Resonant x-ray Bragg diffraction from orbital moments in vanadium sesquioxide (V2O3) and haematite (alpha -Fe2O3)

Resonant x-ray diffraction from V2O3 which has been observed at charge-forb idden reflections (h + k + l odd) is here attributed to orbital moments of the vanadium ion. Furthermore, it is argued that the contribution to intens ity as a function of x-ray energy seen in the experiment on the low energy side of the electric dipole (El) resonance is due to an electric quadrupole (E2) resonance. The absence of the E1 resonance for h odd, and appearance of the E2 resonance at h odd and ii even is accounted for in the calculated structure factor. For h odd the structure factor is proportional to the oc tupole moment of the vanadium ion, which is estimated to have a magnitude s imilar to that of the orbital angular momentum. In the case of E2 resonant x- ray Bragg diffraction From haematite the calculated structure factor con tains three orbital moments, two of odd rank and one of even rank. The calc ulated polarization dependence and azimuthal-angle scans are in complete ac cord with experimental findings. The contribution of even and odd rank mome nts is shown to make scattering depend on circular polarization.