Non-Arrhenius Ionic Conductivity Transitions in Sodium Antiperovskite Ionic Conductors

Abstract

Anti-perovskites materials have recently attracted great interest as a family of solid state electrolytes with high ionic conduction.[1] Typical solid-state ionic conductors have temperature-dependent ionic conductivity following Arrhenius behavior over a substantial range of temperature, corresponding to a constant activation energy Ea. In the Na3OCl antiperovskite (Fig. 1), there are regimes of Arrhenius behavior separated by broad transitions, all while the structure remains cubic, Figure 2. Such non-Arrhenius ionic conductivity of Na3OCl has been observed in previous literature. [2][3] However, there is no clear explanation for this non-Arrhenius transition. Differential scanning calorimetry shows thermal transitions in the vicinity of these conductivity transitions, Figure 3, suggesting order/disorder phase transitions. Reitveld refinement on temperature dependent synchrotron XRD of Na3OCl suggests changes in octahedral tilt systems, which is CmCm in the low temperature region, Pnma in the medium temperature region, and P4/mbm in the high temperature geion, Figure 4. In this work, we use multiple temperature-resolved characterization methods including synchrotron XRD, transport measurements, and calorimetry, and modeling by AIMD and DFT, to understand this behavior and to gain insight into tuning antiperovskites for high ionic conductivity.This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Figure 1