First-Principles Study on Relaxor-Type Ferroelectric Behavior without Chemical Inhomogeneity in BaTaO2N and SrTaO2N

The wide range of applications attracts interest in oxynitride perovskites. BaTaO2N and SrTaO2N have relaxor-type high dielectric permittivities and are promising candidates in many applications especially because Pb is not included, unlike in many relaxor ferroelectrics. There is an urgent need to understand the relation between the anion ordering and the permittivity to facilitate screening and designing materials with higher permittivity, and the chemistry that results in relaxor-type behavior without chemical inhomogeneity. We show using systematic first-principles calculations that stable anion orderings in BaTaO2N and SrTaO2N have two kinds of similar, 3D -Ta-N- coiled chain motifs that can switch to each other, forming a mechanism to break long-range order and increasing the diversity of anion orderings around the pentavalent Ta. Both materials have two sets of low-energy displacements forming opposite polarization directions that can be easily alternated at the picosecond scale. This explanation of the origin of relaxor-type behavior without chemical inhomogeneity currently found only in these two materials will fuel further searching of similar materials.