Anion Mobility and Cation Diffusion in Alkali Metal Borohydrides

Abstract

This study presents a wide spectrum of experimental investigations of alkali borohydrides МВН₄ (М = Li, Na, K, Rb, Cs) and complex hydrides with substituted anions Li(BH₄)_{1– y}I_y, LiLa(BH₄)₃Cl, and Na₂(BH₄)(NH₂) obtained by the nuclear magnetic resonance method, quasielastic neutron scattering spectroscopy, and X-ray diffraction analysis. Activation energies for reorientational motion of anions in alkali borohydrides have been systematized, and possible configurations and types of jumps of ВН₄ groups have been discussed. It has been shown that the activation energy of reorientations of ВН₄ groups change nonmonotonously with the growth of the cation radius. Substitution of halides and amides for anions in complex hydrides leads to an enhancement in the frequency of anion reorientations at low temperatures, a change in the translational diffusion of cations at the expense of a change in the crystalline structure, the appearance of vacancies in the lattice, and influence of the paddle-wheel effect. Interrelation between the type of anion reorientations, cation diffusion, and the crystal lattice has been demonstrated.