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 and 340 K upon cooling) in comparison to the lighter LiCB₁₁H₁₂ and NaCB₁₁H₁₂ analogues. The K⁺ cations in both the low-T ordered monoclinic (P2₁/c) and high-T disordered cubic (Fm3̅m) structures occupy octahedral interstices formed by CB₁₁H₁₂^– 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 30-fold across the transition, yielding a high-T phase value of 3.2 × 10^–4 S cm^–1 at 361 K. However, this is still about 1 to 2 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.