Perovskite oxynitrides (PONs), derivatives of the perovskite oxide structure with both N and O anions, show promise as versatile catalysts in electrochemical reactions. Their versatility arises largely from a high degree of geometric and electronic structure tunability, achievable through anion and cation ordering, composition, defects, and differing structural phases. There are potentially tens of thousands of PON compositions. However, little is known about which stoichiometries are synthesizable or stable under thermal or electrochemical reaction conditions, or the trends in anion or cation choice that could predict this stability. Here we conduct a high-throughput DFT screening study of 295 ABO2N and ABON2 PON structures composed of combinations of 55 A and B cations. For a given pair of A-B cation oxidation states, we analyze relative stability of different anion orderings to understand composition- and configuration-dependent design rules in this chemical space. Our ab initio analysis both confirms the stability of cis ordering of N and O atoms around the B-site octahedra and predicts many previously unknown PONs to be thermodynamically stable. In particular, PONs with B = {Re, Os} and a wide range of A-site chemistries are predicted to be stable (i.e., a decomposition free energy of 10 meV/atom or less). Other PONs with A = {La, Nd} and B = {Nb, Ta} are also predicted to be stable. We also generate theoretical Pourbaix diagrams of promising compositions to understand their aqueous thermodynamic (meta)stability under varying pH and applied potentials. Our work finds that the thermodynamic stability of PONs depends heavily on cation composition and anion ordering, leading to design guidelines to select potentially stable and synthesizable PON materials. Additionally, only a few PONs seem to be stable or metastable under acidic electrochemical conditions, which may limit their utility in many electrochemical systems.

Thermodynamic stability and anion ordering in ABO2N and ABON2 perovskite oxynitrides

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