Selenium, Physical and Chemical Properties
Selenium is a by-product of the extraction of copper and nickel from their sulfide ores. It is concentrated in the anodic slimes of electrorefining of these metals. Red amorphous selenium is produced by reducing selenous acid with sulfur dioxide, by crystallization from solution, or by quenching of selenium vapor or molten selenium. It consists of Se 8 rings similar to elemental sulfur, soluble in carbon disulfide giving red solutions from which selenium can be crystallized. Red selenium is an electrical nonconductor. On heating to >100°C, is transformed into the stable, gray, hexagonal metal-like form in an exothermic reaction. In the vapor phase, red selenium exists mainly as Se 8 rings (Fig. 1a). At higher temperature these decompose into smaller units, eventually forming Se 2 molecules.
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