Chiral Properties of BiFeO3 Inferred from Resonant X-ray Bragg Diffraction

A new chiral phase of ferric ions in bismuth ferrite (BiFeO3), the only material known to support multiferroic behaviour at room temperature, is inferred from extensive sets of data gathered by resonant x-ray diffraction. Values of all ferric multipoles participating in a minimal model of Fe electronic structure are deduced from azimuthal-angle scans. Paramagnetic (700 K) and magnetically ordered (300 K) phases of a single crystal of BiFeO3 have been studied with x-rays tuned near to the iron K-edge (7.1135 keV). At both temperatures, intensities at a Bragg spot forbidden in the nominal space-group, R3c, are consistent with a chiral motif of ferric ions in a circular cycloid propagating along (1, 1, 0)(H). Templeton and Templeton scattering at 700K is attributed in part to charge-like quadrupoles in a cycloid. The contribution is not present in a standard, simplified model of electronic states of the resonant ion with trivial cylindrical symmetry.