Oxygen surface exchange and diffusion studies of La2Mo2O9 in different exchange atmospheres

A combination of 18O tracer isotopic exchange and Secondary Ion Mass Spectrometry (SIMS) measurements was applied to obtain oxygen surface exchange and diffusion coefficients of La2Mo2O9. The isotopic exchange was conducted in dry (18O2) and wet (H218O) exchange atmospheres respectively with the advantages and disadvantages of each exchange discussed. A decrease in the oxygen diffusion coefficients was observed for the wet exchange method which was accompanied by an increase in the oxygen diffusion activation energy, indicating that the humidified and dry atmospheres affect the oxygen transport differently. Changes in the surface exchange coefficients at the phase transition temperature were observed suggesting a different rate-limiting step in the oxygen exchange and diffusion processes. The oxygen diffusivity was correlated to the ionic conductivity using the Nernst–Einstein relation from which the concentration of mobile oxygen ions was estimated to be 5 per unit cell. The result is consistent for both the calculated conductivity from diffusion coefficients and the measured values from the AC impedance spectroscopy.

Research highlights
► Decrease in oxygen diffusion coefficients observed for H218O exchange.
► Rate-limiting step in oxygen exchange and diffusion processes vary with phase.
► No fast grain boundary diffusion.
► Five mobile oxygen ions per unit cell calculated.