Ionic structure, liquid-liquid phase transitions, x-ray diffraction, and x-ray Thomson scattering in shock-compressed liquid silicon in the 100-200 GPa regime.

Abstract

Recent cutting-edge experiments have provided in situ structure characterization and measurements of the pressure (P), density (ρ[over ¯]), and temperature (T) of shock compressed silicon in the 100 GPa range of pressures and up to ∼10000K. We present first-principles calculations in this P,T,ρ[over ¯] regime to reveal a plethora of novel liquid-liquid phase transitions (LPTs) identifiable via discontinuities in the pressure and the compressibility. Evidence for the presence of a highly-correlated liquid (CL) phase, as well as a normal-liquid (NL) phase at the LPTs is presented by a detailed study of one LPT. The LPTs make the interpretation of these experiments more challenging. The LPTs preserve the short-ranged ionic structure of the fluid by collective adjustments of many distant atoms when subject to compression and heating, with minimal change in the ion-ion pair-distribution functions, and in transport properties such as the electrical and thermal conductivities σ and κ. We match the experimental x-ray Thomson scattering and x-ray diffraction data theoretically, and provide pressure isotherms, ionization data, and compressibilities that support the above picture of liquid silicon as a highly complex LPT-driven "glassy" metallic liquid. These novel results are relevant to materials research, studies of planetary interiors, high-energy-density physics, and in laser-fusion studies.