Ferroelectric BaTaON Crystals Grown in a BaCN Flux.

Perovskite-type oxynitride BaTaON 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 BaTaON crystals in the present study. Reddish crystallites of BaTaON of up to 3.1 μm in size were grown using a BaCN flux. Grain growth proceeded through the formation of a Ruddlesden-Popper-type oxynitride from the reaction between BaTaON powder and molten BaCN. 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 -type TaON octahedra.