Anion ordering and vacancy defects in niobium perovskite oxynitrides

Abstract

Niobium perovskite oxynitrides are emerging as promising semiconductor materials for solar energy conversion processes, due to their physical properties and amenability to defect engineering. However, defect engineering in mixed-anion semiconductors such as perovskite oxynitrides is generally hindered by the absence of long-range order in the crystal lattice, a phenomenon known as anion-ordering. We demonstrate how anion ordering influences the stability and mobility of point defects in two exemplar perovskite oxynitrides, BaNbO₂N and LaNbON₂. Accurate first-principles calculations show that fully cis-anion orderings in BaNbO₂N are more stable than fully trans-anion orderings, whereas anion orderings with mixed dimensionality may be more prevalent in the lower-symmetry LaNbON₂. Anion ordering in LaNbON₂ is also influenced by a pronounced A-site coordination sphere effect not observed in BaNbO₂N, whereby local La-(O,N)₁₂ coordination environments give rise to alternating LaO and LaN layers in the bulk material. Anion order was predicted to effect the redistribution of electrons upon anion vacancy creation to the cation sublattice. Diffusion barriers for O²⁻ vacancies in trans-ordered BaNbO₂N were found to be lower than those for N³⁻ vacancies, suggesting that stabilising trans-ordered phases of BaNbO₂N will yield more effective retention of nitrogen content in this material. The reverse is the case for LaNbON₂, with N³⁻ vacancy defects exhibiting more facile diffusion than O²⁻ vacancy defects. We believe these insights will aid the emergent understanding of defect engineering in mixed-anion perovskite oxynitride semiconductors, and specifically help facilitate strategies for stabilising their nitrogen content.