Greenhouse Glass: Squeezing and heating carbon dioxide yields exotic, see-through solid

As ordinary citizens wring their hands over global warming from carbon dioxide in the atmosphere, scientists are wringing new chemical insights from the usually gaseous compound. In the latest extreme exploration, researchers in Italy have for the first time forged solid glass from carbon dioxide.

Mario Santoro and Federico A. Gorelli of the University of Florence and their colleagues made the glass, dubbed carbonia, by intensely squeezing dry ice—a crystalline arrangement of carbon dioxide molecules—between diamond jaws and heating it in a furnace. The severe conditions produce a disorderly, non-molecular arrangement of carbon and oxygen atoms linked by single bonds, Santoro says, instead of carbon dioxide’s typical molecular configuration—a carbon atom double-bonded to each of two oxygen atoms.

The new carbon dioxide glass formed at a pressure of 640,000 atmospheres and a temperature of 700 kelvins, the researchers report in the June 15 Nature. An estimated 10 times as hard as quartz yet softer than diamond, carbonia is “the hardest amorphous material known,” says Santoro.

The transparent solid is similar in structure to the ordinary amorphous silica in window glass. Six years ago, other researchers made another non-molecular form of carbon dioxide, called carbon dioxide 5, which has a crystalline, quartzlike structure.

These non-molecular carbon dioxides may occur naturally in the high-pressure interiors of giant outer planets such as Neptune, Santoro says.

Both carbonia and carbon dioxide 5 might be technologically useful if the substances could be made to persist under everyday conditions, notes Paul F. McMillan of University College London in a commentary accompanying the new report. The materials might also provide a means of consolidating environmentally troublesome, excess carbon dioxide for disposal, he adds.

So far, neither of the non-molecular carbon dioxide solids maintains its structure when the pressure’s off. However, mixing carbonia with silica at even higher temperatures might yield an unusually hard, mixed glass even when returned to room temperature and pressure, Santoro suggests.

The syntheses of carbonia and carbon dioxide 5 are exciting examples of “alchemy under high pressure,” comments Cornell University chemist Roald Hoffmann.

Although carbon and silicon are members of the same family in the periodic table of elements, they seem unrelated because carbon dioxide is typically a gas whereas silicon dioxide, or silica, is commonly crystalline quartz or glass. However, high pressure transforms the chemistry of carbon to be more like that of its cousin silicon, Hoffmann notes, evidenced by carbonia resembling silica and carbon dioxide 5 mimicking quartz.