X-ray birefringence and dichroism in lithium niobate, LiNbO₃

Accurate synchrotron-radiation X-ray transmission measurements have been carried out on hexagonal LiNbO₃ in the vicinity of the Nb K-absorption edge (E = 18.986 keV). The experiments were performed on the two-axis diffractometer at HASYLAB in dedicated mode of DORIS II (3.7 GeV). Single-crystal wafers cut perpendicular to [10.0] were rotated around the monochromatized beam using an experimental set up analogous to the optical polarizing- microscope. Both the horizontally and vertically polarized components of the transmitted radiation were recorded at the same time and analysed in terms of a classical optical model derived from the Jones calculus. Fits to the observations yielded agreement indices between 0.013 and 0.052 supporting the applicability of the model to X-ray energies. X-ray dichroism and birefringence are proved to occur at resonance energies, and all findings show the possibility of simultaneous assessment of anisotropies of the real and imaginary parts of the refractive index in a uniaxial crystal. They also indicate that the polarization of the transmitted radiation (of suitable energy) can be varied considerably in dependence of crystal thickness and orientation with respect to the polarization direction of the incident radiation. For special structures like LiNbO₃, where the tensorial scattering factor of the edge atom (Nb) is invariant against the symmetry operations of the space group, the method yields also the energy-dependent anisotropy of the anomalous-dispersion corrections, f' and f'', in the same experiment.