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Engineering polar oxynitrides: hexagonal perovskite BaWON2.

J. Oró-Solé, I. Fina, C. Frontera, J. Gàzquez, C. Ritter, M. Cunquero, P. Loza-Alvarez, S. Conejeros, P. Alemany, E. Canadell, J. Fontcuberta, A. Fuertes.
Angew. Chem. Int. Ed., 59 (2020) 1-6.

Atomic resolution scanning transmission electron microscopy image of BaWON2 viewed along a-axis. The superimposed structure corresponds to the P63mc model refined from Synchrotron X-ray diffraction.

Known perovskite oxynitrides show centrosymmetric structures, and for some of them high permittivities have been observed and ascribed to local dipoles induced by partial order of nitride and oxide. We report here on the first hexagonal perovskite oxynitride BaWON2, which shows a polar 6H polytype. Synchrotron X-ray and neutron powder diffraction, and annular bright-field in scanning transmission electron microscopy indicate that it crystalizes in the non-centrosymmetric space group P63mc, with a total order of nitride and oxide at two distinct coordination environments in cubic and hexagonal packed BaX3 layers. A synergetic second-order Jahn–Teller effect, supported by first principle calculations, anion order, and electrostatic repulsions between W6+ cations, induce large distortions at two inequivalent face-sharing octahedra that lead to long-range ordered dipoles and spontaneous polarization along the c axis.