Inter-diffusion in lanthanum tungsten oxide

Abstract

Cation diffusion in lanthanum tungsten oxide (LaWO) has been studied on La27W5O55.5–Nd27W5O55.5 and La27W5O55.5–La27Mo1.5W3.5O55 inter-diffusion couples at temperatures between 1150 and 1350 °C in air and 5% H2 in Ar. Inter-diffusion coefficients were derived by fitting concentration profiles of Nd, La, W, and Mo from Electron Probe Micro Analysis to the appropriate solution of Fick's second law. The diffusivity in LaWO follows Arrhenius type behaviour with almost identical bulk diffusion coefficients for La and W site transport, and similar activation energies; 410 ± 30 kJmol- 1 (La) and 450 ± 30 kJmol- 1 (W). This behaviour has been interpreted to reflect that migration occurs via a common migration mechanism for both species, facilitated by vacancies on the lanthanum sublattice. Enhanced grain boundary diffusion was only observed for La site diffusion in the Nd27W5O55.5 phase, with much lower activation energy (170 ± 50 kJmol- 1). The similar bulk diffusivities and relatively slow migration kinetics indicate that LaWO is more stable towards cation diffusion-related degradation than many of the most promising oxygen transport membrane materials.