The Structural and Magnetic Characterisation of Perovskite-Related Oxyfluoride Materials

This thesis explores two n=1 Aurivillius oxyfluoride phases and their structural characterisation and how this relates to their physical properties.

Bi2CoO2F4 was successfully analysed using XRPD, NPD, XANES and SHG techniques. Structural characterisation of NPD data collected at 50 K and at 5 K indicated rotations of CoF6 octahedra about both in-plane and out-of-plane axes, whilst SHG activity supports the assignment of a nocentrosymmetric symmetry. Rietveld refinements suggest a ground state Pca21 symmetry. Low temperature NPD data allowed exploration of the magnetic structure. Analysis indicated the magnetic symmetry was close to a collinear antiferromagnetic structure with in-plane moments;however, two very similar magnetic structures were obtained, one of which has a ferromagnetic component allowed by symmetry, which may explain unusual field dependence found in magnetic susceptibility measurements.

Theory work has highlighted the possibility of anion ordering in heteroanionic materials breaking inversion symmetry and so we carry out a symmetry analysis of possible anion-ordered arrangements for Bi2TiO4F2 assuming mer or fac [TiO3F3]units. Although these units are polar, there are 12 ways of arranging these polar units based on an ideal parent model of I4/mmm symmetry, only 4 of these models are non-centrosymmetric and would allow macroscopic polarisation. Interestingly, further analysis has shown that allowing non-polar octahedral rotations in non-polar anion-ordered models can break inversion symmetry, and this represents a new avenue for designing hybrid-improper materials.