Nuclear magnetic resonance studies of atomic motion in borohydride-based materials: Fast anion reorientations and cation diffusion

• Solid solutions LiBH4–LiI: extremely fast BH4 reorientations down to low T.
• LiLa(BH4)3Cl: Li-ion diffusive jumps and BH4 reorientations at the same frequency scale.
• Dramatic acceleration of B12H12 reorientations in the disordered phase of Na2B12H12.
• Fast Na-ion diffusion in the disordered phase of Na2B12H12.

Two basic types of thermally activated atomic jump motion are known to exist in solid borohydrides and the related systems: the reorientations of complex anions ([BH4]−, [B12H12]2−) and the translational diffusion of metal cations or complex anions. This paper reviews recent progress in nuclear magnetic resonance (NMR) studies of these jump processes in complex hydrides, such as solid solutions of halide anions in borohydrides, bimetallic borohydrides and borohydride–chlorides, borohydride–amides, and B12H12-based compounds. The emphasis is put on the systems showing fast-ion conductivity. For these systems, we discuss a possible relation between the reorientational motion of complex anions and the translational motion of metal cations.