Dr. Jiawei Xian [University of Chinese Academy of Sciences, China]

Title: Properties of liquid iron under extreme conditions from ab initio molecular dynamics

Time: 10:00 - 11:00 AM, Saturday, August 6, 2016

Place: Conference Room 201, HPSTAR (Shanghai)

Host: Dr. Howard Sheng

Abstract:

We performed ab initio molecular dynamics (MD) simulations to compute the entropy and equation of state (EOS) of liquid iron in a wide pressure (up to 940 GPa) and temperature (from melting point to 30000 K) range, relevant to planetary core conditions. The ionic entropy was determined with the two-phase thermodynamic (2PT) model and the accuracy of the 2PT model was evaluated. We found that the 2PT model works best under high temperature-low density conditions while is not accurate enough in the low temperature-high density region. To get accurate entropies in wide thermodynamic conditions we propose the follows: first choose a state from the high temperature-low density region as reference and determine its entropy from the 2PT model, then evaluate the entropies of other states by combining the reference entropy with the entropy differences derived from MD. The differences between the entropies determined in this way and the original 2PT values can lead to 20-40% changes in the predicted melting temperatures. The deficiency of the original 2PT model is attributed to the treatment of gas-like component as hard sphere gas. Interestingly, if the gas-like component is treated as ideal gas, the error in the original 2PT model gets diminished by approximately 50%.

We constructed more accurate EOS models of liquid iron by applying self-consistent corrections to account for the differences between calculations and experiments. The good agreement between the resulting calculated Hugoniot and shock-wave data ensures the reliability of our correction scheme, and confirms the necessity of making corrections to obtain accurate EOS. Based on the corrected EOS, we found that in Sakaiya et al.’s shock-wave experiment, the pressure for the highest density point may be severely underestimated: The measured 810 GPa should be around 940 GPa. Moreover, we pointed out that Ichikawa et al.’s abnormal conclusion for the light element distribution in Earth’s outer core is in fact due to the neglect of errors in ab initio calculations.

Biography of the Speaker:

Dr. Jiawei Xian is currently a post-doctoral research fellow in the School of Earth Science, University of Chinese Academy of Sciences. Dr. Xian obtained his Ph.D degree in the condensed matter sector in 2013 from Scuola Internazionale Superiore di Studi Avanzati (SISSA), Trieste, Italy, and obtained his Bachelor degree in physics from Fudan university in 2008. Dr. Xian has been involved in the development of quantum-mechanics simulation software Quantum-Espresso. Dr. Xian’s research interest covers various topics in computational physics and Earth science, including materials properties under high-pressure and high-temperature conditions and GW calculations of electronic excitations.