Pressure-induced phases of sulfur.
At least three phases of sulfur may be induced at pressures between 16 and 65 kilobars. A fibrous form obtained at pressures of 27 kilobars and higher appears to be closely related to the form obtained when plastic sulfur is chilled and stretched. Crystals of the fibrous form are orthorhombic with a = 13.8, b = 32.4, and c = 9.25 A. Thus far the results are in accord with deductions made by Prins and co-workers that the sulfur atoms are arranged in helices with ten atoms and three turns per 13.8-A period, and that these helices are essentially close-packed. The unit cell contains 16 ten-atom chain lengths. The probable space groups to which the crystal may belong are C c m m, C c 2 m, or C c m 2(1). These imply that the structure must contain both right- and left-handed helices and that at least half the helices have some disorder about their axes. The other two phases appear to have structures related to that of the fibrous form, but analyses of them has not progressed as far. One of these phases appears to be uomegasulfur.