Nitrogen performs under pressure

Scientists at the Carnegie Institution of Washington have created a new form of nitrogen by subjecting ordinary nitrogen gas to pressures of up to 2.4 million times the atmospheric pressure at sea level.

At these pressures the nitrogen is transformed to an opaque, semiconducting solid. The scientists also reported that, once created, the semiconducting solid could remain stable even when the pressure returns to normal.

‘The fact that the major portion of the air has been turned into a semiconducting solid and brought back to be stable at ambient pressure is an important breakthrough for us,’ said team leader Russell Hemley. Hemley and colleagues Mikhail Eremets, Ho-kwang Mao and Eugene Gregoryanz performed the research at Carnegie’s Geophysical Laboratory.

Since the 1930’s theorists have predicted that molecular nitrogen (N2) would become either a semiconductor or a metal if subjected to pressures on the order of a million atmospheres (100 gigapascals).

A similar theory holds for gaseous hydrogen, which is expected to turn into solid metallic hydrogen under similarly high pressures. Solid metallic hydrogen has yet to be produced in the laboratory.

Previous experiments have been limited in the amount of pressure that could be applied to nitrogen, and in the number of measurements that could be performed on the material while under pressure. Last year, the Carnegie scientists reported signs of the material’s transformation at room temperature, using optical techniques alone.

In their recent experiment, the investigators used newly developed techniques that allowed them to measure electrical conductivity at very high pressures and various temperatures.

They found that the non-molecular semiconducting form of nitrogen was stable over a wide pressure range, and some samples – when held at low temperature -retained this state when decompressed to atmospheric pressure.

The observations of the new from of nitrogen are said to suggest that other high-density materials could be created at high pressure and recovered at ambient pressure conditions.