Diffraction Analysis

Summary

Diffraction analysis is a general term used to describe various methods of scattering beams of X-rays, electrons, or neutrons from targeted materials to generate diffraction patterns from which atomic arrangements in gases, liquids, and solids can be precisely determined. Diffraction techniques are useful in identifying and characterizing both natural materials, such as minerals, and engineered materials, such as ceramics. X-ray diffraction played an important role in the discoveries of the three-dimensional structures of molecules such as proteins and deoxyribonucleic acid (DNA), and neutron diffraction has allowed researchers to investigate stresses in automobile and aerospace constituents.

Definition and Basic Principles

In general, diffraction denotes a change in the directions and intensities of waves when they encounter obstacles. All waves, be they water, sound, or light, or even such particles as electrons and neutrons that exhibit wavelike properties, are subject to diffraction. When X-rays, the wavelengths of which range from 0.001 nanometers to 10 nanometers (a nanometer is one billionth of a meter), interact with matter such as a crystal, a pattern is generated that can be photographed and analyzed. This X-ray diffraction analysis has led to the discovery of the precise atomic arrangements in an enormous number of substances, from a simple crystal such as sodium chloride (whose sodium and chloride ions are cubically arranged) to such complex crystals as vitamin B₁₂ (cyanocobalamin), the gigantic molecular structure of which was worked out by Dorothy Crowfoot Hodgkin, helping her to win the 1964 Nobel Prize in Chemistry.

The discovery that moving electrons exhibit wave properties led to a new kind of diffraction analysis employing electron beams. This technique proved to be particularly valuable in studies of the structures of gaseous substances, adsorbed gases, and surface layers. It could also be used to study liquids and solids, and it was often used in tandem with X-ray diffraction analyses. For example, X-ray diffraction studies confirmed that liquid cyclohexane existed in a chair form, and electron diffraction showed that cyclohexane oscillated among two chair forms and a boat form. This led scientists to the discovery of the basic principles of conformational analysis, which proved essential to an understanding of a variety of molecules.

Although X-ray and electron diffraction methods are powerful in elucidating the structures of various substances, they are limited in studying elements that are close together in the periodic table. However, such elements vary greatly in their neutron-scattering ability, and so neutron diffraction analysis can easily distinguish such elements as carbon, nitrogen, and oxygen. Certain details of molecular structure, such as hydrogen bonding, can be more precisely observed by neutron diffraction than by X-ray or electron techniques. Because of this, neutron scattering has proved advantageous in structural studies of colloids, membranes, dissolved proteins, and viruses.

Background and History

The term diffraction owes its origin to Francesco Maria Grimaldi, who derived it from the Latin root diffringere, meaning “to break apart,” in the seventeenth century. Others had observed diffraction patterns when a light beam was broken up by a bird feather. In the early nineteenth century, the English physician and physicist Thomas Young performed his famous double-slit experiment in which he proved that light is a wave phenomenon since interfering waves constituted the sole explanation of the striped light and dark bands that he observed when light managed to traverse the two slits.

The discovery that X-rays also produced interference patterns was made by the German physicist Max von Laue in 1912 when he showed that X-rays passing through a crystal produced a diffraction pattern on a photographic plate. The next year, the British father-and-son team of William Henry Bragg and William Lawrence Bragg used the X-ray diffraction technique to determine the structures of such crystals as sodium chloride and diamond, and William Lawrence Bragg formulated a mathematical equation relating the wavelength of the X-rays, the angle between the incident X-rays and the crystal's parallel atomic layers, and its interplanar spacing.

The Braggs' work led to the flourishing new field of X-ray crystallography, in which the three-dimensional arrangements of a wide variety of materials were determined, for example, fluorite (calcium fluoride) and calcite (calcium carbonate). In 1916, the Dutch physical chemist Peter Debye showed that X-ray diffraction could be extended to powdered solids, and he used this technique to figure out the structure of graphite. During the 1920s and 1930s, the American physical chemist Linus Pauling used X-ray diffraction to determine the structures of more than thirty minerals, including some important silicates. Pauling learned about the electron diffraction of molecules in the gaseous state from its discoverer, Herman Mark, and Pauling, as well as others, determined the structures of many substances that could be studied in the gaseous and liquid states. The techniques of X-ray and electron diffraction were also helpful throughout the second half of the twentieth century in elucidating the structures of proteins and DNA.

How It Works

X-Ray Diffraction. X-ray diffraction is a physical analysis technique used to determine the crystal structure, phase composition, or orientation of a powder, liquid, or solid sample without destroying the substance. Crystals are orderly arrangements of atoms or ions, and crystals constitute about 95 percent of all solid materials. The scattering of X-rays from a crystal is due to its electron densities (atomic nuclei have a negligible contribution). Every crystal bombarded by X-rays gives a unique pattern, and so each pattern acts as a distinctive identifier of the substance. This pattern is a result of secondary waves emanating from the electrons, which are called scatterers, and the phenomenon itself is known as elastic scattering. In a crystal, these scatterers are regularly arrayed, but the reflected X-rays can interfere destructively as well as constructively.

