Crystal structure and charge density analysis of Ca(BH4)2

Calcium borohydride Ca(BH4)2 is one of the promising new hydrogen storage materials because of its large amount of hydrogen desorption capability (9.6 mass%). The crystal structures of α-Ca(BH4)2 (space group: Fddd, lattice constants: a = 8.7782(2) Å, b = 13.129(1) Å, c = 7.4887(9) Å) and β-Ca(BH4)2 (P42/m, a = 6.9509(5) Å, c = 4.3688(3) Å) were refined by synchrotron X-ray diffraction at 300 and 433 K, respectively. The unsolved structures of γ-Ca(BH4)2 (Pbca, a = 7.525(1) Å, b = 13.109(2) Å, c = 8.403(1) Å) and Ca(BH4)2·H2O (Pnma, a = 8.200(1) Å, b = 5.8366(7) Å, c = 11.851(2) Å) were determined. In α-, β- and γ-Ca(BH4)2 structures, six boron atoms around a calcium atom construct CaB6 octahedron. The polymorphism of Ca(BH4)2 is formed by the different connection with adjacent octahedrons sharing vertexes and edges of the CaB6 octahedron. Furthermore, the charge density distribution in α-Ca(BH4)2 was experimentally determined by maximum entropy method. It is clarified that the bonding nature in α-Ca(BH4)2 ionic crystal is constructed from Ca2+ cation and BH4− anion.