Grain-boundary diffusion of cation vacancies in nickel oxide: A molecular-dynamics study

The diffusion of doubly charged nickel vacancies in a Sigma 5(310)[001] tilt grain boundary (GB) of NiO has been studied by molecular dynamics. The simulations are carried out in the NVT ensemble (constant number of particles, volume, and temperature) and the interatomic interactions are described by a rigid ion potential. The analysis of the atomic trajectories shows that the diffusion of the vacancies occurs by jumps between first-neighbor sites; The migration path only involves a restricted number of sites in the GB and the residence times of the vacancy on the most frequently visited sites have been computed. There is no direct relation between the frequencies of visits and the vacancy formation energies calculated for these sites. We have calculated the vacancy jump frequencies and deduced the diffusion coefficient of this defect at high temperature. The vacancy diffusion is slightly faster along the direction parallel to the GB tilt axis than in the perpendicular direction. The value of this anisotropy is consistent with the experimental results obtained for other oxides. The bulk diffusion of the nickel vacancy has been investigated with simulations performed in a perfect crystal. A qualitative analysis of the atomic trajectories shows that the nickel diffusion is enhanced at the boundary. The GB diffusion enhancement has been quantitatively estimated and compared to the experimental results.