X-ray diffraction (XRD) encyclopedia
X-ray diffraction (XRD) encyclopedia

X-ray diffraction (XRD)
The classification and basic principle of X - ray diffractometer
In 1895, German physicist Roentgen discovered X-rays while studying cathode rays. Subsequently, the German physicist Laue in 1912 discovered the X-ray crystal diffraction phenomenon, confirming the nature of the X-ray electromagnetic waves and crystal atoms, molecules, ions are regularly arranged periodically, creating a X-ray diffraction analysis New areas of microstructure. In 1913, the British physicist Prague father and son based on Laurie's discovery of crystal diffraction by crystal plane reflection proposed a concise and easy to use Bragg equation: 2d sinθ = nλ (where λ is the wavelength of X-ray, d is the crystal plane distance, θ is the angle between the incident line and the crystal plane, and n is any positive integer). X-ray crystallography was developed.

When a beam of X-rays is incident on a crystal, it is first scattered by atoms (electrons), each of which is a new source of radiation that radiates electromagnetic waves of the same frequency as the incident waves into space. Since the crystal is composed of atoms, molecules or ions arranged according to a certain regularity of the crystal plane, the spacing of the crystal planes arranged in parallel by the same period of the incident X-ray wavelength, so the X-rays scattered by different crystal planes interfere with each other And produces strong X-ray diffraction in the spatial direction that corresponds to the Bragg equation. The orientation and intensity of the diffracted ray in the spatial distribution are closely related to the crystal structure. The diffraction pattern can be used to determine the type and size of the unit cell and the coordinate position of each atom in the unit cell.

X-Ray Diffractometer (XRD) is currently the most widely used device for studying the crystal structure. Can be divided into X-ray single crystal diffractometer and X-ray polycrystalline diffractometer.

 X-ray polycrystalline diffractometer

X-ray polycrystalline diffraction was proposed by Debye and Scherrer in 1916 and has now become an important material structure research method. X-ray polycrystalline diffractometer has become an indispensable instrument in many fields, industry research institutes and factory laboratories.

X-ray polycrystalline diffractometer, also known as X-ray powder diffractometer, is mainly used to determine the phase composition of a sample. It is based on the Powder Diffraction File (PDF) database and is identified by comparison with a diffraction pattern in a database Whether the crystalline phase exists or is estimated its content, but difficult to determine the unknown phase. For single crystal samples, crushed into powder can also be analyzed and identified. For the sample requirements of lumps, powder can be, easy preparation of the sample, the measurement time is short, the phase identification is relatively simple and fast.

X-ray diffraction analysis of polycrystalline objects is a pile of tiny single-crystal aggregates or polycrystalline, each of which is completely disordered crystal orientation, that is, the same crystal surface of each crystal grain and incident X-ray The angle of intersection can be any angle from 0 ° to 90 ° so that all diffraction rays that satisfy the Bragg equation can occur. From the powder diffraction spectrum can be directly derived diffraction peak position, intensity and peak shape and other physical quantities. Any problem that powder diffraction can solve or any structural parameter that can be obtained is generally based on these three physical quantities.

Materials of different phases have different crystal structures and therefore have different powder diffraction patterns, ie, the positions of the diffraction peaks or the corresponding interplanar spacing and the diffraction intensity are not the same. However, the powder diffraction spectrum of the mixture is a superposition of the phase diffraction spectra of the respective constituents. Therefore, the phase composition of the sample to be analyzed can be qualitatively analyzed by comparing the spectrum of the sample to be measured with that of various pure phases. The diffraction intensity of each phase in the mixing spectrum is proportional to their content in the mixture. Therefore, phase analysis is not only qualitative, but also quantitative. Phase analysis is one of the most widely used X-ray polycrystalline diffraction applications.

The interplanar spacings and lattice constants obtained by the X-ray polycrystalline diffractometer are important structural parameters and can be used to study many problems. The presence of dislocations, stacking faults, reverse domains and other defects in the grains distorts the lattice and produces micro-stresses, as well as the size of the tiny pieces of material that make up the material. The microstructures in these crystals are responsible for diffraction peaks Of the peak shape of the impact, so from the peak shape analysis to infer the structure of the microstructure parameters. In the processing of materials, the orientation of one or several crystal planes of the crystal grains will segregate in every direction of macroscopic processing. This property is called preferential orientation. The non-uniform structure of materials is called texture. Texture will change the material properties, the use of X-ray powder diffraction instrument for texture analysis.

X-ray single crystal diffractometer

X-ray single crystal diffractometer is mainly used to determine the crystal structure of pure substance, the sample must be a single crystal. Diffraction of X-ray onto a single crystal, diffraction and collection of three-dimensional diffraction data can be used to understand the three-dimensional arrangement of atoms in the crystal at the level of atomic resolution, obtaining information on bond length, bond angle, torsion angle, molecular configuration and Conformation, intermolecular interaction and accumulation of a large number of microscopic information, draw the molecular structure diagram and the unit cell map, and from the structural characteristics of some of the possible performance.

Generally when characterizing new compounds, if one can obtain a single crystal sample, it is best to use a single crystal diffractometer for the structure determination. A single-crystal data acquisition takes several hours to several days, parsing a single crystal may take more time. In addition, the cultivation of single crystal is not easy, single crystal growth by many conditions.

If a bunch of monochromatic X-rays are impinged on a single stationary crystal, the incident ray and the crystal planes have a certain angle of intersection, but diffraction may not necessarily occur at an angle that meets the Bragg's formula. In order to make all facets have the opportunity to have diffraction, the most common method is to rotate the crystal. The rotation of the crystal face at different times with the incident line changes the angle of intersection, and therefore at some point in time its direction consistent with the Bragg equation and produce diffraction. Methods for collecting single-crystal diffraction data include turnaround and four-circle diffractometry.

Working crystal is a very early invented diffraction method, its recording medium used in the past is a photographic film, time-consuming and laborious, with the four-circular diffractometer method, with very little use. To the advent of image-plate (IP) recording media and new charge-couple devices (CCDs) in the 1990s, their high sensitivity and accurate recording , And easy to use, experimental time is short, was promoted, the revolving crystal method and reborn.

The four-circle diffractometer was the main experimental method in the seventies and eighties of the twentieth century. The method uses a scintillation counter to record diffraction information point by point. Therefore, it takes a few days or even more than a week to be time-consuming and can not meet the requirement of quickly recording diffraction data of biological macromolecules. At present, Determination of a certain application.
Analytical instruments
Physical property test
Environmental monitoring and analysis
Lab general equipment
About us
About this website
Our service
Product purchase
Contact Us


Make Sure you dont miss interesting happenings by joining our newsletter program.

Contact Us