The Bragg equation describes how these waves add constructively in certain directions. The reason X-rays can do this is that their wavelength is generally similar to the spacing between crystal planes. When a researcher changes the angle between the X-ray beam and a crystal face, the diffraction pattern changes and a series of diffraction photographs taken at different angles allows the investigator to formulate the three-dimensional atomic structure of the crystal.

Powder X-Ray Diffraction. In this technique, experimenters use fine grains of a crystalline substance instead of a single crystal. It is extensively employed for identifying such materials as minerals and chemical compounds, or such engineered materials as ceramics. It is also used for characterizing materials, for studying particles in liquid suspensions or thin films, and for elucidating the structures of components of polycrystalline materials. Because the tiny crystals in the powder sample are randomly oriented, researchers can collect diffraction data either by reflecting X-rays from the sample or by transmitting them through it. Powder X-ray diffractometer systems allow scientists to obtain a diffraction pattern for the substance under investigation, which can then be used to calculate the unit cell of the substance.

Electron Diffraction. This technique depends on the wave nature of electrons, but because electrons are negatively charged, they interact much more intensely with the electromagnetic environment of samples than neutral X-rays or neutrons. Like X-rays, electron beams can be scattered by atoms in a sample, producing patterns that can be registered on a photographic plate or fluorescent screen. To determine the positions of atoms or ions in solids, electron diffraction instruments that can generate high-energy electrons in a thin beam are required. Nevertheless, electrons do not have the penetrating power of X-rays, which leads researchers to use thin slices of solids. Relative to solids, molecules in a gas are far apart, which means that electron diffraction readily generates patterns that allow molecular dimensions to be determined. Because air molecules scatter electrons, these measurements have to be made in a vacuum. The electron diffraction technique has enabled scientists to study films on solid surfaces. The new field of quasicrystals, which exhibit fivefold symmetry in violation of the traditional principles of crystallography, has benefited from electron diffraction analyses.

Despite its usefulness, electron diffraction has its limitations. In electron diffraction experiments, interplanar spacings can be discovered to accuracies in the range of one to one hundred parts per thousand, whereas in powder X-ray diffraction, interplanar spacings can be found to precisions of one to one hundred parts per million.

Neutron Diffraction. For many scientists and engineers, neutron diffraction, when compared with X-ray and electron diffraction, is the least utilized technique for structural determinations and other applications. Nevertheless, as instruments become more sophisticated and techniques more refined, this method has been increasing in popularity. Neutron diffraction differs from X-ray diffraction in terms of scattering points—electrons for X-rays and nuclei for neutrons. This makes neutron diffraction useful in distinguishing certain isotopes and helpful in studies of such compounds as the metal hydrides (hydrogen is difficult to determine by X-ray diffraction). Disadvantages of this technique include the requirement of large crystals and an efficient neutron source. Large crystals can often be difficult to grow, and neutron sources such as nuclear reactors are not as generally available as diffractometers.

Despite these problems, neutron diffraction studies have been successfully performed to measure precisely the carbon-carbon distances in graphite as well as to determine the absolute configuration of atoms in several complex chiral structures (those in which it is impossible to superimpose a configuration on its mirror image). Chemists have also found neutron diffraction studies helpful in illuminating the function of hydrogen bonds in inorganic and organic compounds, the role of magnetism in such classes of compounds as the ferrites and rare-earth nitrides, and the nature of such condensed inert gases as helium, argon, neon, and krypton.

Applications and Products

X-Ray Diffraction. German physicist Wilhelm Conrad Röntgen discovered X-rays in 1895. The power of X-rays to reveal structures within the human body was quickly recognized, leading to many applications. With the discovery of the power of X-rays to uncover the previously hidden atomic structures of crystals in the second decade of the twentieth century, physicists, chemists, and geologists enthusiastically embraced the X-ray diffraction technique as an essential tool in their disciplines. Starting with such simple substances as sodium chloride and diamond, scientists began working out the structures of increasingly complex inorganic materials such as silicates, and the technique was also applied to organic crystals (hexamethylenetetramine was the first to be determined). As instrumentation and methods became more sophisticated, X-ray diffraction, in both its single crystal and powder forms, was most notably applied to specify the molecular structures in living things, such as proteins and DNA. Once the three-dimensional structure of DNA was determined by James Watson and Francis Crick in 1953, an explosion of applications followed, leading to the flourishing field of biotechnology as well as to applications in criminology, genetically modified foods, and medicine.

