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What is X-ray crystallography
X-ray crystallography is a technique for determining the atomic and molecular structure of a crystal in which crystal atoms cause the incident X-ray beam to diffract in many specific directions. By measuring the angle and intensity of these diffracted beams, crystallologists can produce three-dimensional images of the electron density within the crystal. Based on this electron density, the average position of the atoms in the crystal, their chemical bonds, their disorder, and various other information can be determined.
Since many materials can form crystals - such as salts, metals, minerals, semiconductors and various inorganic, organic and biological molecules - X-ray crystallography has become the basis for many scientific fields. During the first few decades of its use, this method determined the size of the atoms, the length and type of chemical bonds, and the atomic scale differences between various materials, especially minerals and alloys. The method also reveals the structure and function of many biomolecules, including nucleic acids such as vitamins, drugs, proteins, and DNA. X-ray crystallography is still the main method of characterizing the atomic structure of new materials and is the main method of identifying materials similar to other experiments. X-ray crystal structures can also explain the unusual electronic or elastic properties of materials, reveal chemical interactions and processes, or serve as a basis for designing drugs to fight disease.
In the single crystal X-ray diffraction measurement, the crystal is mounted on a goniometer. The goniometer is used to position the crystal in the selected orientation. The crystals are illuminated with a finely focused X-ray monochromatic beam, creating a diffraction pattern known as the regularly spaced spots of reflection. Using the mathematical method of the Fourier transform, combined with the known chemical data of the sample, a two-dimensional image taken in different orientations is converted into a three-dimensional model of the electron density in the crystal. If the crystal is too small or the internal composition is not uniform enough, it may result in poor (blur) resolution or even errors.
X-ray crystallography is related to several other methods for determining the atomic structure. Similar diffraction patterns can be generated by scattering electrons or neutrons, which are also explained by the Fourier transform. If a single crystal of sufficient size cannot be obtained, various other X-ray methods can be applied to obtain less detailed information; such methods include fiber diffraction, powder diffraction, and (if the sample is not crystallized) small-angle X-ray scattering (SAXS). If the material under investigation is only available in the form of nanocrystal powders or has a poor crystallinity, then electronic crystallography methods can be used to determine the atomic structure.
For all of the above X-ray diffraction methods, the scattering is elastic; the scattered X-rays have the same wavelength as the incoming X-rays. In contrast, inelastic X-ray scattering methods can be used to study the excitation of a sample rather than the distribution of its atoms.