Chirality-induced `forbidden' reflections in X-ray resonant scattering

It is shown that additional Bragg reflections can appear exclusively owing to the local chirality associated with the left–right asymmetric environment of scattering atoms in non-magnetic crystals. The structure amplitude of these reflections depends on the antisymmetric part of a third-rank tensor describing the spatial dispersion effects. It enhances for resonant near-edge scattering through a mixed multipole transition, which includes a dipole–quadrupole contribution. It is shown that this mechanism works even for centrosymmetric crystals, and some realistic examples are considered in detail (α-Fe₂O₃, LiNbO₃ etc.). For instance, the interference between the dipole–quadrupole and quadrupole–quadrupole terms may be responsible for the threefold symmetry of the azimuthal dependence of the hhh, h = 2n + 1, reflections observed recently in hematite.