Ionic Conductivity Transitions in Sodium Antiperovskite Ionic Conductors

Na₃OCl is a well-studied prototypical fast ion conducting compound representative of the antiperovskite (AP) family, which is of interest for solid-state electrolytes. Here, we observe a two-order-of-magnitude increase in its ionic conductivity (from 10^-1 to 10 mS/cm) with increasing temperature at 305°C. Temperature-dependent synchrotron X-ray diffraction (SXRD), differential scanning calorimetry (DSC), quasi-elastic neutron scattering (QENS), and impedance spectroscopy, are used to show that the increase in conductivity is correlated with a transition from tetragonal to cubic symmetry associated with a change in octahedral tilt disorder. While previous work has shown that octahedral tilting in APs can create ion migration pathways with lower migration energies than in the regular cubic structure, this is the first instance where such a large increase (100 times) in ionic conductivity due to a change in octahedral tilt disorder has been observed. The octahedral tilt disorder may be exploited as a pathway to increasing the ionic conductivity of antiperovskites and structurally related compounds.