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Shock-state temperature and sound-speed measurements on crystalline materials, demonstrate superheating-melting behavior distinct from equilibrium melting. Shocked solid can be superheated to the maximum temperature, T_c′. At slightly higher pressure, P_c, shock melting occurs, and induces a lower shock temperature, T_c. The Hugoniot state, (P_c,T_c), is inferred to lie along the equilibrium melting curve. The amount of superheating achieved on Hugoniot is, Θ_H+=T_c′/T_c−1. Shock-induced superheating for a number of silicates, alkali halides and metals agrees closely with the predictions of a systematic framework describing superheating at various heating rates [Appl. Phys. Lett. 82 (12) (2003) 1836]. High-pressure melting curves are constructed by integration from (P_c,T_c) based on the Lindemann law. We calculate the volume and entropy changes upon melting at (P_c,T_c) assuming the R ln 2 rule (R is the gas constant) for the disordering entropy of melting [J. Chem. Phys. 19 (1951) 93; Sov. Phys. Usp. 117 (1975) 625; Poirier, J.P., 1991. Introduction to the Physics of the Earth’s Interior. Cambridge University Press, Cambridge, 102 pp.]. (P_c,T_c) and the Lindemann melting curves are in excellent accord with diamond-anvil cell (DAC) results for NaCl, KBr and stishovite. But significant discrepancies exist for transition metals. If we extrapolate the DAC melting data [Phys. Rev. B 63 (2001) 132104] for transition metals (Fe, V, Mo, W and Ta) to 200–400 GPa where shock melting occurs, shock temperature measurement and calculation would indicate Θ_H+∼0.7–2.0. These large values of superheating are not consistent with the superheating systematics. The discrepancies could be reconciled by possible solid–solid phase transitions at high pressures. In particular, this work suggests that Fe undergoes a possible solid–solid phase transition at ∼200 GPa and melts at ∼270 GPa upon shock wave loading, and the melting temperature is ∼6300 K at 330 GPa. |
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