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”

•In Ref. [1], the extensive heat treatment of (un)doped a-Al2O3 crystals may have caused near-surface dopant evaporation.•This resulted in a zero measured effect on oxygen diffusivity.•An effect on Al diffusivity could still be observed because of the higher diffusion depth of Al in the Mg-doped crystal.•From calculations, Mgi2+ is found to be the principal charge compensating defect for MgAl1- in Mg-doped a-Al2O3.•This creates a self-buffering effect, explaining the small increase in self-diffusion with respect to dopant concentration.