Average Cubic BaTaO₂N Crystal Structure Formed by 50 nm Size Domains with Polar Nanoregions Consisting of cis-TaO₄N₂ Octahedral Chains

A noncentrosymmetric polar structure in ferroelectric BaTaO2N having a cubic perovskite-type structure was investigated both in theoretical calculation and experimental observations. The most probable coordination around Ta was confirmed to be cis-TaO4N2 in exhaustive energy calculations about the O and N atomic distribution. The cis-TaO4N2 octahedra formed their local stackings during cooling of BaTaO2N melt at 1183 K. Their linkages precipitate as nanosize units forming domains in approximately 50 nm individually having spontaneous polarization as observed in integrated differential phase contrast-scanning transmission electron microscopy. They were observed in a noncentrosymmetric structure with polar space group Pmc21 detected in Raman spectroscopy and supported by the theoretical calculations to form nanopolar regions. The polar nanoregions were three-dimensionally averaged as a centrosymmetric crystal structure in cubic space group Pm3̅m observed in neutron diffraction. The nanoregions in Pmc21 were quenched at room temperature to show their relaxor-type ferroelectricity in the average Pm3̅m structure.