Atomic scale characterization of point and extended defects in niobate thin films

Highlights: • Point and extended defects in alkaline and alkaline-earth niobate thin films were characterized in atomic resolution via STEM-EELS. • Tetragonal tungsten bronze phase was observed to segregate out of perovskite matrix in CaZrO₃ modified (K, Na)NbO₃ thin films. • A sharp [103] boundary consisting of aligned line defects forms the transition region between the perovskite and the tetragonal tungsten bronze phase. • Sr deficient SrNb₂O₆ phase was segregated out from SrNbO₃ matrix with a coherent boundary. -- Abstract: Niobium-based oxides have a wide range of applications owing to their rich crystal and electronic structures. Defects at the atomic scale are always unavoidable and will affect their functionalities, especially when in the form of thin films. Here, atomic resolution scanning transmission electron microscopy and electron energy loss spectroscopy have been performed on various defects (point, line, planar defects and segregated phases) in alkaline and alkaline-earth niobate thin films: CaZrO₃ modified (K, Na)NbO₃ and strontium niobate (SNO), respectively. In CaZrO₃ modified (K,Na)NbO₃ thin films, a tetragonal tungsten bronze phase was found, with a sharp boundary with the perovskite phase. In SNO thin films, several kinds of point defects and antiphase boundaries are commonly observed. In addition, a strongly Sr deficient phase, SrNb₂O₆, precipitates inside the SrNbO₃ phase with a coherent interface. The different oxidation states of Nb in SrNbO₃ and SrNb₂O₆ were revealed from the O K edge. Our characterization of the point defects and extended defects in niobate thin films offers practical guidelines for thin film deposition or discovery of defect-based novel functionalities.