Probing the Stability of hcp Fe Near Melting Using X-ray Diffraction Under Dynamic Compression

Dynamic compression experiments are a powerful tool for investigating the structural and thermodynamic behavior of materials under extreme conditions. At the Omega EP laser facility, we utilized advanced pulse shaping techniques to achieve shock-ramp compression of Fe to pressures exceeding 400 GPa near the melt curve. X-ray diffraction (XRD) was employed to examine the structural stability of the hexagonal close-packed (hcp) phase of Fe under these conditions. Our XRD results show no evidence for the emergence of a high-temperature body-centered cubic (bcc) phase, even at elevated temperatures near melting. By integrating diffraction data with velocity interferometry (VISAR) measurements, we constrained the density, pressure, and structural response of Fe along off-Hugoniot compression paths. These findings provide valuable insights into the phase behavior of Fe at conditions relevant to Earth's core. Complementary EXAFS measurements at similar conditions were used to further constrain the temperature of the the melt curve.