Ferroelectric BaTaO2N Crystals Grown in a BaCN2 Flux.

Perovskite-type oxynitride BaTaO2N has been attracting attention for its large dielectric constant, which is almost independent of the temperature by measurements on its ceramics. Its dielectric characteristics are attributed to polar nanoregions (PNRs) in the average cubic crystal structure. Polarization saturation to produce a butterfly-like piezoresponse force microscopy (PFM) signal was observed on BaTaO2N crystals in the present study. Reddish crystallites of BaTaO2N of up to 3.1 μm in size were grown using a BaCN2 flux. Grain growth proceeded through the formation of a Ruddlesden-Popper-type oxynitride from the reaction between BaTaO2N powder and molten BaCN2. Their electrical property was studied using PFM with special care because of the small size of the crystals. They were found to be much more highly insulating than its ceramics. Ferroelectricity with complete phase inversion was observed on an oxynitride perovskite crystal for the first time. A large coercivity of 50-60 V was observed in the measurement. Such ferroelectricity is ascribed to the PNRs induced by the polar linkages between cis-type TaO4N2 octahedra.