Influence of anion ordering on defect diffusion anisotropy in layered perovskite Sr2TaO3N: implications for oxynitride stability

Abstract

The defect chemistry of heteroanionic semiconductors can have a large effect on ionic conductivity and optoelectronic properties. Of particular interest are oxynitrides, which have found broad interest recently in sunlight driven water splitting due to their favourable band edge positions while being more stable compared to pure nitrides. The mixed-anion content however creates challenges in terms of retaining nitrogen stoichiometry under the oxidizing conditions of the oxygen evolution reaction (OER), either through formation of a passivating oxide layer or bulk nitrogen loss. In comparison to ternary and perovskite oxynitrides, layered perovskites have potential for improved stability against critical performance limiting defects and present an avenue for improving theoretical water splitting efficiency. In this work, the layer and anion ordering specific O²⁻ and N³⁻ defect properties of Ruddlesden–Popper Sr₂TaO₃N perovskite oxynitride were extensively investigated using first-principles calculations. We screen anion orderings, then compare anion defect formation and model the e⁻ redistribution across the Sr²⁺ and Ta⁵⁺ sublattices with neutral V_O and V_N defects. Following this we map the vacancy-mediated anion diffusion pathway barriers with the nudged elastic band (NEB) of O²⁻/V_O and N³⁻/V_N both in the plane of and perpendicular to TaON layers of Sr₂TaO₃N. The findings of this study suggest that cis- to trans- shifts in the local N³⁻–Ta⁵⁺–N³⁻ anion ordering modulate the anisotropy of the vacancy-mediated N³⁻ and O²⁻ diffusion in and out of plane relative to the TaON layer. This work points to the potential of a novel avenue for defect engineering in layered mixed-anion materials and oxynitride photocatalysts.