Low-Temperature X-Ray Diffraction: Apparatus and Techniques

Low-temperature X-ray diffraction (LTXRD) investigations offer many challenges to the diffractionist, not all of which are technical or scientific in nature. LTXRD studies can be frustrating: There are at least two reports of investigations ruined by the loss of crystals (grown with extreme difficulty) because of the widespread power failure and blackout in the northeastern United States in late 1965. LTXRD studies can cause discomfort: In several instances, "low temperatures" have been attained by opening all the windows in the X-ray laboratory. LTXRD studies can be dangerous: It was once reported that a crys- tal was lost because a laboratory assistant fell down a flight of stairs and lay unconscious for about an hour on his way to refilling a liquid-nitrogen (LN2 ) dewar. This last report indicated the disposition of the crystal but not that of the laboratory assistant. However, in general, the results of low-temperature X-ray diffraction investigations cannot be obtained in any other manner, and one is well compensated for the effort expended in constructing and maintaining a low-temperature system. Crystal-structure analyses of solidified liquids and gases, phase transformation investigations, accurate crystal-structure analy- ses and electron-density maps, thermal expansion measurements, and defect structure studies are a few of the many important applications of LTXRD.