Rotational Diffusion in NaBH4

Hydrogen dynamics in complex hydrides comprise long-range translational diffusion as well as localized motions like vibrations, librations, or rotations. All different motions are characterized by their specific length scale and time scales. Here we present a combined experimental and theoretical study oil the rotational diffusion of the (BH4)(-) ions in crystalline NaBH4. The motion is thermally activated and characterized by an activation energy of 117 meV and a prefactor of 11 fs. Thereby the motion is dominated by 90 degrees reorientations of the (BH4)(-) ion around the 4-fold symmetry axis of the cubic crystal. The experimental results are discussed oil the basis of DFT calculations, revealing the potential energy landscape of a (BH4)(-) subunit in the crystalline matrix.