Structural and Dynamical Properties of Potassium Dodecahydro-monocarba-closo-dodecaborate: KCB₁₁H₁₂

MCB₁₁H₁₂ (M: Li, Na) dodecahydro-monocarba-closo-dodecaborate salt compounds are known to have stellar superionic Li⁺ and Na⁺ conductivities in their high-temperature disordered phases, making them potentially appealing electrolytes in all-solid-state batteries. Nonetheless, it is of keen interest to search for other related materials with similar conductivities while at the same time exhibiting even lower (more device-relevant) disordering temperatures, a key challenge for this class of materials. With this in mind, the unknown structural and dynamical properties of the heavier KCB₁₁H₁₂ congener were investigated in detail by x-ray powder diffraction, differential scanning calorimetry, neutron vibrational spectroscopy, nuclear magnetic resonance, quasielastic neutron scattering, and AC impedance measurements. This salt indeed undergoes an entropy-driven, reversible, order-disorder transformation and with a lower onset temperature (348 K upon heating) in comparison to the lighter LiCB₁₁H₁₂ and NaCB₁₁H₁₂ analogues. The K⁺ cations in both the low- $T$ ordered monoclinic ( $P{2}_1/c$ ) and high- $T$ disordered cubic (Fm-3m) structures occupy octahedral interstices formed by the ${\text{CB}}_{11}{\text{H}}_{{12}^{-}}$ anions. In the low- $T$ structure, the anions orient themselves so as to avoid close proximity between their highly electropositive C-H vertices and the neighboring K⁺ cations. In the high- $T$ structure, the anions are orientationally disordered, although to best avoid the K⁺ cations, the anions likely orient themselves so that their C-H axes are aligned in one of eight possible directions along the body diagonals of the cubic unit cell. Across the transition, anion reorientational jump rates change from 6.2×10⁶ s^-1 in the low- $T$ phase (332 K) to 2.6×10¹⁰ s^-1 in the high- $T$ phase (341 K). In tandem, K⁺ conductivity increases by about thirty-fold across the transition, yielding a high- $T$ phase value of 3.2×10^-4 S cm^-1 at 361 K. Yet, this is still about one to two orders of magnitude lower than that observed for LiCB₁₁H₁₂ and NaCB₁₁H₁₂, suggesting that the relatively larger K⁺ cation is much more sterically hindered than Li⁺ and Na⁺ from diffusing through the anion lattice via the network of smaller interstitial sites.