Dimer-mediated cation diffusion in the stoichiometric ionic conductor Li3N

Abstract

Non-equilibrium molecular dynamics has been used to model cation diffusion in stoichiometric Li₃N over the temperature range 50 < T/K < 800. The resulting diffusion coefficients are in excellent agreement with the available experimental data. We present a detailed atomistic account of the diffusion process. Contrary to the conclusions drawn from previous studies, our calculations show that it is unnecessary to invoke the presence of a small concentration of intrinsic defects in order to initiate diffusion. The structure can be considered to consist of alternating layers of composition Li₂N and Li. As the temperature increases an increasing number of cations leave the Li₂N layers and migrate either to the interlayer space or to the Li layer. Those that move into the interlayer space form Li₂ dimers with cations in the Li₂N layers and those that move into the neighboring layer form dimers with cations therein. The two types of dimer are aligned parallel and perpendicular to [001], respectively and have lifetimes of ∼3 ps. The vacancies so created facilitate rapid diffusion in the Li₂N layers and the interlayer cation motion results in slower diffusion perpendicular to the layers.