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

Abstract

Self-diffusion coefficients for 18O in single-crystal MgO have been determined from a novel specimen comprising an epitaxial layer of high-purity Mg 18O 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/( m 2 s −1)= 1.8 −1.1 +2.9 ×10 −10 exp[−(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.