The structure and oxide/fluoride ordering of the ferroelectrics Bi2TiO4F2 and Bi2NbO5F

The structures of two complex layered oxide-fluorides have been determined from the analysis of room temperature X-ray and constant wavelength neutron powder diffraction data. Bi2TiO4F2 and Bi2NbO5F adopt a single layer Aurivillius-type structure with an ordered array of fluoride/oxide anions. The structure of Bi2TiO4F2 has also been studied at 24 K using time-of-flight neutron powder diffraction, which is well below the previously reported ferroelectric transition temperature. The structures of both materials can be described in the tetragonal space group I4/mmm, but show significant atomic displacements from the positions described in the original structural models proposed for these compounds as a result of disordered tilts and/or distortions of the M(O,F)6 octahedra. No three-dimensional long-range ordered structural distortion was observed upon cooling of Bi2TiO4F2 to 24 K. 19F MAS NMR data have been collected and support the presence of only one fluoride site. Bond valence calculations allow the assignment of the oxide and fluoride ions within the structure, and the proposed model has the fluoride ions located in the equatorial sites of the titanium or niobium octahedra.