The rates of 65Zn2+ and Ni2+ diffusion were measured (figures 1 and 2) along the a- and c-axes of monocrystals at temperatures ranging from 758 to 1267C (Zn) and from 900 to 1550C (Ni). The samples were encapsulated in polycrystalline disks during annealing in order to inhibit losses of solute or host material by vaporization. It was found that cation self-diffusion was isotropic, within experimental error, and could be described by:

D(cm2/s) = 7.26 x 10-6exp[-1.80(eV)/kT]

The results for Ni diffusion revealed a small dependence upon the crystallographic direction, with equations of:

D(cm2/s) = 9.89 x 10-5exp[-2.06(eV)/kT]

and

D(cm2/s) = 3.16 x 10-5exp[-2.06(eV)/kT]

for the a- and c-axis directions, respectively. The diffusion of Zn and Ni in polycrystalline samples was strongly enhanced along grain boundaries. At temperatures of up to 1300C, the temperature dependence of D for Ni involved the same activation energy as that found for volume diffusion. The enhanced transport was attributed to the presence of higher defect concentrations near to the boundary. At temperatures above 1300C,  seemed to change with temperature due to incomplete equilibration of the sample. This interpretation was supported by an observed increase in D with the equilibration time in a reducing atmosphere. The increase in D with decreasing O partial pressure supported the suggestion that doubly-ionized interstitial Zn ions were the predominant point defect. Pipe diffusion in this material was also studied (figure 3)

Lattice Diffusion, Grain Boundary Diffusion and Defect Structure of ZnO. Wuensch, B.J., Tuller, H.L.: Journal of the Physics and Chemistry of Solids, 1994, 55[10], 975-84

Figure 1

Diffusivity of 65Zn2+ in ZnO

(closed circles: a-axis, open circles: c-axis)