Local anionic ordering and anisotropic displacement in dielectric perovskite SrTaO2N

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) A and c = 8.0786(1) A 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.