Determination of proton- and oxide ion tracer diffusion in lanthanum tungstate (La/W = 5.6) by means of ToF-SIMS.

Tracer diffusion of protons and oxide ions, as well as chemical diffusion of water, have been determined for the high temperature proton conductor lanthanum tungstate by means of time-of-flight secondary ion mass spectrometry (ToF-SIMS). The oxygen tracer diffusion and surface exchange coefficients, D(O)* and k(O)*, were measured after exchange anneals in water vapor enriched in H(2)(18)O between 350 and 620 °C, and the apparent activation energies were 176 and 82 kJ mol(-1), respectively. The hydrogen tracer diffusion coefficient (D(H)*) was measured between 220 and 320 °C after exchange anneals in D(2)O-containing atmospheres, and the apparent activation energy was 63 kJ mol(-1). The extracted D(H)* agrees with the results of transient conductivity- and TG-measurements. Chemical diffusion and surface exchange coefficients, D(H)δ and k(H)δ, were measured at 250 and 400 °C, and the result confirms that the material is hydrated by ambipolar sluggish transport of protons and oxide ions. The surface exchange coefficients were compared to the result of TG relaxation, suggesting that access to oxygen vacancies limits the overall surface exchange reaction under incorporation of water and oxygen.