Understanding the Appearance of Local and Long-Range Anion Order in Heteroanionic Layered Perovskites

The prediction of the appearance, or lack thereof, or anion ordering in mixed-anion and heteroanionic materials is an important, yet unsolved problem. This issue has a significant impact on the prediction of properties in these materials, especially properties whose appearance is reliant on long-range noncentrosymmetry, such as ferroelectricity, piezoelectricity and nonlinear optical pheonmena such as second harmonic generation. We examine this issue in Ruddlesden-Popper oxyfluorides where both anion ordered and disordered structures exist, such as Sr₂FeO₃F and Sr₂ScO₃F, and fluoride preferentially occupies the apical anion site of the 2D perovskite layers. We use density functional theory calculations to understand and attempt to quantify the contributions of the B-site cation size, covalency, and magnetism to the configurational entropy or the appearance of local and long-range ordered anions.