Figure 3

Molecular dynamics simulation of diffusion in Na_0.8CoO₂ in the stripe phase. The left-hand panel shows the trace of sodium positions over the duration of the simulations (the boxes show the cells used in the MD simulations), the middle panel shows the root-mean-square displacement of individual sodium ions from their initial sites as a function of time, and the right-hand panel shows displacements along the stripe direction. The different colours give trajectories for different ions, with the same colour for a given ion in successive panels from left to right. The simulations were performed at T ~ 350 K for the tri-vacancy stripe phase (a) fully ordered and (b) with an additional vacancy on a 2b site. For the ideal superstructure there are local hops back and forth between 2d and 2b sites perpendicular to the stripe, but there is no bulk self-diffusion. The introduction of additional disorder in the stripes results in multiple hops between 2d and 2b sites with components along the stripes. Chains of hops of different ions results in dynamic irreversibility and self-diffusion along the stripe.