X–ray Crystallography

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X-ray crystallography is a laboratory technique used for the study of the internal structure of crystalline materials. More specifically known as x-ray diffraction, the technique is based on the interference pattern produced as x rays pass through the three-dimensional, repeating pattern of atoms within a crystal lattice. The characteristic interference patterns produced are reflective of the molecular structure of the sample. X-ray diffraction has enabled the measurement of distances between planes of atoms and the determination of the arrangement of atoms within the lattice. Once the characteristic pattern for a substance has been identified, x-ray diffraction may also be used to identify an unknown sample of that same material by matching the diffraction pattern of the unknown to the appropriate known pattern.

Prior to the discovery of x-ray diffraction, crystallographers had no means by which to measure the internal positions of atoms within crystals and could only hypothesize as to the internal structure based upon external and optical features. X-ray diffraction has allowed crystallographers to demonstrate the orderly internal structure of crystals and has profoundly affected science since the inception of the technique.

In 1895, x rays were discovered by German physicist Wilhelm Conrad Roentgen (1845–1923) while experimenting with cathode rays. In 1912, German physicist Max von Laue (1879–1960) suggested that x rays interacting with a crystal could produce a distinctive interference pattern. His hypothesis, for which he was awarded the 1914 Nobel Prize, proved to be correct. The procedure demonstrated the internal order of atoms within a crystal and was the origin of x-ray crystallography. In 1914, father and son team of English physicists William Henry Bragg (1862–1942) and William Lawrence Bragg (1890–1971) refined the analysis of crystalline structure with x-ray diffraction, determined the atomic structure of a simple inorganic substance, common salt (NaCl), and deciphered the mathematical relationships between crystal structure and the associated diffraction pattern. They were jointly awarded the Nobel Prize in 1915; the younger Bragg was the youngest-ever Nobel laureate at age 25 years.

Crystalline substances have an ordered three-dimensional arrangement with a particular spacing of atoms. When x rays strike the atoms within the crystal, the atoms absorb and reemit the energy from the x rays in the form of spherical wave fronts emanating from each atom. The waves traveling outward from each atom interact with other waves in the processes known as constructive and destructive interference. In some directions, the waves cancel each other and little energy remains; in other directions, the energy is reinforced and a zone of increased energy exists. The resulting pattern of constructive and destructive interference is known as a diffraction pattern. The patterns are controlled by the spacing of atoms within the matrix and are unique to that substance.

In its most basic form, a diffractometer consists of three main components; a source of x rays and the means to direct the beam to the sample, a sample holder, and a method for collecting the resultant

KEY TERMS

Crystal —A solid, homogeneous body composed of a single element or compound having a fixed and regular internal atomic arrangement that may be expressed by external planar faces.

Crystal lattice —The ordered, three-dimensional arrangement of atoms in a crystal.

Crystallography —The study of crystals, including their growth, structure, properties, and classification.

Diffraction —The process by which the direction of wave motion is modified by bending around an obstacle.

Diffractometer —The laboratory instrument used for x-ray crystallographic analysis.

Interference —The effect two sets of electromagnetic waves have on each other, and the combined pattern which may be detected as formed by this interaction.

X ray —Electromagnetic radiation of very short wavelength, and very high energy.

radiation and recording the diffraction pattern. In the Laue method, a single crystal is placed in the x-ray beam and the diffraction pattern is captured on photographic film. The crystal is stationary and the method allows for the study of symmetry within the crystal structure. The rotational method of diffraction is similar to the Laue method in that a single, well-formed crystal is used. As the name suggests, however, the crystal is rotated about one axis, allowing the collection of a greater quantity of diffraction data. The difficulties associated with obtaining and orienting well-formed crystals eventually led to the development of the powder method of x-ray crystallography. In this case, the sample is ground to a powder and the diffracted energy from all of the atomic planes within the material are measured simultaneously.

On modern diffractometers, electronic detectors linked to chart recorders have replaced photographic film. The information provided by each of these methods is quite similar, but the automated electronic system has a number of advantages. These advantages include the ability to read the data values directly from the chart without the need for careful measurements, the intensity of the energy peaks is clearly visible on the chart, no need for film developing, and rapid data collection.

X-ray crystallography was initially used to investigate the structure of minerals, confirming and refining the crystallographic descriptions. Use of the technique was expanded to the investigation of metals, alloys, and inorganic and organic chemical substances. More recently, biomedical research has utilized the technique for the investigation of the structure and dynamics of proteins, nucleic acids, and other biological molecules. Research into microelectronics and semiconductors, as well as pharmaceutical research, continue to rely on the qualities of x-ray crystallography.

See also Electromagnetic spectrum; Mineralogy.

Resources

BOOKS

Bragg, William L. The Crystalline State: A General Survey. London: G. Bell and Sons Ltd., 1949.

Clegg, William (editor). Crystal Structure: Principles and Practice. New York: Oxford University Press, 2001.

Hammond, Christopher. The Basics of Crystallography and Diffraction. New York: Oxford University Press, 2001.

OTHER

Rupp, Bernhard. “Crystallography 101.” <http://www.ruppweb.org/Xray/101index.html> (accessed November 7, 2006).

Weiss, Manfred S. “A Short And Simple Overview On What X-ray Crystallography Is All About.” 2001. <http://www.embl-hamburg.de/~msweiss/teach/> (accessed November 7, 2006).

David B. Goings