Thermal Polymorphism in CsCB(11)H(12)

Thermal polymorphism in the alkali-metal salts incorporating the icosohedral monocarba-hydridoborate anion, CB(11)H(12)(−), results in intriguing dynamical properties leading to superionic conductivity for the lightest alkali-metal analogues, LiCB(11)H(12) and NaCB(11)H(12). As such, these two have been the focus of most recent CB(11)H(12)(−) related studies, with less attention paid to the heavier alkali-metal salts, such as CsCB(11)H(12). Nonetheless, it is of fundamental importance to compare the nature of the structural arrangements and interactions across the entire alkali-metal series. Thermal polymorphism in CsCB(11)H(12) was investigated using a combination of techniques: X-ray powder diffraction; differential scanning calorimetry; Raman, infrared, and neutron spectroscopies; and ab initio calculations. The unexpected temperature-dependent structural behavior of anhydrous CsCB(11)H(12) can be potentially justified assuming the existence of two polymorphs with similar free energies at room temperature: (i) a previously reported, ordered R3 polymorph stabilized upon drying and transforming first to R3c symmetry near 313 K and then to a similarly packed but disordered I43d polymorph near 353 K and (ii) a disordered Fm3 polymorph that initially appears from the disordered I43d polymorph near 513 K along with another disordered high-temperature P6(3)mc polymorph. Quasielastic neutron scattering results indicate that the CB(11)H(12)(−) anions in the disordered phase at 560 K are undergoing isotropic rotational diffusion, with a jump correlation frequency [1.19(9) × 10(11) s(−1)] in line with those for the lighter-metal analogues.