Noncentrosymmetric Crystal Structure of the Oxyfluoride PbBiTi4O10F Driven by Cooperative Pb/Bi and O/F Ordering

Mixed-anion chemistry provides unique opportunities for controlling crystal structures through the interplay of chemically distinct anions and cations. Here, we report a newly synthesized oxyfluoride, PbBiTi4O10F, in which Pb2+ and Bi3+, despite their similar chemical characteristics, exhibit long-range cation ordering. Combined synchrotron X-ray and neutron powder diffraction analyses reveal a noncentrosymmetric monoclinic structure with space group Cm, in which Pb/Bi cation ordering is coupled with selective O/F anion occupation. The absence of inversion symmetry is independently confirmed by second-harmonic generation. Bond-valence analysis identifies the origin of this coupling. In the centrosymmetric C2/m structure, Pb and Bi share a common crystallographic site whose coordination environment is unfavorable for both cations: Bi is underbonded, whereas Pb is overbonded relative to their formal valences. The symmetry reduction to Cm splits this site into two crystallographically distinct sites and relieves both bond-valence mismatches simultaneously. It places the two cations in more favorable coordination environments and generates an anion site that is too weakly bonded to accommodate O2– but is well suited to F–. These results indicate that the noncentrosymmetry arises from cooperative cation–anion ordering rather than substantial local structural distortions. The present study demonstrates that selective anion occupation in mixed-anion compounds can provide an additional driving force for cation ordering even when conventional differences in cation size or valence are insufficient, offering a strategy for designing noncentrosymmetric structures.