Spectra of Plastic and Disordered Crystals

Abstract

The crystallization of many almost-spherical molecules (called globular molecules) leads to a new phase of matter which has been called [1–8] a “plastic crystal”. Molecules in plastic crystals appear to possess unusual freedom and the positions of the atoms are less definite than usually found for molecular crystals. Consequently, the X-ray diffraction patterns are less well defined so that the patterns may be limited to only a few reflections. Since disorder, which includes thermal motion, in a crystal effects high order reflections more than low order reflections, and since the molecules in plastic crystals have considerable freedom of movement, a plastic crystal characteristically gives a relatively small number of reflections with considerable background scattering [9]. For example, the X-ray diffraction of cyclobutane in its plastic phase has only two reflections [10]. For cyclohexane, a decrease in the number of lines in the diffraction pattern was found with increasing temperature. The plastic phase I usually has a body centered, or face-centered cubic lattice, but there are many exceptions. For example, cyclopentane [11] has a hexagonal lattice, while methanol [12] is orthorombic in phase I.