Comment on Self-diffusion in high-purity α-Al2O3: Comparison of Ti-doped, Mg-doped and undoped single crystals P. Fielitz, S. Ganschow, K. Klemens, and G. Borchardt, J. Eur. Ceram. Soc., 41, (2021), 663-668

We comment on recent observations of O and Al self-diffusion in single-crystal sapphire with variable doping concentrations of Mg and Ti by Fielitz et al [1]. The paper reports a null effect of aliovalent doping on oxygen diffusivity. We posit that the extensive heat treatment involved in their experimental protocol may have caused dopant evaporation near the surface, and therefore the null result in oxygen diffusivity, whereas an effect on Al diffusivity is still discernible due to a greater diffusion depth of Al. We propose that buffering mechanisms are ultimately responsible for modest increases of self-diffusion with respect to dopant concentrations; in the Mg-doped case, DFT calculations suggest that negatively charged Mg interstitial defects are the principal charge compensating defects for positively charged Mg substitutional ions.

Automated summary

This paper comments on recent observations of O and Al self-diffusion in single-crystal sapphire with variable doping concentrations of Mg and Ti. The study suggests that the null effect of aliovalent doping on oxygen diffusivity may be attributed to dopant evaporation near the surface due to extensive heat treatment, whereas an effect on Al diffusivity is still discernible. Furthermore, buffering mechanisms are proposed to be ultimately responsible for modest increases of self-diffusion with respect to dopant concentrations.