Effect of chemical composition on mass transfer in Y<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> under oxygen potential gradient at high temperatures

The oxygen shielding properties and structural stability of polycrystalline Y2Ti2O7 (YT) solid solution wafers, which were cut from a sintered body and served as models for environmental barrier coatings, were evaluated by the oxygen permeation technique at high temperatures. Here, we examined the effect of compositions by using two samples with slightly different Y/Ti molar ratios. The oxygen permeability constants for YT increased dramatically with a slight increment in the Y/Ti molar ratio, and the activation energy for oxygen permeation markedly decreased. Oxygen permeation in the YT was controlled by the diffusion of oxide ions in the crystal. Almost no decomposition of the YT phase occurred due to migration of the constituent cations at high temperatures, indicating high structural stability under an oxygen potential gradient. The crystal structure was identified as the YT-based pyrochlore structure from Rietveld refinements of synchrotron radiation X-ray powder diffraction data, based on information on the composition dependence of the defect concentrations in the crystals estimated by first principles calculations. The diffusion path for oxide ions in the crystal at high temperature was also investigated based on a spatial distribution map described by the bond valence sum method. Finally, the mechanisms for the uptake of oxygen molecules on the YT surface exposed to a high oxygen partial pressure and the subsequent diffusion of oxide ions in the YT crystals were clarified.