Phase transitions in LiCoO ₂ thin films prepared by pulsed laser deposition

Microstructures of epitaxial LiCoO ₂ thin films formed on the (0 0 0 1) surface of sapphire (α-Al ₂O ₃) substrates by pulsed laser deposition at room temperature and annealed at 600°C in air were investigated by a combination of selected-area electron diffraction, high-resolution transmission electron microscopy, spherical-aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, and electron energy-loss spectroscopy. As-deposited LiCoO ₂ thin films consisted of epitaxial grains of the fully cation-disordered phase (γ) with a cubic rock-salt structure. During annealing, this cubic-structured phase transformed into the fully ordered trigonal (α) phase oriented with its basal plane parallel to the surface of the sapphire substrate. Although overall the film appeared to be a single crystal, a small number of Co ₃O ₄ grains were also observed in annealed thin films, indicating that some Li and O had been lost during processing. The atomically sharp interface between the film and substrate also became rougher during annealing, with step defects being formed, suggesting that a localized reaction occurred at the interface.