Tensor X-ray optical properties of the bromate ion

Linearly polarized synchrotron radiation has been used to observe X-ray dichroism of the bromate ion near the bromine K edge by transmission absorption spectroscopy using a crystal of potassium bromate, and to measure the anomalous scattering of bromine and its anisotropy in diffraction experiments with sodium bromate. The principal values of f" are as high as 12 and 17 electrons atom^-1 and of f' as low as -13 and -15 for polarization respectively parallel and perpendicular to the threefold axis. The anisotropy is as much as 6.6 for f" and 4.6 for f', or a few percent more after correction for thermal motion and for incomplete polarization of the beam. A consequence of this large anisotropy is that reflections forbidden by the screw-axis rules are observed. Applications and possible adverse consequences of these effects in structure research are discussed. An energy calibration gives 13 482.1 (10) eV or 0.91960 (7) Å for the peak of the first K-edge absorption line in sodium bromate. These results suggest that X-ray optical activity in sodium bromate can be observed most readily near 0.9205 Å, where the magnitude is estimated to be of the order of 0.1° cm^-1. Crystal structures were redetermined for KBrO₃, R3m, a = 6.011 (1), c = 8.152 (2) Å, Z = 3, 295K, R = 0.010 for 308 unique reflections, and for NaBrO₃, P2₁3, a = 6.705 (2) Å, Z = 4, 296 K, R = 0.013 for 433 unique reflections.