Diffusion within α-CuI studied using ab initio molecular dynamics simulations

Abstract

The structure and dynamics of superionicα-CuI are studied in detail by means of ab initio Born–Oppenheimer molecular dynamicssimulations. The extreme cation disorder and a soft immobile face centred cubic sublatticeare evident from the highly diffuse atomic density profiles. The Cu–Cu pair distributionfunction and distribution of Cu–I–Cu bond angles possess distinct peaks at 2.6 Å and60° respectively, which are markedly lower than the values expected from theaverage cationic density, pointing to the presence of pronounced short-rangecopper–copper correlations. Comparison with lattice static calculations showsthat these correlations and the marked shift in the cationic density profile in the⟨111⟩ directions are associated with a locally distorted cation sublattice, and that the movementswithin the tetrahedral cavities involve rapid jumps into and out of shallow basins on thesystem potential energy surface. On average, the iodines are surrounded by three copperswithin their first coordination shell, with the fourth copper being located in a transitionzone between two neighbouring iodine cavities. However, time-resolved analysis revealsthat the local structure actually involves a mixture of threefold-, fourfold- andfivefold-coordinated iodines. Examination of the ionic trajectories shows that the copperions jump rapidly to nearest neighbouring tetrahedral cavities (aligned in the⟨100⟩ directions) following a markedly curved trajectory and often involving short-lived (∼1 ps) interstitial positions. The nature of the correlated diffusion underlying theunusually high fraction of coppers with short residence time can be attributedto the presence of a large number of ‘unsuccessful’ jumps and the likelihood ofcooperative motion of pairs of coppers. The calculated diffusion coefficient at 750 K,DCu = 2.8 × 10−5 cm2 s−1, is in excellent agreement with that found experimentally.