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 BaTaO 2 N 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 -TaO 4 N 2 in exhaustive energy calculations about the O and N atomic distribution. The cis -TaO 4 N 2 octahedra formed their local stackings during cooling of BaTaO 2 N 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 Pmc 2 1 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 Pm 3 ̅ m observed in neutron diffraction. The nanoregions in Pmc 2 1 were quenched at room temperature to show their relaxor-type ferroelectricity in the average Pm 3 ̅ m structure.