The crystal symmetry and powder pattern of hexamethylbenzene below 116 K

Laue patterns of very thin single-crystal platelets of hexamethylbenzene at a temperature (80 K) below the cold-phase transition (116 K) have been recorded. In this colder phase (Ph. III), whose flat plate pattern is interpretable as a precession photograph because of the probable ordered fagmentation of the sample, the crystal symmetry appears to be hexagonal (or trigonal), as confirmed also by previous optical and spectroscopic observations. The X-ray diffraction spectrum of the powder taken at the same temperature gives 3.59 Å as the distance between the planes containing the molecular rings, and was indexed on the basis of a hexagonal cell with a = 31.5, c = 3.59 Å, containing 12 molecules (probable space group P6mm, with all molecules lying in special positions); the length of the basal axis is exactly four times that of the elementary mesh obtained from the hexagonal grid on which the single-crystal pattern lies.