research papers
The wavelength dispersion in a graphite (002) monochromated Mo Kα beam was analyzed. A wavelength window was found with Å, i.e. . The very large dispersion leads to systematic errors in caused by scan-angle-induced spectral truncation. A limit on the scan angle during data collection is unavoidable, in order that an ω2θ measurement should not encompass neighboring reflections. The systematic intensity errors increase with the Bragg angle. Therefore they influence the refined X-ray structure by adding a truncational component to the temperature factor: B(X-ray) = B(true) + B(truncation). For an Mo tube at 50 kV, we find B(truncation) = 0.05 Å2, whereas a value of 0.22 Å2 applies to the same tube but operated at 25 kV. The values of B(truncation) are temperature independent. The model bias was verified via a series of experimental data collections on spherical crystals of nickel sulfate hexahydrate and ammonium hydrogen tartrate. Monochromatic reference structures were obtained via a synchrotron experiment and via a `balanced' tube experiment.
Supporting information
Portable Document Format (PDF) file https://doi.org/10.1107/S0108767300002853/sh0139sup1.pdf | |
Structure factor file (SHELXL table format) https://doi.org/10.1107/S0108767300002853/sh0139sup2.sft |