Helicity-dependent resonant X-ray scattering in CuB₂O₄

Abstract

Exploitation of X-ray circular polarized beams to study forbidden Bragg reflections and new information that could be obtained in these experiments are discussed. It is shown that the intensities of such reflections can be different for the right- and left-circular polarizations (i.e. exhibiting circular dichroism) even for the dipole–dipole resonant transitions involved in the scattering process. This difference can be observed only in crystals having no center of inversion. Here, this approach is used to study helicity-dependent resonant diffraction in copper metaborate CuB2O4 single crystal, which is non-centrosymmetric but achiral. Nonetheless, a strong circular dichroism has been observed for hh0 forbidden reflections in the vicinity of the Cu K-edge. This effect is shown to originate from dipolar transitions in Cu atoms occupying the 8(d) Wyckoff position only.

Determination of Absolute Structure of Chiral Crystals Using Three-Wave X-ray Diffraction

“…This caused a reduction of the degree of circular polarization by a factor of 0.7. The experimental set-up is essentially a high-vacuum three-circle diffractometer operating in horizontal scattering geometry with a sample and detector arm rotations driven from outside the vacuum chamber by the Huber goniometer with a very high accuracy [30].…”

“…Such effects were observed in crystals with helical screw axes such as quartz (SiO 2 ) [26], berlinite (AlPO 4 ) [27] and tellurium [28,29]. However, the difference between the energy spectra of forbidden reflections for the right and left circularly polarized incident radiation can be observed not only in chiral crystals, but also for all substances without inversion center [30], and not only in forbidden reflections [31]. A study of a dichroism in forbidden reflections was also used for the visualization of chiral domains in DyFe 3 (BO 3 ) 4 [32].…”