We have prepared pure $\mathrm{Ca}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_2$ without any solvent adducts and determined its structural parameters by powder x-ray diffraction measurement. The crystal structure of $\mathrm{Ca}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_2$ 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 $\mathrm{Ca}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_2$. The interaction between Ca atoms and $\mathrm{B}{\mathrm{H}}_4$ complexes has an ionic character while the internal bonding of $\mathrm{B}{\mathrm{H}}_4$ is essentially covalent. It is confirmed that $\mathrm{Ca}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_2$ 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 (2006)].

• Received 26 June 2006

DOI:https://doi.org/10.1103/PhysRevB.74.155122