jp0c05038_si_005.zip (709.89 kB)
Structural and Dynamical Properties of Potassium Dodecahydro-monocarba-closo-dodecaborate: KCB11H12
dataset
posted on 2020-08-07, 14:35 authored by Mirjana Dimitrievska, Hui Wu, Vitalie Stavila, Olga A. Babanova, Roman V. Skoryunov, Alexei V. Soloninin, Wei Zhou, Benjamin A. Trump, Mikael S. Andersson, Alexander V. Skripov, Terrence J. UdovicMCB11H12 (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.