The isotopic exchange of O, between a monocrystal of Mn-substituted hexa-aluminate and 18O-enriched gas, was monitored by means of secondary ion mass spectrometry. The observed 18O concentration profiles showed that the diffusion rate was higher along the [110] direction than along the [001] direction. It was deduced that the pseudo-layer structure of hexa-aluminate affected atomic diffusion. That is, the loosely packed intermediate monolayer between closely-packed spinel blocks, spread parallel to the (001) plane, was a preferred diffusion route for O2- ions.

M.Machida, T.Shiomitsu, K.Eguchi, H.Arai, Y.Shimizu: Journal of Solid State Chemistry, 1991, 95[1], 220-3