![]() In the third regime, at temperatures above 70 ☌, the thermal energy increases above the background energy fluctuations and the activation energy decreases to 0.34 eV, reflecting the local energy barrier for noncorrelated ion diffusion. This mechanism of so-called correlated ion diffusion originates from the coupling of the cation and anion motion due to short-range ion–ion interactions combined with background energy fluctuations, which we can associate through quasi-elastic neutron scattering experiment to fast librations of the anions. The second regime of faster diffusion above −50 ☌ is characterized by an apparent activation energy of 0.6 eV, higher than expected from the local microscopic barrier of 0.35 eV observed by, e.g., 23Na nuclear magnetic resonance spin-lattice relaxation. In the first regime, at temperatures below −50 ☌, conductivity remains low before a glasslike transition identified by X-ray diffraction and calorimetry causes a faster increase of sodium conductivity through site disordering. We find from electrochemical impedance spectroscopy that the temperature-dependent conductivity is characterized by three distinct regimes of conductivity. The conduction mechanism of Na 2(B 12H 12) 0.5(B 10H 10) 0.5, a particularly promising solid-state electrolyte for sodium-ion batteries, is elucidated. ![]()
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