Polymorphism and solid solutions of trimethylammonium monocarboranes

Metal closo-boranes have recently received significant attention as solid-state electrolytes due to their high thermal and electrochemical stability, and the weak interaction between the cat- and anion, facilitating fast ionic conductivity. Here we report a synthesis method for obtaining a novel mixed closo-carborane compound, [NH(CH3)(3)][(CB8H9)(0.26)(CB9H10)(0.66)(CB11H12)(0.08)]. The crystal structures are investigated for [NH(CH3)(3)][CB9H10] and [NH(CH3)(3)][(CB8H9)(0.26)(CB9H10)(0.66)(CB11H12)(0.08)], revealing that the latter forms a solid solution isostructural to [NH(CH3)(3)][CB9H10]. The compounds exhibit polymorphism as a function of temperature, and we report the discovery of four polymorphs of [NH(CH3)(3)][CB9H10] and four isostructural solid solution [NH(CH3)(3)][(CB8H9)(0.26)(CB9H10)(0.66)(CB11H12)(0.08)], along with a high-temperature decomposition intermediate of the latter. The alpha-polymorph is an ordered structure, with increasing amounts of disorder for the beta- and gamma-polymorphs, while the high temperature delta- and epsilon-polymorphs at T > 476 K are fully disordered on both the cation and anion site. These new compounds may be used as precursors for new types of solid-state ionic conductors.