Anion Disorder in K3BH4B12H12 and Its Effect on Cation Mobility

Abstract

Mixed anion borohydride, closo-borane of potassium, K3BH4B12H12, has been synthesized using mechanochemistry and characterized by combination of temperature dependent synchrotron radiation X-ray powder diffraction, solid state nuclear magnetic resonance, thermal analysis, electrochemical impedance spectroscopy, topology analysis, and ab initio solid state calculations. At RT the compound crystallizes in the monoclinic superstructure (P2/c) of the cubic antiperovskite prototype. At 565 K it transforms by first order phase transition into a rhombohedral (R-3m) deformation of the cubic prototype, which further transforms at 680 K by a second order phase transition into a cubic (P23) antiperovskite structure. While the monoclinic polymorph is observed for the first time among mixed anion salts, the rhombohedral and cubic polymorphs are known among other alkali metal and ammonium halides (or borohydrides), closo-boranes. The first phase transition is related to the repulsive homopolar H–H contacts between BH4–...