Abstract
In this work, we resolve a long-standing issue concerning the local structure of molten MgCl2 by employing a multimodal approach, including X-ray scattering and Raman spectroscopy, along with the theoretical modeling of the experimental spectra based on ab initio molecular dynamics (AIMD) simulations utilizing several density functional theory (DFT) methods. We demonstrate the reliability of AIMD simulations in achieving excellent agreement between the experimental and simulated spectra for MgCl2 and 50 mol % MgCl2 + 50 mol % KCl, and ZnCl2, thus allowing structural insights not directly available from experiment alone. A thorough computational analysis using five DFT methods provides a convergent view that octahedrally coordinated magnesium in pure MgCl2 upon melting preferentially coordinates with five chloride anions to form distorted square pyramidal polyhedra that are connected via corners and to a lesser degree via edges. This is contrasted with the results for ZnCl2, which does not change its tetrahedral coordination on melting. Although the five-coordinate MgCl53- complex was not considered in the early literature, together with an increasing tendency to form a tetrahedrally coordinated complex with decreasing the MgCl2 content in the mixture with alkali metal chloride systems, current work reconciles the results of most previous seemingly contradictory experimental studies.
Original language | English |
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Pages (from-to) | 5971-5982 |
Number of pages | 12 |
Journal | Journal of Physical Chemistry B |
Volume | 125 |
Issue number | 22 |
DOIs | |
State | Published - Jun 10 2021 |
Funding
This work was supported as part of the Molten Salts in Extreme Environments (MSEE) Energy Frontier Research Center funded by the U.S. Department of Energy Office of Science. MSEE work at Iowa was supported via subcontract from Brookhaven National Laboratory (BNL). BNL and ORNL operate under DOE contracts DE-SC0012704 and DE-AC05-00OR22725, respectively. M.B. acknowledges financial support by the Deutsche Forschungsgemeinschaft (DFG) through project Br 5494/1-1. This research used resources of the Compute and Data Environment for Science (CADES) at the Oak Ridge National Laboratory and the National Energy Research Scientific Computing Center (NERSC), which are supported by the Office of Science of the U.S. Department of Energy under Contracts No. DE-AC05-00OR22725 and No. DE-AC02-05CH11231, respectively. This research used resources of the Advanced Photon Source operated by Argonne National Laboratory under contract no. DE-AC02-06CH11357. F.W., S.S., and C.J.M. acknowledge the computational resources at the University of Iowa high-performance computing facility.