Shock-Compressed Diamond: Melt, Insulator-Conductor and Polymeric-Atomic Transitions

Temperatures measured on the shock-Hugoniot of diamond reveal melting between 650 ({+-} 60) GPa and 9000 ({+-} 800) K and 1090 ({+-} 50) GPa and 8400 ({+-} 800) K, with a heat of fusion of {approx} 25 {+-} 10 kJ/mole and a negative Clapeyron slope {partial_derivative}T/{partial_derivative}P|{sub melt} = -5 {+-} 3 K/GPa. Thus, the fluid is denser than the compressed solid, and optical reflectivity measurements show it to be metallic. Hugoniot-temperature measurements extending to over 4000 GPa (40 Mbar) and 115,000 K suggest de-polymerization of a dense covalently-bonded fluid to an atomic state between 10,000 and 30,000 K. These experimental results indicate that carbon present deep inside planets such as Uranus and Neptune could be solid for through-going convection, whereas stable stratification would allow for the presence of fluid metallic carbon at depth; in either case, the presence of carbon could potentially affect planetary seismic normal modes.