The diffusion of doubly charged nickel vacancies in a Σ5(310)[001] tilt grain boundary was studied (table 21). The simulations were carried out in the NVT ensemble (constant number of particles, volume, and temperature) and the interatomic interactions were described by a rigid ion potential. Analysis of the atomic trajectories showed that the diffusion of vacancies occurred by jumps between first-neighbor sites. The migration path involved only a restricted number of sites in the grain boundary and the residence times of the vacancy on the most frequently visited sites were computed. There was no direct relationship between the frequencies of visits and the vacancy formation energies calculated for these sites. The vacancy jump frequencies were calculated and the diffusion coefficient of this defect at high temperature was deduced. The vacancy diffusion was slightly faster along the direction parallel to the grain boundary tilt axis than in the perpendicular direction. The value of this anisotropy was consistent with experimental results obtained for other oxides. The bulk diffusion of the nickel vacancy was investigated with simulations performed in a perfect crystal. A qualitative analysis of the atomic trajectories showed that nickel diffusion was enhanced at the boundary.

Grain-Boundary Diffusion of Cation Vacancies in Nickel Oxide: a Molecular-Dynamics Study. Karakasidis, T., Meyer, M.: Physical Review B, 1997, 55[20], 13853-64

Table 21

Nickel vacancy diffusion coefficients

parallel and perpendicular to the tilt axis in NiO