From ionic to superionic conductivity: The influence of cation order on sodium diffusion in Na3Zr2Si2PO12

For many solid electrolytes, the introduction of structural disorder is beneficial for the ionic conductivity, since it breaks the localisation of the mobile species. Yet, in the case of the Na+ superionic conductor (NASICON) structure, we show that an ordered configuration of the cations can both diminish or escalate the ionic conductivity. Here, the key factor is the arrangement of the P5+ ions with respect to the sodium diffusion channels. Engineering this factor, NASICON can be tuned from a poor, two-dimensional conductor to a superionic, three-dimensional conductor. Since experimentally only general Si/P lattice positions can be determined, the distribution of P5+ in current samples is not known. Based on our calculations of conductivities and activation energies, we conclude that experimental samples possess no long-range cation order yet. Consequentially, we propose that NASICON type materials have not been utilised to their fullest extend. By tailoring their cation order, an increase in ionic conductivity of nearly a factor of four can be enabled. Supporting this statement, we present a systematic study on the influence of cation order on the tracer diffusion coefficient D-tr, the sodium diffusion pathways, the charge diffusion coefficient D-s, the ionic conductivity sigma, the Haven ratio H-R and the activation energy EA obtained by Molecular Dynamics simulations.