PARIS, 15 June 2006 - Scientists in Italy have discovered that carbon dioxide can form a glass, the British journal Nature reports today.

Under extremely high pressures of up to half a million atmospheres, molecules of carbon dioxide (CO₂) form a glassy crystalline solid, they found.

Carbon is a member of the same chemical group as silicon and germanium.

These latter elements combine with oxygen to provide glass - one is silicon dioxide, which is used for windows, bottles and other everyday glasses, and the other is germanium dioxide, which is added to silica to provide the fibres used in optical communication.

Carbon, though, is the odd one out, because it combines with oxygen to provide the poisonous gas carbon monoxide or the suffocating gas carbon dioxide. CO₂, when frozen and squeezed, also forms the "dry ice" which provides atmospheric smoke for stage shows.

The new material has been baptised amorphous carbonia, or a-CO₂.

At present, a-CO₂ is a curiosity because it cannot be tested or used outside the pressure chamber. The CO₂ that in these extraordinary conditions takes up a chaotic "amorphous" structure, becoming glass, reverts to orderly molecules of CO₂ under decompression.

The first challenge will be to develop a form of a-CO₂ that can survive in room temperatures.

"Carbonia-based minerals and glasses could give rise to useful technological materials, if we can recover them to ambient conditions," says Paul McMillan, a University College London chemist.

The discovery of a-CO₂ raises intriguing questions about the huge gas planets in the outer Solar System.

In theory, the enormous pressures of Jupiter, Saturn and the other gas giants could turn the planetary interior into hard, stiff a-carbonia.

The paper is lead-authored by Federico Gorelli and Mario Santoro of the European Laboratory for Non-Linear Spectroscopy (LENS), based in Florence, Italy.