On the influence of the coherence length on the ionic conductivity in mechanochemically synthesized sodium-conducting halides, Na3−xIn1−xZrxCl6

Abstract

The Na⁺ ionic conductivity of ball milled Na_3−xIn_1−xZr_xCl₆ rises with increasing Zr content, while that in the subsequently annealed Na_3−xIn_1−xZr_xCl₆ compounds reaches a maximum conductivity at an intermediate substitution degree (x = 0.5). To clarify the underlying mechanism causing the differing trends, the local structure and coherence length of the ball milled Na_3−xIn_1−xZr_xCl₆ solid solutions were investigated by pair distribution function analyses. The structural evolution in the ball milled Na_3−xIn_1−xZr_xCl₆ series resembles those found in their annealed counterpart, however, its structural coherence length decreases with higher Zr content. By further investigating the transport properties using impedance spectroscopy, this work uncovers a correlation between the coherence length and the ionic conductivity in ball milled Na_3−xIn_1−xZr_xCl₆, in which lower structural coherence leads to higher ionic transport. This work indicates an influence of the microstructure beyond unit cell scale onto macroscopic transport properties in these sodium-conducting halide solid electrolytes.