Abstract
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.
Highlights
Iron is the stable product of nuclear burn in massive stars and highly abundant on many planets in and outside our solar system
Using the Linac Coherent Light Source facility at the Stanford Linac Coherent Light Source National Accelerator Laboratory, we have observed x-ray scattering from iron compressed with laser-driven shocks to earth-core-like pressures above 400 GPa
Our x-ray-scattering measurements were taken on samples of warm dense iron created using laserdriven shock compression at the Matter in Extreme Conditions (MEC) end station of the Linac Coherent Light Source (LCLS) x-ray free-electron laser [21,22]
Summary
Iron is the stable product of nuclear burn in massive stars and highly abundant on many planets in and outside our solar system. The position of the melting transition between the solid inner core and the liquid outer core determines the inner structure of the planet and the generation of its magnetic field and restricts the abundance of light elements in the core To reach these extreme conditions, shock-wave experiments are commonly applied, often with optical diagnostics. As shock heating is in excess of the equilibrium melt temperature, these data demonstrate important timescale effects during the phase changes: We propose superheating of the solid and a slow transition into a new lattice as an explanation These results have important implications for the interpretation of experiments and modeling of matter at high strain rates.
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