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The perovskite-type oxynitride BaTaO2N is a promising lead-free relaxor ferroelectric material. However, this material releases a part of its nitrogen during its sintering above 1350 degrees C. Subsequent annealing of the resulting sintered BaTaO2N0.85 ceramics under an ammonia flow is necessary to recover an insulating, stoichiometric BaTaO2N ceramics. Very recently, molten BaCN2 was reported to be a useful flux for the preparation of BaTaO2N microcrystals without the loss of nitrogen. In the present work, BaTaO2N powder was sintered with a BaCN2 additive under mechanical pressure, using a spark plasma sintering system. Nitrogen loss from the BaTaO2N ceramic was avoided during liquid phase sintering at approximately 900 degrees C. The stoichiometric BaTaO2N ceramic product with a relative density of 79.8 % was an electrically insulator and it showed its relative dielectric constants, epsilon(r) in the range from 320 to 650 and a loss tans values from 0.04 to 0.19 at room temperature.

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 mu m 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.