Morphology of Spherulites and Single Crystals of Trigonal Selenium

Abstract

The crystallization of amorphous selenium is shown to occur by the formation of lamellas in which the molecular axis lies across the wide face and perpendicular to the growth direction. Below about 200°C the lamellas grow radially from a common nucleus branching noncrystallographically to produce a spherical crystalline mass. The theoretical selenium chain length in the quenched amorphous phase is much longer than the average lamella widths observed at large supercoolings. This indicates that at the edges the chains must fold back into the lamella if stress accumulation due to interconnection of the two phases of different density is to be avoided. It is suggested that such an array of chain folds introduces a dilation of the lattice which is accommodated if the lamella twists. Where it is cooperative, such twisting within the radial array of lamellas is shown to be responsible for the annular ring structure of selenium spherulites. In crystallization above 200°C the radial symmetry begins to breakdown and the lamella width approaches the mean chain length. Some fractionation then appears to take place to produce sets of lamellae of different widths which presumably contain fully extended chains. Clearer evidence of such fractionation was shown in a crystal grown under high pressure where the melt viscosity is lower. These smaller supercoolings also led to the growth of leaf-shaped single crystals having the same internal structure as the lamellas but with no set width. It is surmised that these are produced by a bundle-like accretion of chains. Such leaf crystals were observed within the bulk of `single' crystals grown by the traveling solvent or the Czochralski method. It is pointed out that the breakdown of periodicity in the selenium covalent bonding at impurity terminated chain ends, estimated at about 1016 to 1017/cc, or at chain folds, must have an appreciable influence on the charge transport properties of single crystals of trigonal selenium.