A molecular dynamics study of ionic conduction in CuI .2. Local ionic motion and conduction mechanisms

The ionic motions in molecular dynamics simulations of the gamma-, beta- and alpha-phases of CuI have been studied in detail. In the cubic gamma-phase the Cu+ ions have large anharmonic vibrations along [111] type directions, towards the face centres of the tetrahedral cages defined by the neighbouring four I- ions. However the diffusion pathway is along [100] type directions, towards the edges of these cages. The diffusion mechanism involves a correlated motion of 'chains' of several Cu+; this explains the experimental observation of a break-down of the jump diffusion model. As temperature increases the number of diffusing 'chains' also increases; interactions between these chains then leads to a rapid increase in the diffusion rate and a transition to the fast ion conducting alpha-phase. In the hexagonal beta-phase Cu+ have similar behaviour; they vibrate towards the cage faces but diffuse in the direction of the cage edges. However the situation is more complicated because in this structure two cages share a common face. This produces a complex behaviour where the average sites for Cu+ are in the cage centres over shorter times, but in the (shared) face centres over longer times.