Engineering Polar Oxynitrides: Hexagonal Perovskite BaWON2

Non‐centrosymmetric polar compounds have important technological properties. Reported 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. Reported here is 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. The new oxynitride is a semiconductor with a band gap of 1.1 eV and a large permittivity.
This work was supported by the Ministerio de Ciencia e Innovación, Spain (MAT2017‐86616‐R, MAT2017‐85232‐R, SEV‐2015‐0496, MDM‐2017‐0767, PGC2018‐096955‐B‐C44 and PGC2018‐093863‐B‐C22) and the Generalitat de Catalunya (2017SGR1377, 2017SGR581, 2017SGR1506 and 2017SGR1289). We thank Alba and ILL (experiments 2018082958 and 5‐23‐721, https://doi.org/10.5291/ILL‐DATA.5‐23‐721) for beam time provision, and Dr. François Fauth for assistance in data collection at Alba. I.F and J.G. acknowledge the Ramon y Cajal contracts RYC‐2017‐22531 and RYC‐2012‐11709. S.C. was supported by FONDECYT, Chile (project 11171063). ICTS‐CNME at UCM is acknowledged for offering access to STEM and expertise.