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Ultramicroscopy. 2018 Oct;193:64-70. doi: 10.1016/j.ultramic.2018.06.008. Epub 2018 Jun 15.

Domain structures and Prco antisite point defects in double-perovskite PrBaCo2O5+δ and PrBa0.8Ca0.2Co2O5+δ.

Author information

1
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, United States. Electronic address: yong.ding@mse.gatech.edu.
2
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, United States.
3
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, United States. Electronic address: zhong.wang@mse.gatech.edu.

Abstract

Owing to the excellent mixed-ionic and electronic conductivity and fast oxygen kinetics at reduced temperature (<800 °C), double-perovskite oxides such as PrBaCo2O5+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 PrBaCo2O5+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 PrCo 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 PrCo 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 PrBaCo2O5+δ to improve the structure stability, we observed tweed structures in the PrBa0.8Ca0.2Co2O5+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 PrCo antisite defects on the ionic and electric conductivities must be considered.

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