Under Pressure, Nitrogen Breaks Its Bonds

Nitrogen atoms love company. They always hook up with other atoms, and the covalent bond between two nitrogen atoms is among the strongest of its kind. Now scientists have succeeded in splitting the atoms apart. They squeezed nitrogen so hard that it became nonmolecular, and as they suspected, the debris was also a semiconductor--although it's unlikely to be a useful one.

Like hydrogen, oxygen, and iodine, nitrogen normally exists as two-atom molecules in the form of a gas at room temperature. In 1986, scientists found that iodine molecules split into atoms at very high pressures, and another experiment suggested that fiercer conditions might break up nitrogen molecules. Intense shock waves heated molecules to almost 6000 degrees Celsius and squeezed them to pressures a tenth of that at Earth's core. A sudden change in the behavior of the liquid nitrogen led researchers to suspect they'd separated the molecules. Following up on these hints, a team led by Ho-kwang Mao at the Carnegie Institution of Washington in Washington, D.C., tried to create this new form of nitrogen at room temperature.

Mao's team used a diamond anvil that can inflict the highest pressures ever created in a laboratory. It works about like the name suggests: Two flat-topped diamonds are mechanically pressed together. Between the diamonds is a metal gasket with a small hole, where a nitrogen sample is loaded. Squeezing the diamonds together flattens the gasket and subjects the nitrogen to pressures nearly half that at the center of Earth. Mao could watch the sample through the diamonds.

At this high pressure, nitrogen lost all signs of being a molecule--infrared light that normally is absorbed by nitrogen-nitrogen bonds passed right through the sample, they report in the 7 August Physical Review Letters. Another light-absorption test showed that the sample was a semiconductor--it conducted electrical currents, although the extreme pressure needed to create nonmolecular nitrogen makes it unlikely it could ever serve as a useful semiconductor, says team member Alexander Goncharov.

Sandro Scandolo, a theorist at Princeton University in New Jersey, says that the successful observation of nitrogen's nonmolecular structure, although it was predicted, came as a pleasant surprise. "A new form of matter is created," he says. Scandolo foresees more high-pressure experiments with hydrogen or oxygen that will create unexpected forms of matter.

Related sites
A profile of nitrogen
The Physical Review Letters abstract