Anion order in perovskite oxynitrides AMO₂N (A = Ba, Sr, Ca; M = Ta, Nb): a first-principles based investigation

Perovskite-type oxynitrides have attracted a lot of research interest as emerging functional materials with promising wide applications. The ordering of O/N anions in perovskite oxynitrides plays an important role in determining their physical properties, while it is still challenging to characterize the actual anion order in a particular material and understand the underlying physics. In this work, we have investigated anion order in a series of perovskite oxynitrides AMO₂N (A = Ba, Sr, Ca; M = Ta, Nb) through first-principles calculations and the cluster-expansion-model-based Monte Carlo simulations. In terms of cluster correlation functions, it can be explicitly demonstrated that short-range anion order is present in all these perovskite oxynitrides. In addition, the anion order varies with the temperature of thermal equilibrium and depends on the cation type. Special quasi-ordered structures are then constructed as representative structures by taking the calculated anion order at finite temperature into consideration and their band gaps and dielectric tensors are predicted by first-principles calculations and compared to experimental values.