Dry ice forms ultrahard glass
Jun 14, 2006
Scientists have discovered a novel form of solid carbon dioxide. The new material, which was made by applying extreme pressures to normal solid carbon dioxide, resembles window glass on the atomic scale. Dubbed amorphous carbonia, the substance could be important for understanding the interiors of gas-giant planets in which carbon dioxide is squeezed at high pressures. It could also be used to make ultrahard glass because it is expected to be very stiff, like diamond (Nature 441 857).
Carbon is unlike the other elements in group IV of the periodic table because it forms a gas -- carbon dioxide -- when reacted with oxygen at room temperature. The other group IV elements, in contrast, form solids when combined with oxygen. Silicon, for example, forms crystalline silica (the mineral quartz) as well as amorphous silica glass (one of the main constituents of ordinary window glass), in which the silicon and oxygen atoms form a disordered network.
Although carbon dioxide can be solidified to form "dry ice", it only does so when squeezed under high pressure or cooled to low temperature. Moreover, dry ice is a molecular crystal, in which the crystal lattice consists of molecules of carbon dioxide rather than of individual carbon or oxygen atoms. A team led by Mario Santoro and Federico Gorelli of the University of Florence and the INFM has now been able to make amorphous carbon dioxide for the first time, in which the individual carbon and oxygen atoms for a continuous, disordered network structure, as in silica.
The researchers made the new a-carbonia by squeezing normal solid carbon dioxide to pressures of around 400,000 to 500,000 atmospheres (or 40 to 50 GPa). Infrared and laser Raman spectroscopy, along with X-ray diffraction, confirmed that the material was no longer made up of discrete molecules but had a disordered network structure.
The new material could have implications for planetary physics because the interiors of gas-giant planets, like Jupiter, contain carbon dioxide under pressures of more than 40GPa. "Another important implication is that mixtures of a-carbonia and a-silica could, in principle be used to make new amorphous glasses that would be very hard and stiff and likely stable at room temperature," adds Santoro. "Small amounts of these new glasses could be of interest for technology applications like hard and resistant coatings for micro-electronics, for example."
The team now plans to study a-carbonia at pressures higher than 80GPa to investigate whether or not it transforms into an amorphous material with a carbon coordination number greater than four, that is, each carbon is connected to more than four oxygen atoms. "This is known to happen for a-SiO2 and a-GeO2 and is crucial for providing insights into the fundamental thermodynamics of the whole class of network-forming systems to which a-carbonia belongs," explains Santoro.
About the author
Belle Dumé is science writer at PhysicsWeb