Observation of Anion Order in Pb₂Ti₄O₉F₂

The observation of anion order is indispensable for the investigation of oxyfluorides. However, the negligible contrast between O^2– 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–O^2– 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.