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 R 3 polymorph stabilized upon drying and transforming first to R 3 c symmetry near 313 K and then to a similarly packed but disordered I 43 d polymorph near 353 K and ( ii ) a disordered Fm 3 polymorph that initially appears from the disordered I 43 d polymorph near 513 K along with another disordered high-temperature P 6 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.