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Home , Molecular solid, Polyamorphism

NATURE|Vol 441|15 June 2006 NEWS & VIEWS

by Daw et al.1, exploring these questions 1. Daw, N. D. et al. Nature 441, 876–879 (2006). ambient conditions, preventing further study will require interdisciplinary approaches, in 2. Lee, D. Curr. Opin. Neurobiol. 16, 191–198 (2006). of its physical properties. During decompres- 3. Sutton, R. S. & Barto, A. G. Reinforcement Learning: An which the computational theories inspire Introduction (MIT Press, Cambridge, MA, 1998). sion from the conditions under which it is and guide behavioural and neurobiological 4. Ramnani, N. & Owen, A. M. Nature Rev. Neurosci. 5, formed, the transforms back to an amor- ■ experiments. 184–194 (2004). phous version of molecular CO2. Devel- Daeyeol Lee is in the Department of Brain and 5. Schweighofer, N. & Doya, K. Neural Netw. 16, 5–9 (2003). oping carbonia that can be recovered to Cognitive Sciences, University of Rochester, 6. Soltani, A., Lee, D. & Wang, X.-J. Neural Netw. (in the press). 7. Doya, K. Neural Netw. 15, 495–506 (2002). ambient conditions will be one of the first Rochester, New York 14627, USA. 8. Aston-Jones, G. & Cohen, J. D. Annu. Rev. Neurosci. 28, challenges for future research. e-mail: [email protected] 403–450 (2005). These results1 have implications for - state physics, and inform us about the condi-

tions under which CO2 undergoes various phase transitions, including melting7. There SOLID-STATE CHEMISTRY has been much discussion of the newly recog- nized phenomenon of polyamorphism — the ability of a material to exist in several different A glass of dioxide amorphous forms — and of phase transitions Paul F. McMillan between distinct liquid states of a single sub- stance. These are driven by density or entropy Carbon is unusual in its family of elements because it has gaseous oxides. changes between glassy or liquid forms at con- 8 But under high pressure, carbon dioxide forms crystalline and can stant chemical composition . It has been sug- gested that such transitions occur for many become a glass — so revealing the chemical family resemblance. and glasses, including , silica and germania. The observation by Santoro et al.1 Everyone is familiar with the common forms tion to 80 GPa at ambient temperature3. that, upon decompression, glassy carbonia of dioxide (SiO2, silica), such as the Recently, however, experiments showed that transforms from a dense crystalline version known as quartz, the major under simultaneous high-pressure and high- containing networks of single bonds, to an component of sand. When melted and cooled temperature conditions, CO2 amorphous solid containing molecular CO2, rapidly, and usually mixed with oxides, undergo bond-breaking and re-formation reac- represents a dramatic example of polyamor- sand forms silica-based glasses that we use to tions, producing a three-dimensional network phic behaviour, analogous to that observed for 9,10 make many useful things, ranging from win- of polymerized tetrahedral CO4 units. This net- amorphous silicon or liquid . It dows to champagne bottles. A glassy dioxide work is analogous to the crystalline silica struc- follows that anomalies are to be expected in of (GeO2, germania) is also tures found in minerals such as cristobalite, the melting behaviour of crystalline CO2 known, and is added to the kilometre-long tridymite or quartz4,5, although it reverts to solids, or that a may occur in silica glassy fibres used in optical telecommu- solids containing CO2 molecules at pressures the liquid state at high pressure, between the nications, to control their refractive index. below 1 GPa. Other - and -con- molecular and the polymeric forms. Carbon belongs to the same family of elements taining molecules also undergo solid-state The discovery of mineral-like polymeric as silicon and germanium, but carbon dioxide chemistry at high pressure6. These results have and ionic solids that form at high pressure,

(CO2, carbonia) glass has remained a theoret- generated great excitement among chemists. based on the light elements carbon, oxygen ical possibility. On page 857 of this issue, The newly prepared materials might have and nitrogen, opens up a new area in solid- however, Santoro and colleagues1 show that useful properties for technological applications. state chemistry. Carbonia-based minerals and carbonia glass can be made when a high- Studies on the polymeric, silica-like form of glasses could give rise to useful technological density form of solid CO2 is melted and cooled CO2 suggest that it is ‘super-hard’ and that it is materials, if we can recover them to ambient under high-pressure conditions. an optically nonlinear material — for example, conditions. These findings will also help set Carbon, silicon and germanium are the first it causes doubling of the frequency of light from the rules for understanding structure, bonding three members of group IV of the periodic a laser4,5. The discovery also has implications for and thermodynamic properties as we move table, which also includes the heavier elements geochemistry, because the conditions found in our experiments into the high-pressure, high- tin and lead. These last two elements form both Earth’s mantle could induce the formation of temperature conditions mimicking those deep monoxides and dioxides, which have been used these newly discovered forms of carbon oxides. inside planetary interiors. ■ 2 as pigments since ancient times . In contrast, Incorporation of CO2 into solid-state com- Paul F. McMillan is in the Department of only the dioxides of silicon and germanium pounds formed under high temperature and Chemistry, Christopher Ingold Laboratory and occur as stable solids. Once we reach carbon, pressure may even lead to methods for disposal Materials Chemistry Centre, University College the lightest member of the series, we find that of this environmentally problematic gas. London, 20 Gordon Street, London WC1H 0AJ, both carbon monoxide and carbon dioxide are So, a crystalline silica-like form of CO2 has UK, and in the Davy–Faraday Laboratory, Royal stable molecules, but these exist as gases at been made, but how about a glassy version? Institution of Great Britain, London. ambient temperature and pressure. As we rise Enter Santoro and colleagues1, who have pre- e-mail: [email protected] up a group of the periodic table, such anom- pared a dense, polymeric amorphous form of alous jumps in the properties of compounds this compound that is analogous to silica- and 1. Santoro, M. et al. Nature 441, 857–860 (2006). 2. Ball, P. Bright Earth: The Invention of Colour (Viking, London, formed from the elements of that group often germania-based glasses. They did this by com- 2001). occur. Smooth changes in chemical and physi- pressing molecular solid CO2 to pressures of 3. Liu, L.-G. & Bassett, W. A. Elements, Oxides, Silicates: High- cal properties are generally observed among the 40–65 GPa with heating, then cooling it to Pressure Phases with Implications for the Earth's Interior heavier members of the group, but the lightest ambient temperature. Their result is at least as (Oxford Univ. Press, 1986). 4. Iota, V. et al. Science 283, 1510–1513 (1999). element behaves quite differently. exciting as the discovery of the polymeric crys- 5. Yoo, C.-S. et al. Phys. Rev. Lett. 83, 5527–5530 (1999). Carbon dioxide is a linear with talline carbon dioxide solid, and the chemistry 6. Eremets, M. I. et al. J. Chem. Phys. 121, 11296–11300 (2004). strong carbon–oxygen double bonds. Freezing of carbonia-based glasses is now open for 7. Tschauner, O. et al. J. Phys. Rev. Lett. 87, 075701 (2001). or high-pressure treatment condenses this exploration. 8. McMillan, P. F. J. Mat. Chem. 14, 1506–1512 (2004). 9. Katayama, Y. et al. Nature 403, 170–173 (2000). gas into solid forms, in which the molecules So far, glassy carbonia is like its crystalline 10. McMillan, P. F., Wilson, M., Daisenberger, D. & Machon, D. remain intact, even under extreme pressuriza- equivalent in that it is not yet recoverable to Nature Mater. 4, 680–684 (2005).

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