Diffraction and spectroscopic study of pyrochlores Bi2−xFe1+xSbO7

• Fe rich pyrochlores of the type Bi2−xFe1+xSbO7 were prepared by solid state methods.
• Structures determined using a combination of neutron and synchrotron X-ray diffraction.
• Fe partially occupies the 8-coordinate site.
• Dispacive disorder of the Bi cations observed as a consequence of the 6s2 electrons.
• Non-Vegard behaviour seen at low Fe contents due to disorder.

The structural and electronic properties of the series Bi2−xFe1+xSbO7 (0 ⩽ x ⩽ 0.6) were investigated using a combination of diffraction and spectroscopy. Synchrotron and neutron diffraction analysis show that Fe3+ cations substitute for Bi3+ onto the A site with increasing x, which was further confirmed by analysis of the Fe K/L-edge X-ray absorption near-edge spectra. The diffraction analysis indicated the presence of displacive disorder along the A2O chains, likely the result of the Bi3+ 6s2 lone pair, as well as non-Vegard-like behaviour of the lattice parameters in the Fe-poor region. Fe K-edge extended X-ray absorption fine-structure analysis of Bi2FeSbO7 confirmed the displacive disorder of the Bi3+ cations as well as Sb5+ and Fe3+ disorder on the B site.