Oxygen self-diffusion in single-crystal MgO: Secondary-ion mass spectrometric analysis with comparison of results from gas-solid and solid-solid exchange

Self-diffusion coefficients for ¹⁸O in single-crystal MgO have been determined from a novel specimen comprising an epitaxial layer of high-purity Mg¹⁸O upon a single crystal substrate of normal MgO. Heating the specimen in air produced a gas-solid exchange gradient at the sample surface as ¹⁸O in the epitaxial layer exchanged with ¹⁶O in air. A solid-solid interdiffusion gradient was produced between the substrate crystal and the ¹⁸O-enriched epitaxial layer. SIMS analysis of gas-solid exchange gradients prepared in the temperature range 1000-1650 °C provided diffusion coefficients that could be described as D/(m²s^-1) = (1.8_-1.1^+2.9) × 10^-10exp[-(3.24±0.13)eV/kT]. Interdiffusion gradients produced by annealing at 1100 and 1200 °C yielded the self-diffusion coefficients that were comparable to those obtained from gas-solid exchange, indicating that the surface exchange reaction is fast enough. The results are interpreted in terms of a defect model in which oxygen diffusion occurs by an interstitial type of defect as a result of suppression of anion vacancy concentration by large concentrations of extrinsic cation vacancies.