(696e) Thermodynamic Stability and Anion Ordering in ABO2n and ABON2 Perovskite Oxynitrides

Perovskite oxynitrides (PONs), derivatives of the perovskite oxide structure with both N and O anions, show promise as versatile catalysts (e.g., thermal catalysis and electrocatalysis). 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 perovskite oxynitride chemistries are synthesizable and stable under catalytic reaction conditions, or the trends in anion or cation choice that might predict this stability.

Here we report a DFT screening study of 295 ABO₂N and ABON₂ PON structures composed of combinations of 55 A and B cations. Our workflow enumerates PON structures and discovers trends related to stability under reaction conditions (Figure 1). 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. 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 variety 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 possibly stable. Theoretical Pourbaix diagrams of promising compositions are generated to understand their aqueous thermodynamic (meta)stability under varying pH and applied potentials, which has relevance to electrocatalytic applications. Generally, we find 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, suggesting that PON catalysts may function better in non-aqueous solvents for many electrochemical systems.