A glass of carbon dioxide

Carbon is unusual in its family of elements because it has gaseous oxides. But under high pressure, carbon dioxide forms crystalline solids and can become a glass — so revealing the chemical family resemblance.

Everyone is familiar with the common forms of silicon dioxide (SiO₂, silica), such as the crystalline version known as quartz, the major component of sand. When melted and cooled rapidly, and usually mixed with metal oxides, sand forms silica-based glasses that we use to make many useful things, ranging from windows to champagne bottles. A glassy dioxide of germanium (GeO₂, germania) is also known, and is added to the kilometre-long silica glassy fibres used in optical telecommunications, to control their refractive index. Carbon belongs to the same family of elements as silicon and germanium, but carbon dioxide (CO₂, carbonia) glass has remained a theoretical possibility. On page 857 of this issue, however, Santoro and colleagues¹ show that carbonia glass can be made when a high-density form of solid CO₂ is melted and cooled under high-pressure conditions.

Carbon, silicon and germanium are the first three members of group IV of the periodic table, which also includes the heavier elements tin and lead. These last two elements form both monoxides and dioxides, which have been used as pigments since ancient times². In contrast, only the dioxides of silicon and germanium occur as stable solids. Once we reach carbon, the lightest member of the series, we find that both carbon monoxide and carbon dioxide are stable molecules, but these exist as gases at ambient temperature and pressure. As we rise up a group of the periodic table, such anomalous jumps in the properties of compounds formed from the elements of that group often occur. Smooth changes in chemical and physical properties are generally observed among the heavier members of the group, but the lightest element behaves quite differently.

Carbon dioxide is a linear molecule with strong carbon–oxygen double bonds. Freezing or high-pressure treatment condenses this gas into solid forms, in which the molecules remain intact, even under extreme pressurization to 80 GPa at ambient temperature³. Recently, however, experiments showed that under simultaneous high-pressure and high-temperature conditions, CO₂ molecules undergo bond-breaking and re-formation reactions, producing a three-dimensional network of polymerized tetrahedral CO₄ units. This network is analogous to the crystalline silica structures found in minerals such as cristobalite, tridymite or quartz^4,5, although it reverts to solids containing CO₂ molecules at pressures below 1 GPa. Other nitrogen- and oxygen-containing molecules also undergo solid-state chemistry at high pressure⁶. These results have generated great excitement among chemists.

The newly prepared materials might have useful properties for technological applications. Studies on the polymeric, silica-like form of CO₂ suggest that it is ‘super-hard’ and that it is an optically nonlinear material — for example, it causes doubling of the frequency of light from a laser^4,5. The discovery also has implications for geochemistry, because the conditions found in Earth's mantle could induce the formation of these newly discovered forms of carbon oxides. Incorporation of CO₂ into solid-state compounds formed under high temperature and pressure may even lead to methods for disposal of this environmentally problematic gas.

So, a crystalline silica-like form of CO₂ has been made, but how about a glassy version? Enter Santoro and colleagues¹, who have prepared a dense, polymeric amorphous form of this compound that is analogous to silica- and germania-based glasses. They did this by compressing molecular solid CO₂ to pressures of 40–65 GPa with heating, then cooling it to ambient temperature. Their result is at least as exciting as the discovery of the polymeric crystalline carbon dioxide solid, and the chemistry of carbonia-based glasses is now open for exploration.

So far, glassy carbonia is like its crystalline equivalent in that it is not yet recoverable to ambient conditions, preventing further study of its physical properties. During decompression from the conditions under which it is formed, the glass transforms back to an amorphous version of molecular solid CO₂. Developing carbonia glasses that can be recovered to ambient conditions will be one of the first challenges for future research.

These results¹ have implications for liquid-state physics, and inform us about the conditions under which CO₂ undergoes various phase transitions, including melting⁷. There has been much discussion of the newly recognized phenomenon of polyamorphism — the ability of a material to exist in several different amorphous forms — and of phase transitions between distinct liquid states of a single substance. These are driven by density or entropy changes between glassy or liquid forms at constant chemical composition⁸. It has been suggested that such transitions occur for many liquids and glasses, including water, silica and germania. The observation by Santoro et al.¹ that, upon decompression, glassy carbonia transforms from a dense amorphous solid containing networks of single bonds, to an amorphous solid containing molecular CO₂, represents a dramatic example of polyamorphic behaviour, analogous to that observed for amorphous silicon or liquid phosphorus^9,10. It follows that anomalies are to be expected in the melting behaviour of crystalline CO₂ solids, or that a phase transition may occur in the liquid state at high pressure, between the molecular and the polymeric forms.

The discovery of mineral-like polymeric and ionic solids that form at high pressure, based on the light elements carbon, oxygen and nitrogen, opens up a new area in solid-state chemistry. Carbonia-based minerals and glasses could give rise to useful technological materials, if we can recover them to ambient conditions. These findings will also help set the rules for understanding structure, bonding and thermodynamic properties as we move our experiments into the high-pressure, high-temperature conditions mimicking those deep inside planetary interiors.