Insight into the energetics and properties of oxyfluorides through anion ordering

Ordered cation and oxygen vacancies is an established route to tune the properties of transition metal oxide double perovskites. However, less is known about what role these structural vacancies can play in materials comprising two unique anions, for example, transition metal oxyfluorides. Here we use the experimentally known double perovskite [ ]KNaNbOF₅ with P4/nmm symmetry to generate all possible unique anion orderings. We generated 346 structures based on combinations of cation and anion vacancies in the presence of octahedral rotations. We found 94% of the structures are non-centrosymmetric, which could possibly support interesting electronic phenomena. Furthermore, a subset of these structures exhibit octahedral tilting without any external stimuli. By analyzing structural features in conjunction with orbital interactions, we explain how control of the anion ordering can lead to changes in the energetics and electronic properties of oxyfluorides.