Tellurium, allotropy

Sulphur, selenium and tellurium exhibit allotropy, and in certain of their crystalline forms the elements are isomorphous. As would be expected from the increased positive character of tellurium, the allotropy of this element is less well defined. In the liquid condition the elements arc miscible with one another and yield mixed crystals the ternary system, S— Sc—Tc, exhibits neither the formation of compounds nor ternary eutectics, but contains two zones of complete miscibility in which there exist mixed crystals of selenium and tellurium with sulphur, and of sulphur and tellurium with selenium.2... 

Liquid selenium appears to exhibit allotropy, hut tellurium does not. 

Solid selenium likewise exists m several allotropic forms. The i crystalline variety is labile, and may possibly occur in two modification both of which arc monoclmic.2 The grey crystalline form appears consist of two varieties, SeA and ScD, in dynamic equilibrium with ea other.1 Solid tellurium manifests allotropy, but to a much less pi nounccd degree. 

Sulfur exhibits allotropy and its structure in all phases is quite complex. The common crystalline modification, rhombic sulfur, is in equilibrium with a triclinic modification above 96°C. Both have structures based on Sg-rings but the crystals are quite different. If molten sulfur is poured into water a dark red plastic form is obtained in a semielastic form. The structure appears to be a helical chain of S atoms. Selenium and tellurium both have a gray metal-like modification but sulfur does not have this form. 

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