Pressuring oxygen to turn red.

Applying tremendous pressure to crystals can force drastic changes in the chemical bonds between atoms. At pressures greater than 10 gigapascals (GPa), for example, solid oxygen undergoes a significant volume reduction and a dramatic shift to a deep red color, signaling the formation of a new molecular structure. Now, physicists report spectroscopic evidence suggesting that in this red, high-pressure form, neighboring pairs of oxygen molecules, each consisting of two oxygen atoms, combine into four-atom units. Federico A. Gorelli and Roberto Bini of the University of Florence and their coworkers report their findings in the Nov. 15 PHYSICAL REVIEW LETTERS.

The formation of [O.sub.4] molecules may serve as a crucial step in oxygen's transformation into long chains, then into a metal at even higher pressures, the researchers conclude. Last year, a Japanese team reported that solid molecular oxygen becomes a superconducting metal at pressures exceeding 100 GPa, about 1 million times atmospheric pressure (SN: 7/18/98, p. 47).