For example, a cube-shaped crystal with cubic integrant molecules green color in the above figure can be transformed to a dodecahedral crystal with rhombic faces. To do this, we can add new layers of integrant molecules on each surface of the cubic crystal gray color in the above figure but each time diminish one line of molecules from each side. If we just diminish one molecule for two opposite sides while diminish two molecules for the other two sides when each time we overlay a layer of integrant molecules, the cubic crystal will be transformed to a dodecahedral crystal with pentagonal faces not an orthogonal dodecahedron.
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Top row: one type of close-packed structure of equal spheres. Bottom row: crystal structures similar to sodium chloride left and to cesium chloride right. Scientists predicted some possible crystal structures before the invention of X-ray diffraction method. One good example as a series of crystal structures published by Barlow in For a crystal that only consisted of one type of atoms, he regarded all atoms as solid spheres. As a result, the crystal structure was equivalent to the closest packing of such solid spheres.
There are actually two types of closest packing but only one of them is shown in the above figure. Barlow further analyzed the closest packing of two types of spheres and correctly predicted the structures similar as NaCl and CsCl. Dublin Soc. In , 17 years after Roentgen discovered X-ray, von Laue suddenly came up with a splendid idea.
He wondered if diffraction would occur when a beam of X-ray pass through a crystal since the characteristic wavelength of X-ray is close to the spacing between atoms in a crystal. In a couple of months, he proved this assumption with the help of two excellent experimentalists and they acquired the first photo of X-ray diffraction. The upper left image illustrates the X-ray diffraction photo of sphalerite took by Laue and his colleagues in Einstein highly appraised X-ray diffraction as one the greatest physics experiments.
Nature , ].
Bragg and his son, W. Laue incorrectly interpreted the diffracted X-ray as an excitation consequence when the incident X-ray interacted with the crystal. However, W. Bragg and W. According to this principle, they changed the configuration of the apparatus from transmission to reflection. Therefore, solving a crystal structure finally became a reality with their contributions and the structural analysis of NaCl and diamond were very significant results at that time.
Afterwards, the X-ray diffraction technology was developed rapidly. Scientist now can use it to solve the structures of inorganic materials, organic molecules, DNA, and proteins We will see the related applications later in this book. X Rays and Crystal Structure ]. Top left: edge dislocation, top right: crystal plane schematic of edge dislocation, bottom left: screw dislocation, bottom right: crystal plane schematic of screw dislocation.
There are always a variety of defects in crystal structures. We will introduce one type of defects, dislocation, which includes two basic types: edge dislocation and screw dislocation. The concept of dislocation was firstly suggested by Tyler et al. In , Hirsch et al. The mechanical properties of metal are affected to dislocations. When plastic deformation applied to metals, new dislocations are generated and the gliding of dislocations occurs. In addition, screw dislocations influence crystal growth and cause the helical structure that could be observed under a microscope.
Dislocations in Crystals ]. In , D. Shechtman observed a diffraction pattern that he hardly believed when he was using electron diffraction to study a fast quenched Al-Mn alloy above image.
List of Crystallographers
Substances including inorganic salts and minerals, semiconductors, and organic and biological compounds can form crystals under suitable and specific conditions. The method is useful in determining the structure of molecules, which allows researchers to characterize and understand their behavior and function. This method of structure determination has provided the most reliable evidence scientists have about the way molecules are shaped and what their bonds angles and lengths are. The best x-ray crystallographic structures are derived from the purest crystal samples, meaning samples that contain only molecules of one type and as few impurities as possible.
Spectroscopy in Organic Chemistry: Crystallography
Crystal samples contaminated with impurities, samples that are too small, and samples that are not uniform may result in the formation of imperfect crystals, whose defects affect the quality of the data that can be obtained. This process reveals the geometry of the atoms within the molecules. The x-ray beams are diffracted in a characteristic pattern that gives rise to reflections, dark spots on the detector which represent places where constructive interference of the diffracted light has occurred.
The detector records the reflections on a two-dimensional surface. The crystal is typically rotated with respect to different axes and shot again with X-rays, so that diffraction patterns from all angles of the X-rays hitting the crystal are recorded. Then mathematical algorithms are applied in order to decode the information contained within the recorded reflections.
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A map is constructed to describe the electron density of the molecules in the crystal. Atomic models of the molecules are also created; these can explain the experimentally observed electron density. The final result is the three-dimensional structure of the molecules in the crystal.