The c-axis compressive strength of pure and Ti-doped sapphire was measured at 600C in air. Monocrystals were doped with both Ti3+ and Ti4+ ions from mixtures which initially contained 0.05 to 0.25wt%Ti2O3 in Al2O3. It was shown that both the valence state and the concentration of the Ti were important in slowing down twin boundary movement under c-axis compression.

The Effect of Titanium Doping on the High-Temperature Rhombohedral Twinning of Sapphire. E.Savrun, W.D.Scott, D.C.Harris: Journal of Materials Science, 2001, 36[9], 2295-301