Crystallography is the study of materials in which the atoms stack in a three-dimensionally ordered geometric arrangement. In a single crystal a single pattern extends throughout the entire material. A particular variant of crystallography, x-ray crystallography is the study and determination of crystalline structures through x-ray diffraction techniques. In 1953, Watson and Crick used x-ray crystallography data gathered by other scientists to confirm their model of the double helical structure of DNA.
Detailed analyses of crystal structures are carried out by X-ray diffraction. In 1912, Max von Laue predicted that the spacing of crystal layers is small enough to diffract light of the appropriate wavelength. William Henry Bragg and his son, William Lawrence Bragg, were awarded the Nobel Prize in chemistry (1915) for their development of crystal structure analysis using X-ray diffraction. The Braggs' found that when X-ray radiation was scattered by a crystalline material both constructive and destructive interference occurred; this interference occurs because the wavelength of the X-rays is of similar magnitude to the spaces between the atoms of the crystal. Analysis of the resulting diffraction pattern, the position of the lines of the scattered radiation along with their relative intensities, is the basis of the X-ray diffraction technique; this analysis allows the determination of the precise location of the atoms in the crystal. Polycrystalline materials have discontinuities in the periodicity of the material. In amorphous materials, there is still less periodicity within the material; the amount of non-ordered atoms in amorphous materials is at least comparable with that exhibiting periodicity.
The X-ray diffraction technique has been one of the most important structural methods throughout the twentieth century. It has expanded our knowledge by providing detailed structures of vitamins, proteins (enzymes, bacterial membranes, liquid crystals, polymers, organic compounds and inorganic compounds.
The idea that a crystal is composed of identical structural subunits was first proposed in 1784 based on observations of the cleavage of calcite. Subsequent investigations have shown that the structures of these subunits can be inferred from a crystal's symmetry. Even casual observation suggests that the symmetry of a crystal as a whole is related to some smaller subunit within it. The subunit is called a unit cell and it contains all of the essential information, such as the symmetry and elemental composition, of the crystal. Repeated translation along the edges of the unit cell can be used to derive the entire crystal lattice; in other words, the crystal lattice is the unit cell repeated many times in a periodic fashion.
The energy associated with crystal formation, the lattice energy, can be calculated by consideration of the different types of bonds within the solid: van der Waals bonds, ionic bonds, hydrogen bonds, covalent bonds, and/or metallic bonds. In the case of van der Waals solids (e.g., Ne, CO2), the lattice energy can be calculated by summing up the pair potentials of interacting atoms using a secondary bond potential for atomic interactions. For ionic solids (e.g., NaCl, ZnS), the Coulomb interaction, supplemented with a strongly repulsive force, is used in place of the secondary bond potential. In covalent (e.g., diamond, graphite) and hydrogen-bonded (e.g., H2O) materials, the calculation of the lattice energy is much more complicated, and the lattice energy cannot simply be calculated as a sum over pair potentials acting between atoms.
The growth and size of any crystal depends on the conditions of its formation. Temperature, pressure, the presence of impurities, etc., will affect the size and perfection of the crystal. As a crystal grows, different imperfections may occur which can be classified as either point defects, line defects (or dislocations), or plane defects. Point defects include missing atoms or substituted atoms; line defects are defects that extend along straight or curved lines in a crystal; plane defects extend along true planes or curved surfaces within crystals.
Experiments in decreased gravity conditions aboard the space shuttles and in Spacelab I demonstrated that proteins formed crystals rapidly, and with fewer imperfections, than is possible under normal gravitational conditions. This is important because macromolecules are difficult to crystallize, and usually will form only crystallites whose structures are difficult to analyze. Protein analysis is important because many diseases (including Acquired Immune Deficiency Syndrome, AIDS) involve enzymes, which are the highly specialized protein catalysts of chemical reactions in living organism. The analysis of other biomolecules important in cellular or genetic regulatory mechanisms may also benefit from these experiments.
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