We have prepared pure Ca͑BH 4 ͒ 2 without any solvent adducts and determined its structural parameters by powder x-ray diffraction measurement. The crystal structure of Ca͑BH 4 ͒ 2 is found to be orthorhombic with space group Fddd ͑No. 70͒. Using this structural information, the first-principles calculations have been performed to investigate the fundamental properties of Ca͑BH 4 ͒ 2 . The interaction between Ca atoms and BH 4 complexes has an ionic character while the internal bonding of BH 4 is essentially covalent. It is confirmed that Ca͑BH 4 ͒ 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 et al., Phys. Rev. B 74, 045126 ͑2006͔͒.

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