Domain structures and Pr_co antisite point defects in double-perovskite PrBaCo₂O_5+δ and PrBa_0.8Ca_0.2Co₂O_5+δ

Owing to the excellent mixed-ionic and electronic conductivity and fast oxygen kinetics at reduced temperature (<800 °C), double-perovskite oxides such as PrBaCo₂O_5+δ exhibit excellent properties as an oxygen electrode for solid oxide fuel cells (SOFCs). Using transmission electron microscopy (TEM), we revealed high-density antiphase domain boundaries (APBs) and 90° domain walls in PrBaCo₂O_5+δ grains. Besides the regular lamellar 90° domain walls in {021} planes, irregular fine 90° domains are attached to the curved APBs. Electron energy-loss spectroscopy (EELS) reveals the composition variation across some of the 90° domain walls. There are fewer Co and more Ba ions approaching the 90° domain walls, while the changes in Pr and O ions are not detectable. We assume that the extra Ba²⁺ cations replace the Pr³⁺ cations, while the Pr³⁺ cations go to the Co site to form Pr_Co antisite point defects and become Pr⁴⁺. In this case, the Pr⁴⁺ cations will help to balance the local charges and have compatible ionic radius with that of Co³⁺. The local strain field around the 90° domain walls play a crucial role in the stabilization of such Pr_Co antisite point defects. The antisite point defects have been observed in our high-resolution TEM images and aberration-corrected high-angle annular dark-field (HAADF) scanning TEM images. After Ca²⁺ doped into PrBaCo₂O_5+δ to improve the structure stability, we observed tweed structures in the PrBa_0.8Ca_0.2Co₂O_5+δ grain. The tweed structure is composed of high-density intersected needle-shaped 90° domain walls, which is linked to a strong local strain field and composition variation. Even when the temperature is increased to 750 °C, the domain structures are still stable as revealed by our in situ TEM investigation. Therefore, the influence of the domain structures and the Pr_Co antisite defects on the ionic and electric conductivities must be considered.