Structure and dynamics in liquid iron at high pressure and temperature. A first principles study

Abstract

We have studied the evolution of structural and dynamic properties of liquid Fe as a function of pressure for 11 thermodynamic states close to the melting line. The pressure range considered goes from ambient pressure to 323 GPa, and the study has been carried out by using the ab-initio molecular dynamics technique. The agreement between the calculated static structure and the available experimental data is very good, including details like an asymmetric second peak, which remains over most of the whole pressure range and suggests a significant local icosahedral short-range order in the liquid. The dynamical structure is studied through the characteristics of the propagating density fluctuations and the associated longitudinal and transverse particle currents. The transverse dispersion relations expose two branches of modes for all pressures, whose range of appearance is analyzed and put in connection with the double-peak structure of the Fourier spectra of velocity autocorrelation functions. We have also investigated the existence of fingerprints of transverse acoustic excitation modes in the dynamic structure factor for the high pressure states similar to those observed in the inelastic X-ray scattering intensity data of liquid Fe at ambient pressure. The calculated electronic density of states shows that with increasing pressure there is a widening of the conduction band along with a decreasing significance of spin polarization. Finally, we also report results for transport coefficients like self-diffusion, shear viscosity and adiabatic sound velocity, which are compared with the available experimental data.