At the beginning of diffraction analysis, the instruments and other products associated with this technique (X-ray diffractometers, photographic film, and various crystals) were relatively simple and inexpensive. As the field evolved, instruments became more complex and expensive. In the early years, analyses of diffraction photographs were time-consuming and labor-intensive (some complex structures took years to figure out). With the development of efficient mathematical techniques and ever-more-powerful computers, complex structures could be determined in days or even hours.

A major application of X-ray diffraction analysis has been the study of defects in metals, alloys, ceramics, and other materials. These studies can be done more quickly and efficiently, and methods have become so refined that they can be applied to defects in nanocrystalline materials. Although the use of diffraction techniques in such industries as aerospace, iron, and steel is well known, not-so-well known is their use in the manufacture of such common household products as cleansers. Diffraction techniques enable researchers to monitor the effectiveness of the abrasive minerals in these cleansers.

Electron- and Neutron-Diffraction Analysis. Like X-ray diffraction, electron and neutron analyses have applications in the areas of characterization of materials, substance identification, measurement of purity, texture description, and so on. Electron diffraction is better than X-ray analysis in studying such substances as membrane proteins because, unfortunately, X-rays pass through these thin layers without forming a diffraction pattern. Electron crystallographic analysis has been used to determine the atomic arrangements in certain proteins. More common is determining structures of inorganic crystals, including complex materials such as zeolites. As with X-ray analysis, the instruments and other products associated with electron diffraction methods have become more advanced, efficient, and expensive, but certain applications, such as the use of electron diffraction in electron microscopy, have led many scientists, from mineralogists to biologists, to expand and deepen their discoveries.

Applications of neutron analysis developed much later than those found in X-ray and electron diffraction. Like electron diffraction, neutron analysis has often been used in conjunction with X-ray techniques. For example, researchers were able, in such a combined study, to elucidate the internal dynamics of protein molecules. Independently, neutron diffraction has allowed investigators to study the details of atomic movements in substances. This basic scientific knowledge has helped others to develop better products, including window glass, semiconductors, and other electronic devices. In industry, neutron diffraction has been used to study the stresses, strains, and textures of various building materials. For example, metal alloys and welds often exhibit cracks or expansion as well as shrinkage, which limits the value of the respective products. Indeed, this practice is so prevalent that it has been named “engineering diffraction.”

Careers and Course Work

Associations such as the American Crystallographic Association provide detailed information on the education needed to pursue a career in diffraction analysis. Courses in physics, chemistry, and higher mathematics form the basis for later study in specialized diffraction techniques, mineralogy, or metallography. If, for example, a person wishes to pursue a career in failure analysis, courses in fracture mechanics, fractography, and corrosion testing and engineering form part of the curriculum. On the other hand, a student desiring a career in diffraction analysis as it applies to biotechnology would take courses in molecular biology, molecular biophysics, molecular genetics, and biostatistics.

Many careers are possible for students who complete programs in diffraction analysis, from being an X-ray diffraction technician to becoming the head of an integrated imaging facility. Some companies want applicants with interdisciplinary expertise, for example, someone with a mastery of mineralogy, X-ray diffraction, and modern computer modeling methods. Many universities have X-ray diffraction facilities and will hire research assistants as well as tenured professors. The increasing number of forensic science laboratories hire large numbers of scientists and technicians, many of whom should have expertise in diffraction techniques.

Social Context and Future Prospects

In the twenty-first century, determining structures by diffraction techniques has become faster, more efficient, and more accurate than determinations in the twentieth century. Most techniques take less than twenty minutes with minimal sample preparation required, but the analysis remains accurate and reliable. Another established trend is the continual application of these techniques to more complex structures with easier-to-use analytical software. Programs like XRDWIN 2.0, DIFFRAC.SUITE, Empyrean, X’Pert³ Powder, or X’Pert³ MRD allow even non-experts to perform analysis.

Nanotechnology is flourishing in the twenty-first century, and diffraction techniques have been applied to smaller and smaller crystals. In the realm of environmental concerns, diffraction techniques provide powerful tools for identifying, characterizing, and developing a detailed structural understanding of pollutants and also of the chemical compounds that make up the life forms that are increasingly threatened by these pollutants. As scientists develop more earth-friendly substances, diffraction analysis allows comparisons and understanding of suitable alternatives to previously used, harmful materials.

Another trend is the improvement of traditional techniques and instruments and the discovery of new ones. In the late twentieth century, computer-controlled diffractometers were widely used because they facilitated the collection and processing of digitized diffraction patterns. According to Moore's law, the computing power of integrated circuits doubles about every eighteen months. Thus, the computerization of diffraction technologies has become more powerful and less expensive than ever. More structures are determined by X-ray, electron, and neutron diffraction, and larger databases are created, producing a treasure trove of useful information. The International Center of Diffraction Data (ICDD) maintains a comprehensive database of compounds.

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