Rotational motion of BH{sub 4} units in MBH{sub 4} (M=Li,Na,K) from quasielastic neutron scattering and density functional calculations

We present a combined experimental and theoretical study on the rotational motion of the (BH{sub 4}){sup -} ions in alkaline tetrahydroborides MBH{sub 4}, M=(Li,Na,K). The BH{sub 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 NaBH{sub 4} and KBH{sub 4} phases and in the high-temperature phase of LiBH{sub 4}, the motion of the (BH{sub 4}){sup -} ion is dominated by 90 deg. reorientations while it displays a threefold jump behavior in the low-temperature phase of LiBH{sub 4}. The experimental results are discussed on the basis of density functional theory calculations, revealing the potential-energy landscape of a (BH{sub 4}){sup -} subunit in the crystalline matrix and relating the hydrogen dynamics to the structural properties of the respective compounds.