Structural and magnetic characterisation of Aurivillius material Bi2Sr2Nb2.5Fe0.5O12

The n=3 Aurivillius material Bi2Sr2Nb2.5Fe0.5O12 is investigated and combined structural refinements using neutron powder diffraction (NPD) and X-ray powder diffraction data (XRPD) data reveal that the material adopts a disordered, tetragonal (I4/mmm) structure at temperatures down to 2 K. Significant ordering of Fe3+ and Nb5+ over the two B sites is observed and possible driving forces for this ordering are discussed. Some disorder of Sr2+ and Bi3+ over the M and A sites is found and is consistent with relieving strain due to size mismatch. Highly anisotropic thermal parameters for some oxygen sites suggest that the local structure may be slightly distorted with some rotation of the octahedra. Magnetic measurements show that the material behaves as a Curie–Weiss paramagnet in the temperature range studied with no evidence of any long-range magnetic interactions. Solid solutions including Bi3−xSrxNb2FeO12, Bi2Sr2−xLaxNb2FeO12 and Bi2Sr2Nb3−xFexO12 were investigated but single-phase materials were only successfully synthesised for a narrow composition range in the Bi2Sr2Nb3−xFexO12 system.