The monoclinic-to-trigonal transformation in selenium

Abstract

The conversion of monoclinic selenium to trigonal selenium is studied by x-ray diffraction, optical microscopy, and thermal analysis between 343 and 487 K. Below the melting temperature of the monoclinic crystal the process is topochemical, but not topotactic. The melting of the monoclinic crystals to the pure Se8- ring melt occurs at 413 K (with a heat of fusion of 2.8 kJ/mole Se, and an entropy of fusion of 6.8 J/K/mole Se). The nucleation of the trigonal phase in the monoclinic single crystal is athermal and occurs in crystalline register with the monomer, either through the nature of the nucleating defect or through epitaxy. The time for the appearance of the fixed temperature-independent number of nuclei is governed by an activation energy of about 100 kJ/mole. The subsequent growth occurs by crystallization during polymerization in the direction of the macromolecular helix axis, independent of the host crystal. Above 360–380 K, the trigonal phase branches to spherulitic crystals with radial c-axis orientation. The linear crystal growth is described over the whole analyzed temperature range by the same exponential (−1/T) rate law with an activation energy of 113 kJ/mole, which indicates that ring opening is the rate-determining step.