Structure and Phase Transformation of Rhombohedral Selenium Composed of Se₆ Molecules

A new allotrope of selenium has been found. X-ray analysis has revealed that the new form consists of hexameric molecules, Se₆, with symmetry D_3d. It has been ascertained by rotational and Weissenberg methods that the crystal has a rhombohedral unit cell with lattice parameters in hexagonal axes, a=11.362±0.001 A, c=4.429±0.008 A, and belongs to space group R. Refinement of atomic parameters using the full matrix least squares method gives the following structural quantities: x=0.16021±0.00048, y=0.20227±0.00047, z=0.12045±0.00120, bond length 2.356±0.009 A, bond angle 101.1±0.3°, and dihedral angle 76.2±0.4°. Studies on the transformation by DSC and X-ray measurements revealed that the rhombohedral phase is irreversibly converted to the polycrystalline trigonal phase at temperatures above 105°C.