Chemical inhomogeneity in the near-surface region of KTaO3 evolving at elevated temperatures

Abstract

Measurements by x-ray diffraction, secondary ion mass spectrometry and atomic force microscopy are reported for single crystalline KTaO3 after thermal treatment under oxidizing conditions in the temperature range of 700 to 1100 degrees C. The formation of a series of new phases KO*(KTaO3)(n) of Ruddlesden-Popper type is observed. Atomic force microscopy of (100) surfaces reveals corresponding dramatic changes in the surface morphology. Secondary ion mass spectrometry gives evidence of a redistribution of material leading to an enrichment of K with respect to Ta close to the surface and a corresponding depletion in deeper parts of the nearsurface region. The results are in strong contrast to standard point defect models. Instead, our experimental observations have to be interpreted in terms of an evolving chemical inhomogeneity due to segregation processes in the near-surface region of the crystals and intergrowth of KO, which results in the formation of the Ruddlesden-Popper phases. The additional effect of K2O evaporation above 900 degrees C is discussed and the corresponding topographical changes induced are documented.