Divergent-beam X-ray photography of crystals

TLDR

Divergent‑beam X‑ray photography of single crystals in transmission reveals extinction at Bragg angles and provides absorption‑line intensity and geometry that inform crystal texture. The study aims to use divergent‑beam X‑ray photography to determine crystal orientation and, where possible, to measure lattice constants or X‑ray wavelengths precisely without precision apparatus. The method employs divergent‑beam X‑ray transmission, requiring accurate wavelength–crystal size matching, and allows orientation determination and, in favorable cases, precise lattice constant or wavelength measurement without precision apparatus. Photographs show many organic crystals are unexpectedly perfect, rapid cooling to liquid‑air temperatures increases mosaicity, while Type I diamonds and natural ice near melting remain highly perfect; measurements reveal C–C distances in diamonds vary with impurity, and Zn Kα₁ wavelength relative to Cu Kα₁ is 1 432.21 pm, agreeing with precision methods.

Abstract

Divergent-beam X-ray photography of single crystals by transmission can be used to study the ‘extinction’, that is, the diminution of the transmitted radiation that takes place at the Bragg reflexion angles. The intensity and geometry of the absorption lines observed give useful information about the texture of the crystal. Divergent beam photographs have shown that many crystals of organic compounds are unexpectedly perfect, and that sudden cooling to liquid-air temperatures will increase the mosaic character of their structure by an important factor and make them more suitable for structural analysis by the usual methods. Type I diamonds, and natural ice even near to its melting-point, are also found to possess a high degree of perfection, which cannot be removed by liquid-air treatment. The divergent beam method may be used for the determination of orientation, but it is important that the wave-length of X-rays employed should be correctly related to the size and nature of the crystal. In certain favourable cases it is possible to make precision measurements of lattice constant or of wave-length from divergent beam photographs, without the use of any kind of precision apparatus. By such means it has been shown that the C—C distance in individual diamonds varies from 1541.53(± 0-02) to 1541.27X, (1.54465-1-54440A), a difference presumably due to varying impurity content. Using diamond and a brass anticathode, the Zn Ka 1 wave-length, relative to Cu K Ka 1 as 1537.40X, is found to be 1432.21 ( ± 0-04) X. Temperature control would improve the accuracy of this measurement, which is, however, in good agreement with the latest value obtained by orthodox precision methods.