Local anionic ordering and anisotropic displacement in dielectric perovskite SrTaO₂N

Abstract

The perovskite oxynitride SrTaO₂N as a promising candidate for lead-free dielectric materials has been studied with a special emphasis on the structural characterization using neutron powder diffraction. SrTaO₂N powders were prepared by means of ammonia nitridation via precursors obtained from a soft chemistry method and a solid state reaction route. All the products crystallized in tetragonal I4/mcm space group with only slight variation in the lattice constant, e.g., a = 5.7023(1) Å and c = 8.0786(1) Å for the soft-chemistry derived sample. Neutron diffraction analysis suggested the short-range O/N ordering involving a cis configuration in TaO₄N₂ octahedra together with large anionic displacement within the ab plane on their axial position in the average crystal lattice. These facts are associated with local tilting of the TaO₄N₂ octahedra which is likely to play a key role in the unusual dielectric behavior of SrTaO₂N.