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

Abstract

Total structure factors (Bragg and diffuse scattering) for CuI in and -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 ions. The RMC results in the and -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 directions, directly between tetrahedrally coordinated sites. The octahedral site is not occupied, though there is a very small level of diffusion through this site, along directions, at the highest temperatures. There are small differences between RMC, MD and RR in the -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 in the structure factor for the and -phases and the existence of a short-distance peak in in the RMC models.