Solvent-free trivalent rare-earth metal borohydrides $R{\left(\mathrm{B}{\mathrm{H}}_4\right)}_3$ ($R=\mathrm{Y}$, Dy, and Gd) were synthesized from $R{\mathrm{Cl}}_3$ and $\mathrm{Li}\mathrm{B}{\mathrm{H}}_4$ through solid-state metathesis reactions and characterized by powder x-ray or neutron diffraction measurement and Raman spectroscopy combined with first-principles calculations. The crystal structure of $R{\left(\mathrm{B}{\mathrm{H}}_4\right)}_3$ was clarified to adopt a primitive cubic structure with $\mathrm{Y}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_3$: $a=10.852\left(1\right)\mathrm{\AA }$, $\mathrm{Dy}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_3$: $a=10.885\left(3\right)\mathrm{\AA }$, and $\mathrm{Gd}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_3$: $a=10.983\left(5\right)\mathrm{\AA }$ in space group $Pa\overline{3}$ (No. 205), the ${\left[\mathrm{B}{\mathrm{H}}_4\right]}^{-}$ complex anions of which locate on the edges of a distorted cube composed of $R^{3+}$. Based on the crystal structure, the observed Raman scattering positions are theoretically assigned such that the $\mathrm{B}{\mathrm{H}}_4$ bending is at $1050–1300{\mathrm{cm}}^{-1}$ and $\mathrm{B}{\mathrm{H}}_4$ stretching is at $2250–2400{\mathrm{cm}}^{-1}$, respectively. In addition, the computational studies on $\mathrm{Y}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_3$ suggested it to be an insulator that occupied $\mathrm{B}2s,2p$ and $\mathrm{H}1s$ orbitals with little contribution from Y, and the heat of formation was $\Delta H=-113\mathrm{kJ}∕\mathrm{mol}$ $\mathrm{B}{\mathrm{H}}_4$, which was estimated from $(1∕3)\mathrm{Y}+\mathrm{B}+2{\mathrm{H}}_2\to (1∕3)\mathrm{Y}{\left(\mathrm{B}{\mathrm{H}}_4\right)}_3$.

• Received 2 September 2007

DOI:https://doi.org/10.1103/PhysRevB.77.104114