Sulfur: cool, compact, and conductive.

Squeezed under enormous pressures and cooled to a chilly 17 kelvins, sulfur turns into a superconductor, according to a new study. Although many of the new ceramic superconductors work at 100 kelvins or more, sulfur sets a record high temperature for a pure element conducting electricity without resistance.

Using a device called a diamond anvil cell, Russell J. Hemley of the Carnegie Institution of Washington (D.C.) and his colleagues compressed sulfur under about 1.6 million times atmospheric pressure (SN: 10/26/96, p. 261). The Carnegie group and a coworker from the Russian Academy of Sciences in Troitsk report their findings in the Nov. 27 Nature.

"I'm very excited about these results," says Marvin L. Cohen of the University of California, Berkeley. Two years ago, he and his colleagues developed computer models showing that sulfur should turn into a metal at high pressures and superconduct at low temperatures.

Originally, not many people believed the predictions. "You think of sulfur as that yellow powder that you mix into gunpowder," he says. "You don't think of it as a material that would be a metal, particularly a superconductor."

An experiment in the 1970s indicated that sulfur could turn metallic and superconductive, but no one had been able to duplicate those results. X-ray diffraction studies at Cornell University in 1993 showed that sulfur assumes different crystal structures at high pressures. Soon afterward, Cohen's group modeled the behavior of the structural transitions that the element should undergo. The Carnegie experiments now seem to bear out those predictions.

Measuring the electrical conductivity of a material in a diamond anvil cell is tricky. Because of the high pressures and tiny quantities involved, attaching two wires to the sample isn't feasible. Instead, the Carnegie group applied a magnetic field to the cell and monitored the changes in sulfur's magnetic properties that signal its transformation into a superconductor.

Selenium and tellurium, elements chemically similar to sulfur, also superconduct but at much lower temperatures and pressures. Sulfur, a simpler element, may serve as a good test material to study superconductivity theories, says Hemley. Researchers are interested both in superconductivity of the more complex ceramics and in the effects of high pressure on another pure element, hydrogen, which was condensed into a metal for the first time last year (SN: 4/20/96, p. 250).