Tl3[CO3][CB11H12]: An anti-Perovskite-Type Dodecahydro-closo-Carbadodecaborate Double Salt with an Interesting Phase Transition

The remarkable structural versatility and chemical stability of closo-(carba)borates makes them attractive building blocks for the design of new inorganic materials. In this study, the reaction of [(CH3CH2)3NH][CB11H12] and Tl2[CO3] yields single-phase Tl3[CO3][CB11H12] crystallizing in transparent, needle-shaped crystals, which exhibit a weak yellow luminescence under UV light. Single-crystal structure analysis reveals a hexagonal anti-perovskite-type structure, which is isotypic with other closo-borate double salts like Cs3[NO3][B12H12]. At room temperature, the title compound crystallizes in the orthorhombic system (space group: Pnma) with lattice parameters of a = 818.67(5) pm, b = 1030.38(6) pm and c = 1708.51(9) pm (Z = 4). The Raman spectrum is dominated by the vibrational bands of the quasi-icosahedral closo-carbaborate anions [CB11H12]−. Nevertheless, oxocarbonate vibrations can also be identified. Thermal analyses of Tl3[CO3][CB11H12] display a first-order phase transition with an onset temperature at 148 °C and its decomposition at 378 °C. In the crystal structure of the high-temperature phase, the anions are rotationally disordered, which leads to an increase in space group symmetry to P63/mmc. The study of the solid decomposition products showed that Tl3[CO3][CB11H12] decomposes to elemental thallium under nitrogen atmosphere, but after the decomposition under argon atmosphere thallium(I) metaborate is also detectable.