  Using the LCLS facility at the SLAC National Accelerator Laboratory, we have
observed X-ray scattering from iron compressed with laser driven shocks to
Earth-core like pressures above 400GPa. The data shows shots where melting is
incomplete and we observe hexagonal close packed (hcp) crystal structure at
shock compressed densities up to 14.0 gcm-3 but no evidence of a
double-hexagonal close packed (dhcp) crystal. The observation of a crystalline
structure at these densities, where shock heating is expected to be in excess
of the equilibrium melt temperature, may indicate superheating of the solid.
These results are important for equation of state modelling at high strain
rates relevant for impact scenarios and laser-driven shock wave experiments.
