ISOTOPIC EXCHANGE AND DIFFUSION OF OXYGEN IN OXIDES WITH DIFFERENT BULK AND SUBSURFACE DIFFUSIVITIES

A theoretical model has been proposed to describe the kinetics of oxyg en isotopic exchange between gas and oxides with subsurface regions di fferent from the bulk in the rate of oxygen transfer (surface phases, grain boundaries, etc.). Exact solutions valid for an arbitrary ratio between the exchange and diffusion rates have been obtained for both t he time dependence of the total O-18 tracer concentration and the depe ndences of (OO)-O-16-O-16, (OO)-O-18-O-16, (OO)-O-18-O-18 molecules co ncentrations in a gas. The latter makes it possible to analyse the con tributions to the kinetics from various types of exchange taking diffu sion into account. The results obtained also allow the determination o f the exchange rate and diffusivity from the experiment with high enou gh accuracy, as shown by the example of the oxide YBa2Cu3O6+x.