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

Abstract

MCB 11 H 12 (M: Li, Na) dodecahydro-monocarba-closo-dodecaborate salt compounds are known to have stellar superionic Li + and Na + conductivities in their hightemperature disordered phases, making them potentially appealing electrolytes in all-solidstate 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 KCB 11 H 12 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 LiCB 11 H 12 and NaCB 11 H 12 analogues. The K + cations in both the low-T ordered monoclinic (P2 1 /c) and high-T disordered cubic (Fm3̅ m) structures occupy octahedral interstices formed by CB 11 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 × 10 6 s −1 in the low-T phase (332 K) to 2.6 × 10 10 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 LiCB 11 H 12 and NaCB 11 H 12 , 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.

Interplay between the Reorientational Dynamics of the B3H8– Anion and the Structure in KB3H8

“… 10 The simultaneous increase of the conductivity and the decrease of the relaxation time suggest that the anion reorientations play a vital role in promoting the cation transport by flattening the potential energy landscape as reported for M 2 B x H x and MCB x –1 H x (M = Li and Na, x = 10 and 12) 31 , 32 and more recently for KCB 11 H 12 . 29 With respect to this latter material, both KB 3 H 8 and KCB 11 H 12 possess identical cubic disordered structures, rapidly reorienting anions, and full K + cation occupation of the octahedral interstices. One key difference that one might expect to significantly impact the ultimate cation conductivity is the relative sizes of the anions, which directly affects the stable unit cell size.…”

“…Such a broad Lorentzian reflects a more localized, order-of-magnitude more rapid reorientational “rattling” of the [B 3 H 8 ] − anion around its equilibrium position. 29 , 30…”

“…One key difference that one might expect to significantly impact the ultimate cation conductivity is the relative sizes of the anions, which directly affects the stable unit cell size. From simple steric arguments, assuming spherical anions with the cubic-close-packed lattice constants for KB 3 H 8 and KCB 11 H 12 (≈ 8.017 Å 10 and ≈10.150 Å, 29 respectively), one can show that the octahedral and tetrahedral interstices tend to be larger for KCB 11 H 12 than for KB 3 H 8 , which in effect affords more room for cation translations and is consistent with the roughly 4-orders-of-magnitude higher conductivity observed at 373 K for disordered KCB 11 H 12 than for KB 3 H 8 . Yet, we do acknowledge that other factors such as the larger deviation from a spherical shape of the B 3 H 8 – anion and the differences in anion reorientational mobilities and mechanisms may also be affecting the conductivities in unknown ways.…”

Comparative Molecular Dynamics Study of the Roles of Anion–Cation and Cation–Cation Correlation in Cation Diffusion in Li2B12H12 and LiCB11H12

“…For the H-density isosurface, each high density was connected by another four neighboring high densities, indicating a four-fold symmetric distribution, which has also been recently reported in a FPMD study. 42 We also examined the B and C isosurface distributions, which were nearly the same because B and C are connected by strong B-C covalent bonds. There was a slight difference because of the different valences, bond lengths, and bond strengths.…”

“…For instance, a larger cation size can reduce the transition temperature. 42,43 Recently, remarkable reduction in the transition temperature has been reported in C-substituted closo-boranes. 44 In particular, alloying of two different C-substituted closo-boranes could decrease the transition temperature to close to room temperature.…”