Insights into low electronic and ultrahigh ionic conductivities of sodium tantalum oxychlorides NaTaO<i>x</i>Cl6−2<i>x</i> in crystalline and amorphous phases

Anion mixing and amorphization are recognized as an effective strategy to develop solid-state electrolytes with low electronic but high ionic conductivities. Here, we present atomistic insights into ultrahigh ionic conductivities with very low electronic conductivities of oxychlorides NaTaOxCl6−2x (0≤x≤1) using ab initio calculations and molecular dynamics simulations. We observe a flat feature in the conduction bands, which leads to low electronic conductivities on the order of 10−9 S/cm. Furthermore, our calculations reveal that oxygen mixing increases Na ionic conductivity to a higher degree than amorphization, which is attributed to the shrinkage of Ta-centered polyhedra by oxygen substitution.