Thermodynamical stability of calcium borohydride Ca(BH4)2

We have prepared pure Ca(BH4)(2) without any solvent adducts and determined its structural parameters by powder x-ray diffraction measurement. The crystal structure of Ca(BH4)(2) is found to be orthorhombic with space group Fddd (No. 70). Using this structural information, the first-principles calculations have been performed to investigate the fundamental properties of Ca(BH4)(2). The interaction between Ca atoms and BH4 complexes has an ionic character while the internal bonding of BH4 is essentially covalent. It is confirmed that Ca(BH4)(2) obeys the linear relationship between the heat of formation and the Pauling electronegativity of the cation, which has been proposed in a previous study [Nakamori , Phys. Rev. B 74, 045126 (2006)].