The photocatalytic and dielectric behaviors of Aurivillius oxyfluorides such as Bi 2 TiO 4 F 2 depend sensitively on their crystal structure and symmetry but these are not fully understood. Our experimental work combined with symmetry analysis demonstrates the factors that influence anion order and how this might be tuned to break inversion symmetry. We explore an experimental approach to explore anion order, which combines Rietveld analysis with strain analysis.

Aurivillius oxides have been a research focus due to their ferroelectric properties, but by replacing oxide ions by fluoride, divalent magnetic cations can be introduced, giving Bi 2 MO 2 F 4 (M = Fe, Co, and Ni). Our combined experimental and computational study on Bi 2 CoO 2 F 4 indicates a low-temperature polar structure of P2 1 ab symmetry (analogous to ferroelectric Bi 2 WO 6 ) and a ferrimagnetic ground state. These results highlight the potential of Aurivillius oxide-fluorides for multiferroic properties. Our research has also revealed some challenges associated with the reduced tendency for polar displacements in the more ionic fluoride-based systems.

The Aurivillius materials are well known for their ferroelectric properties [1] and associated structural distortions. [2] They form a class of layered perovskite-related phases with general formula Bi2An-1BnX3n+3 (X is usually oxide, but halides are also known), with structures built up from alternating fluorite-like [Bi2O2] 2+ layers and [An-1BnX3n+1] 2perovskite-like layers. The search for magnetoelectrics, with coupled magnetic and ferroelectric order, has motivated investigations to introduce magnetic ions into the B cation sites. However, this has been challenging and the concentrations of magnetic B cations in Aurivillius oxides is typically low. [3][4][5] Redirecting research away from oxides and towards mixed-anion systems, including Aurivillius oxyfluorides, opens up a wider compositional range, as well as the possibility of tuning structure and properties by anion order. [6,7] This presentation describes work on n = 1 Aurivillius oxyfluorides including Bi2TiO4F2 and Bi2CoO2F4. Our symmetry analysis [8] of possible anion-ordered structures highlights the challenges of packing polar heteroanionic units to break inversion symmetry, as well as means by which this might be achieved for Bi2TiO4F2. We also explore methods to determine anion ordering in materials with anions with similar scattering lengths. [9] Increasing the fluoride content in these oxyfluorides gives access to phases with lower oxidation states for B cations, and the report of Bi2CoO2F4, with long-range magnetic order of the Co 2+ sublattice,[10] motivated our investigation using neutron powder diffraction. We've explored its nuclear structure and in particular, the anion sublattice and structural distortions, and determined its magnetic structure. [11] This gives insight into its physical properties and opens the door to designing and preparing new multiferroics.