H diffusion in Mg- and Be- doped ⍺Al2O3 (corundum) single crystals

The diffusivity of hydrogen, as protons, in Mg- and Be-doped corundum has been determined from 544 to 1007 °C, by annealing single crystals in CO₂-H₂ or N₂-H₂ mixes at ambient pressure. The addition of hydrogen leads to decolorization of the crystals, which is attributed to the associated removal of electron holes. Spatially resolved semi-quantitative profiles of hydrogen concentration versus distance were recorded using Fourier transform infrared spectroscopy, and/or Cr luminescence lifetime spectroscopy. These show hydrogen diffusion associated with trapping by Mg or Be, which leads to characteristic step shaped (broadly sigmoidal) forms of concentration-distance profiles. Numerical modelling of this diffusion-plus-trapping process allows hydrogen diffusion coefficients to be extracted, which are several orders of magnitude higher than any diffusion coefficients that have been previously determined in this system. This discrepancy is attributed to previous studies not taking trapping behaviour into account. Re-analysis of some published data, now considering trapping, can explain a ∼ 4-5 orders of magnitude discrepancy in calculated diffusion coefficients.