Pressure-induced complexity in dynamically compressed calcium

Abstract

In this project we studied the series of structural phase transitions that occurs in the alkali earth metal Ca as it transforms to increasingly complex arrangements with pressure. The experiments were designed specifically for the ultra-bright X-ray source of the LCLS and the optical capabilities of the MEC nanosecond drive laser. We studied in detail the unusual structural behavior near the minimum of Ca’s melt curve along the Hugoniot and investigated possible metastable phase formation as an explanation for large discrepancies in pressure-volume data from previous shock and static experiments. In total 200 data shots were taken. Using the laser pulse shaping capabilities for the MEC nanosecond laser permitted two thermodynamic compression paths to be explored: (i) Along the principal Hugoniot within the solid and liquid phases, (ii) along a double-shock path to access a cooler P-T compression path within the solid state. Probing the compressed Ca sample with the LCLS XFEL permitted structural determination of the Ca through x-ray diffraction. Pressure was determined with Doppler Interferometry (MEC VISAR diagnostic) and the known equation-of-state of Ca. These data represent the first direct measurements of lattice level rearrangement in Ca under shock compression. We observe the onset of melt, structure within the liquid and an evolution of crystal structure along off-Hugoniot paths up to 100 GPa. These experiments yield important insights into non-equilibrium phase formation under dynamic loading and the deviation away from nearly free electron like behavior, which has been found to be ubiquitous among the elements.