Superionic α-CuI was studied in detail by means of ab initio Born-Oppenheimer simulations. Extreme cation disorder and a soft immobile face-centred cubic sub-lattice were revealed by highly diffuse atomic density profiles. The Cu-Cu pair distribution function and the distribution of Cu-I-Cu bond-angles exhibited distinct peaks at 2.6 and 60° respectively. These were markedly lower than the values expected from the average cationic density; indicating the presence of pronounced short-range copper-copper correlations. A comparison with lattice static calculations showed that these correlations and a marked shift in the cationic density profile in the <111> directions were associated with a locally distorted cation sub-lattice, and that movements within the tetrahedral cavities involved rapid jumps in and out of shallow basins on the system potential energy surface. On average, the iodine ions were surrounded by three copper ions within their first coordination shell; with the copper being located in a transition zone between two neighbouring iodine cavities. Time-resolved analysis revealed that the local structure involved a mixture of threefold-, fourfold- and fivefold-coordinated iodine ions. Examination of the ion trajectories showed that the copper ions jumped rapidly to nearest-neighbour 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 copper ions with short residence time was attributed to the presence of a large number of so-called unsuccessful jumps and the likelihood of cooperative motion of pairs of copper ions. The calculated diffusion coefficient at 750K, of 2.8 x 10-5cm2/s, was in excellent agreement with experiment.

Diffusion within α-CuI Studied using ab initio Molecular Dynamics Simulations. Mohn, C.E., Stølen, S., Hull, S.: Journal of Physics - Condensed Matter, 2009, 21[33], 335403