Practical Determination of Structure

Abstract

Although optical microscopy was an invaluable tool in the early days of crystallography, the information that it revealed was rather limited. Measurement of the angles between the faces of well defined naturally occurring mineral crystals enabled the overall symmetry of the crystal structure to be inferred. However, the large wavelength of visible radiation meant that the resolution obtainable was much too poor to resolve detail on an atomic scale, and no informaton as to the size of the unit cell and the actual positions of the atoms could be deduced. Evaluation of this detailed structure had to await the advent of X-rays during the first few years of this century. X-rays are electromagnetic waves like visible light but with a much smaller wavelength. This wavelength is typically 10⁻¹⁰ m and is of the same order of magnitude as the interatomic spacings in crystals, and so the network of atoms in solids acts like a naturally occurring three-dimensional diffraction grating. The techniques of X-ray crystallography have enabled the actual atomic positions in a vast number of simple and complex structures to be determined and, in addition, X-ray diffraction has provided valuable information on many other aspects of structure including preferred orientation in polycrystalline sheets or wires, and crystal imperfections such as grain boundaries and dislocations.