We present a combined experimental and theoretical study on the rotational motion of the ${\left({\text{BH}}_4\right)}^{-}$ ions in alkaline tetrahydroborides $M{\text{BH}}_4$, $M=\left(\text{Li},\text{Na},\text{K}\right)$. The ${\text{BH}}_4$ motions are thermally activated and characterized by activation energies in the order of 0.1 eV, typical frequencies are in the terahertz range at temperatures of about 400 K. In the cubic ${\text{NaBH}}_4$ and ${\text{KBH}}_4$ phases and in the high-temperature phase of ${\text{LiBH}}_4$, the motion of the ${\left({\text{BH}}_4\right)}^{-}$ ion is dominated by $90°$ reorientations while it displays a threefold jump behavior in the low-temperature phase of ${\text{LiBH}}_4$. The experimental results are discussed on the basis of density functional theory calculations, revealing the potential-energy landscape of a ${\left({\text{BH}}_4\right)}^{-}$ subunit in the crystalline matrix and relating the hydrogen dynamics to the structural properties of the respective compounds.

• Received 29 March 2010

DOI:https://doi.org/10.1103/PhysRevB.81.214304