LaTiO 3.5 is a layered compound made up of four LaTiO 3 units:âtwo regular perovskite units stacked between two distorted units that share an additional oxygen layer. In a single-crystalline thin film the (100) plane can be aligned parallel to the substrate surface; i.e.; the additional oxygen layers are inserted parallel to the substrate surface. A detailed structural characterization reveals planar defects originating from substrate surface steps. These results; in combination with the in situ reflection high-energy electron-diffraction observations; suggest an oxidation mechanism using such antiphase boundaries as oxidation paths.