Influence of Br^-/S^2- site-exchange on Li diffusion mechanism in Li₆PS₅Br: a computational study

We investigate how low degrees of [Formula: see text] site-exchange influence the [Formula: see text] diffusion in the argyrodite-type solid electrolyte [Formula: see text] by ab initio molecular dynamics simulations. Based on the atomic trajectories of the defect-free material, a new mechanism for the internal [Formula: see text] reorganization within the [Formula: see text] cages around the [Formula: see text] sites is identified. This reorganization mechanism is highly concerted and cannot be described by just one rotation axis. Simulations with [Formula: see text] defects reveal that [Formula: see text] interstitials ([Formula: see text]) are the dominant mobile charge carriers and originate from Frenkel pairs. These are formed because [Formula: see text] defects on the [Formula: see text] sites donate one or even two [Formula: see text] to the neighbouring cages. The [Formula: see text] then carry out intercage jumps via interstitial and interstitialcy mechanisms. With that, one single [Formula: see text] defect enables [Formula: see text] diffusion over an extended spatial area explaining why low degrees of site-exchange are sufficient to trigger superionic conduction. The vacant sites of the Frenkel pairs, namely [Formula: see text], are mostly immobile and bound to the [Formula: see text] defect. Because [Formula: see text] defects on [Formula: see text] sites act as sinks for [Formula: see text] they seem to be beneficial only for the local [Formula: see text] transport. In their vicinity T4 tetrahedral sites start to get occupied. Because the [Formula: see text] transport was found to be rather confined if [Formula: see text] and [Formula: see text] defects are direct neighbours, their relative arrangement seems to be crucial for effective long-range transport. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.