Observation of Anion Order in Pb2Ti4O9F2

The observation of anion order is indispensable for the investigation of oxyfluorides. However, the negligible contrast between O2– and F– in both X-ray and neutron diffraction obscures the distinct anion sites for Rietveld refinement. Therefore, the difference in the chemical bonding of M–O2– and M–F– is the key to determining anion order. In this study, bond-valence-sum calculations and determination of the electron density distribution by the maximum entropy method illustrated anion order in the newly synthesized oxyfluoride Pb2Ti4O9F2. These results demonstrate a promising method to determine anion order in mixed anion systems.