Neutron diffraction study of molecular motion in solid deuterium chloride

Mean square thermal displacements of the chlorine and deuterium atoms were derived both in the ordered orthorhombic and in the disordered cubic phase of deuterium chloride by a least-squares study of the neutron powder diffraction patterns recorded at four temperatures over the 4.2 to 111.5 K range. The atomic displacements were subsequently interpreted as due to the superposition of the internal stretching vibrations and of the rigid-body translational and angular vibrations of the molecules. The analysis of the results revealed substantial rigid-body vibrational components in the zero-point motion of the molecules and a rapid rise in the amplitude of the in-plane angular vibrations on approaching the transition point. It also showed a small but possibly significant lengthening of the CI—D bond as the temperature decreases. Finally, it gave a fuller understanding of the disordered cubic phase as a dynamic mixture of short-lived polymers of varying lengths and shapes.