Metastability in shocked iron: Controversy with regard to sound velocity and temperature measurements

A discrepancy still exists between key shock temperature measurements and corresponding results from laser-heated diamond anvil cells for iron. Shock melting temperatures for iron deduced from anvil interface temperature measurements exceed the melting curve from static experiments. Also, current shock sound velocity work is not reproducing earlier shock sound velocity experimental results indicating a solid-solid phase transition at 200 GPa along the Hugoniot. It has been suggested that the disparity in these measurements is due to metastability and superheating of shocked solid phases beyond equilibrium phase boundaries on the short dynamic time scale. Indeed, various shock temperature data sets on alkali halides, quartz and forsterite have exhibited superheating of these ionic and covalent solids beyond the normal melting curves. However, metals such as iron have different interatomic potentials than do atoms, in, say, ionic solids, and one would expect less metastability in shocked states. I examine these claims of metastability in light of available data and molecular dynamics calculations of the region behind the shock front.