the current literature. Therefore, we detailed research on the behavior of Ca(BH4)2under high pressure by using ab initio plan wave pseudopo-tential method. Focusing on the crystal structure of the high-pressure β-Ca(BH4)2, we explore the nature of the pressure-induced phase transition of Ca(BH4)2and investigate the hydrogen desorption properties. The research provides theoretical guidance for the hydrogen storage performance of complex metal hydrides under high pressure.In conclusion, the pressure-induced structural transition of Ca(BH4)2from α-Ca(BH4)2(space group:Fddd) to β-Ca(BH4)2occurs at3.72GPa, the crystal struc tures of β-Ca(BH4)2is found to be tetragonal with space group P4. This transition is identified as first-order with volume collapse of4.6%. The predicted β-Ca(BH4)2structure is dynamically stable as found in the phonon-dispersion curve. The elec-tronic structure reveals the covalent bonds interaction between atoms of B and H. Both phases have nonmetallic character with broadband gap. In order to explore the phase transformation mechanism, we have performed calculations to determine the phonon-dispersion curves and phonon density of states. The results show that pressure-induced softening of phonon frequencies is found in α-Ca(BH4)2at3.72GPa. The average net charge and bond overlap population, which have calculated on the basis of the Mulliken population. The calculation reveals that in α and β phases of Ca(BH4)2complex, the charges transfer mainly from Ca2+to the [BH4]-subunit and there have ionic bonding of Ca2+and [BH4]-anion. The activation energy of the β-Ca(BH4)2phase is higher than the α-Ca(BH4)2phase. So the β-2structure needs high energies to decomposition release hydrogen.