Secondary-ion mass spectrometric analysis of anisotropic oxygen self-diffusion in barium hexaaluminate single crystals

Abstract

Oxygen self-diffusion in a barium hexaaluminate (Ba_0.75Al₁₁O_17.25) single crystal has been analysed by secondary-ion mass spectrometric analysis (SIMS) after diffusion annealing the sample in ¹⁸O₂. Depth profiles of the isotopic concentration in the crystal showed that oxygen penetration normal to the c axis (⊥c) was higher than that along the c axis (//c). Diffusivities of oxygen in the ⊥c direction calculated from the depth profile were an order of magnitude larger than those in the //c direction. The diffusion coefficient in the //c direction was close to that in MgAl₂O₄ spinel oxide. These results indicate that the pseudo-layer structure of barium hexaaluminate influences the diffusion of oxide ions, i.e. loosely packed intermediate monolayers between closely packed spinel blocks, which spread in parallel to the (001) plane, are likely to be the preferential diffusion route of oxide ions.