X-ray Investigation of the Thermal Transitions in Some Molecular Crystals.

Abstract

Various phase changes which occur in crystals of hexamethylbenzene and beryllium oxyacetate have been investigated by X-ray methods.The crystal structure of hexamethylbenzene at high temperatures has been determined by single crystal oscillation method, (Cu Ka, λ=1.54 kX), in order to obtain some information about the anomalous volume change at high temperatures. It belongs to the orthorhombic system. The dimensions of the unit cell which contains four molecules of C6 (CH3) 6 are a=9.06 kX, b=15.70 kX and c=7.52 kX at 155°C. The space group was found to be D232h-Fmmm and the determined atomic parameters are shown in Table 2. Powder photographs were also taken at various temperatures, from which the change of the dimensions of the unit cell was deduced. The determined structure was discussed comparing with the structure at room temperatures which has been very accurately analyzed by Robertson and Brockway. The mechanism of the anomalous volume change was briefly discussed taking the thermal movements of the molecules into account.The crystal structures of different modifications of beryllium oxyacetate have been investigated by X-ray methods, (Cu Ka) . An attempt has been made to derive the expression for the change of the Bragg and diffuse reflections of X-rays associated with the transition about 40°C, from the viewpoint of the thermal movements of the acetate groups. It was shown that the derived intensity expression could account for the characteristic feature of the observed X-ray scattering.At 147°C the phase change of the first order occurs and when the temperature is raised above this point the substance assumes gradually the appearance of soft wax, though it melts sharply at 285°C. Powder photographs were taken at various temperatures. Most of the lines observed could be explained as due to a rhombohedral unit cell with dimensions a=9.72 kX. ar=54°20', containing one molecule in it. It will be noteworthy that this unit cell dimension is closely related to the cubic closest packing of spheres (rhombohedral lattice with ar=58°12') . This fact, combined with the observation that the X-ray intensity distribution in the powder photograph taken just below the melting point showed much resemblance to the liquid halo slightly above the melting point, suggests that the liquid state near the melting point may be considered to be cubic closest packing of molecules. Another modification of Be-oxyacetate can be obtained by sublimation, the crystal data of which are given.