Engineering Polar Oxynitrides: Hexagonal Perovskite BaWON₂.

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• Non-centrosymmetric polar compounds are crucial for technological applications.
• Existing perovskite oxynitrides typically display centrosymmetric structures.
• High permittivities in some oxynitrides are linked to local dipole ordering.

Purpose of the Study:

• To report the synthesis and structural characterization of the first hexagonal perovskite oxynitride.
• To investigate the origin of its polar nature and potential technological properties.
• To explore its electronic and dielectric characteristics.

Main Methods:

• Synchrotron X-ray and neutron powder diffraction for structural determination.
• Annular bright-field scanning transmission electron microscopy (ABF-STEM) for microstructural analysis.
• First-principles calculations to support theoretical understanding of electronic structure and bonding.

Main Results:

• Discovery of a novel hexagonal perovskite oxynitride, BaWON2, with a polar 6H polytype.
• Determination of its non-centrosymmetric space group P63mc, featuring ordered nitride and oxide ions.
• Identification of a synergetic second-order Jahn-Teller effect, anion order, and cation repulsions driving spontaneous polarization.

Conclusions:

• BaWON2 is the first reported hexagonal perovskite oxynitride with a polar structure.
• Its non-centrosymmetric nature arises from specific ordering of anions and electronic effects.
• The material is a semiconductor with a 1.1 eV band gap and exhibits large permittivity.