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Melting temperature of diamond at ultrahigh pressure

2009; Nature Portfolio; Volume: 6; Issue: 1 Linguagem: Inglês

10.1038/nphys1438

1745-2481

J. H. Eggert, D. G. Hicks, P. M. Celliers, D. K. Bradley, R. S. McWilliams, Raymond Jeanloz, James E. Miller, T. R. Boehly, G. W. Collins,

Geological and Geochemical Analysis

Measurements of the melting point of diamond at pressures of around 10 million atm suggest it could be present in crystalline form in the interiors of giant planets. At even higher pressures and temperatures about 50,000 K, diamond melts to form an unexpectedly complex, polymer-like fluid phase. Since Ross proposed that there might be ‘diamonds in the sky’ in 1981 (ref. 1), the idea of significant quantities of pure carbon existing in giant planets such as Uranus and Neptune has gained both experimental2 and theoretical3 support. It is now accepted that the high-pressure, high-temperature behaviour of carbon is essential to predicting the evolution and structure of such planets4. Still, one of the most defining of thermal properties for diamond, the melting temperature, has never been directly measured. This is perhaps understandable, given that diamond is thermodynamically unstable, converting to graphite before melting at ambient pressure, and tightly bonded, being the strongest bulk material known5,6. Shock-compression experiments on diamond reported here reveal the melting temperature of carbon at pressures of 0.6–1.1 TPa (6–11 Mbar), and show that crystalline diamond can be stable deep inside giant planets such as Uranus and Neptune1,2,3,4,7. The data indicate that diamond melts to a denser, metallic fluid—with the melting curve showing a negative Clapeyron slope—between 0.60 and 1.05 TPa, in good agreement with predictions of first-principles calculations8. Temperature data at still higher pressures suggest diamond melts to a complex fluid state, which dissociates at shock pressures between 1.1 and 2.5 TPa (11–25 Mbar) as the temperatures increase above 50,000 K.