Characterized Allotropes and Mixtures

In Section II, the structures of fourteen well-defined allotropes have been described. About forty other allotropes have been reported, but for these structural data are incomplete or contradictory. The reason for this varies. Some allotropes, such as r-sulfur that forms only in 2 out of 5000 experiments (29), are hard to prepare or are unstable. Others, such as w-sulfur are easy to prepare and reasonably stable, but X-ray data are contradictory and have led to much confusion and controversy. 

Molecule Color a b c Angle P(deg) Space cell Space group 

The reason for the lack of progress on this allotrope, which was first reported in 1939, and on which over a dozen publications have appeared, is often misunderstood. The difficulty is intrinsic. The existence of a great variety of intramolecular and intermolecular allotropes proves that many species must have comparable stability, i.e., that they can coexist. Thus, many contradictory allotropes are mixtures. We know now that the challenge of preparing and identifying a new allotrope does not end with its sjmthesis, but includes isolation of the metastable species from a mixture of equally metastable compounds, a process that 

Reagent Molecular species Well-established Inconclusive or mixture 

Solid sulfur Cyclo-Sg catena-Sgo Sg, Laminar, u, orange, metallic, cubic 

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