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 {\it in situ} structure characterization and measurements of the pressure ($P$), density ($\bar{\rho}$) and temperature ($T$) of shock compressed silicon in the 100 GPa range of pressures and upto $\sim$10,000K. We present first-principles calculations in this $P,T,\bar{\rho}$ 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 $\sigma$ and $\kappa$. 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.