Effect of crystal orientation on the segregation of aliovalent dopants at the surface of La₀.₆Sr₀.₄CoO₃

Abstract

The effect of crystal orientation on aliovalent dopant segregation at the surface of La₀.₆Sr₀.₄CoO₃ as a model perovskite oxide was investigated. La₀.₆Sr₀.₄CoO₃ pellets were produced and then annealed in air at 800 °C for 2–5 hours to drive the cation segregation at the surface. To quantify the chemical state of the pellets' surface, including cation composition and secondary phase precipitates rich in Sr, the pellets were characterized by X-ray photoelectron spectroscopy and Auger electron spectroscopy. To systematically assess the correlation between crystal orientation and dopant cation segregation, selected regions at the surface of the annealed pellets were analyzed by scanning electron microscopy and electron backscatter diffraction. Investigation of more than 300 grains revealed that the area coverage of the Sr-rich precipitates on grains having a {001} orientation was more than 3 times higher compared to that on the {101} and {111} orientations. On the other hand, the number density of precipitates was very similar on each grain orientation. This grain orientation dependent behavior indicates that the exposed facet plays an important role in dopant cation segregation, especially in the growth of secondary phases, likely by altering the surface energy and charged defect concentration. The present study provides insights into the importance of atomic arrangements in determining the surface stability and cation segregation tendency on perovskite oxides.