Sub-nanosecond phase transition dynamics in laser-shocked iron

Iron is one of the most studied chemical elements due to its socio-technological and planetary importance, hence understanding its structural transition dynamics is of vital interest. By combining a short pulse optical laser and an ultra-short free electron laser pulse available at PAL-XFEL, we have observed, for the first time, the sub-nanosecond structural dynamics of iron from high quality X-ray diffraction data measured at 50 ps intervals up to 2500 ps. We unequivocally identify a three-wave structure during the initial compression and a two-wave structure during the decaying shock, involving all of the known structural types of iron (α, γ and ε-phases). In the final stage, negative lattice pressures are generated by the propagation of rarefaction waves leading to the formation of expanded phases and the recovery of γ-phase. Our observations demonstrate the unique capability of measuring the atomistic evolution during the entire lattice compression and release processes at unprecedented time and strain rate.