TRAPPED HYDROGEN DIFFUSION IN MG-DOPED CORUNDUM

Hydrogen has been known to move fast in acceptor-doped corundums, which have been intensively studied by material scientists for applications such as proton conductors, hydrogen fugacity sensor, or hydrogen permeation barriers. Consequently, hydration of gem corundums (sapphires and rubies) can be accomplished in a very short period of time under relatively low temperatures, and may dramatically alter the color of the stones. However, the published diffusivities of hydrogen show huge discrepancies among each other, indicating complications in the diffusion process. A series of Mg-doped corundums are partially hydrated at 543-1007°C, and have been carefully characterized in this study in order to understand the diffusion mechanism of hydrogen in corundum.

The Mg-doped corundums show unaltered brownish cores surrounded by colorless rims after partial hydration, as the light absorbing electron holes are neutralized by hydrogen atoms. The diffusion profile of hydrogen, which is best characterized by the luminescence lifetime variation of Cr³⁺, show smooth sigmoidal shapes instead of simple error functions. This indicates that hydrogen atoms do not diffuse freely in the Mg-doped corundum but are partially trapped into some immobile configurations as they diffuse through the lattice. This trapping mechanism results in much lower apparent diffusivities of hydrogen, which explains the huge discrepancies among previously published results. Slightly anisotropic profiles at higher temperatures suggests the diffusivity of hydrogen is also more anisotropic than that of the electron holes.