Structure and ionic conduction in CuI: diffuse neutron scattering and RMC modelling

Total structure factors (Bragg and diffuse scattering) for CuI in gamma-, beta- and alpha-phases have been measured by powder neutron diffraction. Reverse Monte Carlo (RMC) modelling of the data has been used to obtain detailed information on the distribution of mobile Cu+ ions. The RMC results in the gamma- and alpha-phases are in good agreement with molecular dynamics (MD) simulation and Rietveld refinement (RR) of high-resolution (Bragg scattering) powder neutron diffraction data; they confirm that the dominant diffusion pathway is in [100] directions, directly between tetrahedrally coordinated Cu+ sites. The octahedral site is not occupied, though there is a very small level of diffusion through this site, along [111] directions, at the highest temperatures. There are small differences between RMC, MD and RR in the beta-phase. In all phases the radial distribution functions obtained from experiment are significantly different from the MD results. It is shown clearly that there is a relationship between the experimental observation of a broad diffuse scattering peak at Q similar to 1 Angstrom(-1) in the structure factor for the beta- and alpha-phases and the existence of a short-distance peak in g(CuCu)(r) in the RMC models.