X-ray Diffraction Instruments
X-ray diffraction instruments are used to measure crystal structure, grain size, texture and/or residual stress of materials and compounds through the interaction of X-ray beams and the sample. The wavelengths of X-rays are of the same order of magnitude as the distances between atoms or ions in a molecule or crystal (Å, 10-10 m). Crystals diffract X-rays passing through them at specific angles depending on the X-ray wavelength, the crystal orientation, and the structure of the crystal. As X-rays are predominantly diffracted by electron density, analysis of the diffraction angles can be used to produce an electron density map of a given crystal or crystalline structure. Electron X-ray diffraction instruments and neutron X-ray diffraction instruments are sensitive to nuclei and are often used to accurately determine hydrogen positions; as hydrogen atoms have very little electron density, determining their positions requires extensive refinement of the diffraction pattern.
X-ray diffraction instruments consist of X-ray generators, goniometers, sample holders, and X-ray detectors such as photographic film or a movable proportional counter. X-ray generators or tubes produce X-rays by bombarding a metal target with high-energy (10 - 100 keV) electrons, which bounces electrons out of the materials core into its outer shells. As electrons fill holes in the outer shells, the inner shells emit X-ray photons. Two common targets are molybdenum (Mo) and copper (Cu), which have strong K() X-ray emissions at 0.71073 and 1.5418 Å, respectively. Decelerating electrons in a target or a synchrotron ring also generate X-rays. These sources produce a continuous spectrum of X-rays and require X-ray diffraction instruments with crystal monochromators to select a single wavelength.
X-ray diffraction instruments perform their function using a number of diffraction methods. These include powder, Laue or single crystal diffraction, and rotating crystal. Less common methods or variations of typical X-ray diffraction instruments, such as Lang, Borrman, and rocking curves, can be used to for crystal quality and/or perfection measurement, to map internal topographies, for textural analysis and residual stress studies.
