Microstructure and secondary phases in epitaxial LaBaCo₂O_{5.5 + δ} thin films

Aberration-corrected scanning transmission electron microscopy was employed to investigate the microstructures and secondary phases in LaBaCo₂O_{5.5 + δ} (LBCO) thin films grown on SrTiO₃ (STO) substrates. The as-grown films showed an epitaxial growth on the substrates with atomically sharp interfaces and orientation relationships of [100]_LBCO//[100]_STO and (001)_LBCO//(001)_STO. Secondary phases were observed in the films, which strongly depended on the sample fabrication conditions. In the film prepared at a temperature of 900 °C, nano-scale CoO pillars nucleated on the substrate, and grew along the [001] direction of the film. In the film grown at a temperature of 1000 °C, isolated nano-scale Co₃O₄ particles appeared, which promoted the growth of {111} twinning structures in the film. The orientation relationships and the interfaces between the secondary phases and the films were illustrated, and the growth mechanism of the film was discussed.