X-ray powder methods

X-ray powder methods

Physical techniques used for the identification of substances, and for other types of analyses, principally for crystalline materials in the solid state. In these techniques, a monochromatic beam of x-rays is directed onto a polycrystalline (powder) specimen, producing a diffraction pattern that is recorded on film or with a diffractometer. This x-ray pattern is a fundamental and unique property resulting from the atomic arrangement of the diffracting substance. Different substances have different atomic arrangements or crystal structures, and hence no two chemically distinct substances give identical diffraction patterns. Identification may be made by comparing the pattern of the unknown substance with patterns of known substances in a manner analogous to the identification of people by their fingerprints. The analytical information is different from that obtained by chemical or spectrographic analysis. X-ray identification of chemical compounds indicates the constituent elements and shows how they are combined.

The x-ray powder method is widely used in fundamental and applied research; for instance, it is used in the analysis of raw materials and finished products, in phase-diagram investigations, in following the course of solid-state chemical reactions, and in the study of minerals, ores, rocks, metals, chemicals, and many other types of material. The use of x-ray powder diffraction methods to determine the actual atomic arrangement, which has been important in the study of chemical bonds, crystal physics, and crystal chemistry, is described in related articles. See X-ray crystallography, X-ray diffraction

There are many types of powder diffractometer available ranging from simple laboratory instruments to versatile and complex instruments using a synchrotron source. Specialized instruments allow recording of diffraction patterns under nonambient conditions, including variable temperature, pressure, and atmosphere. Completely automated equipment for x-ray analysis is available. Most laboratory instruments consist of a high-voltage generator which provides stabilized voltage for the x-ray tube, so that the x-ray source intensity varies by less than 1%. A diffractometer goniometer is mounted on a table in front of the x-ray tube window. Electronic circuits use an x-ray detector to convert the diffracted x-ray photons to measurable voltage pulses, and to record the diffraction data.