The element sulfur is found in the form of several allotropes, two of which, monoclinic and rhombic sulfur, are shown below.
Left: Rhombic sulfur (Courtesy of Ken O’Donoghue), Right: Monoclinic sulfur (Courtesy of E.R. Degginger)

Both monoclinic and rhombic sulfur consist of cyclic molecules of eight sulfur atoms.

Cyclic molecules of solid sulfur (S₈)

The forms of sulfur found at ordinary temperatures and pressure act as insulators of electric current. However, studies of sulfur at high pressure show evidence for transitions to different structures that constitute metallic phases (superconductivity is associated with structural changes from one metallic crystal structure to another metallic structure, and that the second structure is distorted compared to the first. Special electromagnets based on superconducting materials are widely used in medicine for magnetic resonance imaging (MRI). In general, superconducting materials only exhibit this property at extremely low temperatures, i.e., lower than the temperature of liquid hydrogen (20 K).

The results with sulfur are particularly important because its metallic phases have the highest critical temperature for the onset of superconductivity of any solid element so far investigated. Moreover, this critical temperature increases with pressure, which is unusual. For example, selenium and tellurium, which are members of the same family as sulfur, show the opposite effect (4). Nobody knows why this is so. The significance of the results described here is that sulfur provides expanded opportunities for testing theories of superconductivity. Researchers are planning to go to even higher pressures to study these unusual effects.

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