The structure and dynamics of superionic α-CuI were studied in detail by means of

ab initio Born–Oppenheimer molecular dynamics simulations. The extreme cation

disorder and a soft immobile face centred cubic sub-lattice were evident from the

highly diffuse atomic density profiles. The Cu–Cu pair distribution function and

distribution of Cu–I–Cu bond angles possess distinct peaks at 2.6Å and 60°

respectively, which were markedly lower than the values expected from the

average cationic density, pointing to the presence of pronounced short-range

copper–copper correlations. Comparison with lattice static calculations showed that

these correlations and the marked shift in the cationic density profile in the <111>

directions were associated with a locally distorted cation sub-lattice, and that the

movements within the tetrahedral cavities involve rapid jumps into and out of

shallow basins on the system potential energy surface. On average, the iodine

atoms were surrounded by three copper atoms within their first coordination shell,

with the fourth copper atom being located in a transition zone between two

neighbouring iodine cavities. However, time-resolved analysis revealed that the

local structure actually involves a mixture of threefold-, fourfold-and fivefoldcoordinated

iodine. Examination of the ionic trajectories showed that the copper

ions jump rapidly to nearest neighbouring tetrahedral cavities (aligned in the <100>

directions) following a markedly curved trajectory and often involving short-lived

(~1ps) interstitial positions. The nature of the correlated diffusion underlying the

unusually high fraction of coppers with short residence time can be attributed to the

presence of a large number of 'unsuccessful' jumps and the likelihood of

cooperative motion of pairs of coppers. The calculated diffusion coefficient at

750K, DCu = 2.8 x 10−5cm2/s, was in excellent agreement with that found

experimentally.

Diffusion within α-CuI Studied Using Ab Initio Molecular Dynamics Simulations.

C.E.Mohn, S.Stølen, S.Hull: Journal of Physics - Condensed Matter, 2009, 21[33],

335403