Ever wonder how scientists find out the structure, physical properties, and the chemical composition of different substances and elements? The answer is through x-ray diffraction. Using x-ray diffraction, scientists get a better picture of what the world looks like at a atomic and subatomic level, helping them to solve problems and make technological progress.
Introduction to Diffraction
X-ray diffraction is the bombardment of a substance with x-rays in order to determine its structure. As the photons in the x-ray collide with the electrons in the material being analyzed, some of the photons are deflected off of the initial trajectory in a phenomenon known as wave scattering. Sometimes the electron will absorb some of the energy of the x-ray beam and the scattered x-rays that bounce away after this absorption have different wavelengths than they did when they first hit the material and its electrons (inelastic scattering). At other times, the wavelengths of the x-rays do not change after colliding with the material but their momentum does (elastic scattering). In diffraction, these x-rays whose wavelengths that have not changed are measured, and the point of momentum difference indicates the presence of an electron. From these measurements, the location of the electrons in the material being studied can be located and depicted.
The most common method of x-ray diffraction is powder diffraction, which bombards a powdered form of the material being analyzed with x-rays. This is exceptionally useful for measuring crystalline solids.
How Diffraction Works
Most x-ray diffraction experiments are conducted using a diffractometer. These devices come in a variety of shapes and sizes, but typically they include a radiation source, a monochromator that allows the scientist to select the wavelength of the x-rays with which to bombard the experiment’s subject, a device to adjust the shape of the x-ray beam, and a sensor or detector that can pick up the wavelengths and momentum of the x-ray after they hit the material being tested.
X-ray crystallography is concerned with the determination of crystalline structures using x-ray diffraction. As the x-ray strikes the solid, diffraction patterns occur at many different orientations, and as the wavelengths reverberate out, they collide with each other forming many peaks and patterns. As these are measured, an electron density map can be composed and compared against theories of crystalline structure to determine what the crystal looks like on an atomic level. Bragg’s law allows scientists to measure the angles of coherent and incoherent scattering that occur when x-rays hit the crystal lattice — the regular pattern that occurs again and again in the crystal. Measuring these angles is essential to determining the final structure.
Other Techniques in Diffraction.
Besides the aforementioned techniques, neutron diffraction can also be used to depict atomic structure. Neutrons bombard the nuclei and not the electron clouds in this case, providing a different look at the atomic makeup of the material being tested. In electron diffraction, electrons bombard the sample material and the interference with the electrons in the element being studied is measured to discover electron placement.
For more information on x-ray diffraction, please consult the following:
- Basics of Diffraction — This is a good pdf that provides a thorough introduction to x-ray diffraction.
- Bragg’s Law — A simple explanation and depiction of Bragg’s Law can be found via this link.
- Electron Diffraction — Here is a page that introduces the process and concept of electron diffraction, including Bragg’s law and more.
- Introduction to X-Ray Diffraction — The University of California, Santa Barbara, provides a thorough introduction to x-ray diffraction and its use in characterizing various materials.
- ISIS: Neutron Diffraction — The Science and Technology Facilities Council of the UK explains the processes of neutron diffraction.
- Powder X-Ray Diffraction — Read an explanation of the powder x-ray diffraction used by mineralogists on this page.
- Single Crystal X-Ray Diffraction — Carleton College has this good overview on single crystal x-ray diffraction.
- Squeezing More Out of Ultrafast X-Ray Measurements — This article explains how new methods are being used to study interactions between ultrafast lasers and matter.
- X-Ray Analysis of a Solid — Information on x-ray diffraction in solids, including Bragg’s law, can be found here.
- X-Ray Diffraction and Instrumentation — This page has some very basic information on x-ray diffraction and the use of diffractometers.