Role of vacancy anharmonicity on non-Arrhenius diffusional behavior

Curvature in the Arrhenius plot of self-diffusion, usually attributed to the coexistence of two or more separate mobile defects, is shown to be explainable in terms of a single highly relaxed vacancylike defect, in which the anharmonicity of the lattice modes gives rise to a large thermal-expansion coefficient for the defect. A model based on this approach gives excellent agreement for all cases in which curvature has been observed, as well as for others in which no curvature has been detected. (U.S.)