X-ray and molecular dynamics study of the temperature-dependent structure of molten NaF- Zr F4

Anubhav Wadehra, Rajni Chahal, Shubhojit Banerjee, Alexander Levy, Yifan Zhang, Haoxuan Yan, Daniel Olds, Yu Zhong, Uday Pal, Stephen Lam, Karl Ludwig

Research output: Contribution to journalArticlepeer-review

Abstract

The local atomic structure of NaF-ZrF4 (53-47 mol%) molten system and its evolution with temperature are examined with x-ray scattering measurements which are then used to validate the quality of ab initio and neural network-based molecular dynamics (NNMD) calculations in the temperature range 515-700°C. The machine-learning enhanced NNMD calculations offer improved efficiency while maintaining accuracy at higher distances compared to ab initio calculations. Looking at the evolution of the pair distribution function with increasing temperature, a fundamental change in the liquid structure within the selected temperature range, accompanied by a slight decrease in overall correlation is revealed. NNMD calculations indicate the coexistence of three different fluorozirconate complexes: [ZrF6]2-, [ZrF7]3-, and [ZrF8]4-, with a shift in the dominant coordination state from the 7-coordinated Zr cation toward a 6-coordinated cation with increasing temperature. The study also highlights the metastability of different local coordination structures, with frequent interconversions between the 6- and 7-coordinate states. Analysis of the Zr-F-Zr angular distribution function reveals the presence of both "edge-sharing"and "corner-sharing"fluorozirconate complexes with specific bond angles and distances in accord with previous studies, while the next-nearest-neighbor cation-cation correlations demonstrate a clear preference for unlike cations as nearest-neighbor pairs, emphasizing nonrandom arrangement. These findings contribute to a comprehensive understanding of the complex local structure of the molten salt, providing insights into temperature-dependent preferences and correlations within the molten system.

Original languageEnglish
Article number075402
JournalPhysical Review Materials
Volume8
Issue number7
DOIs
StatePublished - Jul 2024

Funding

This research at Boston University, University of Massachusetts-Lowell, and the Worcester Polytechnic Institute was supported by the U.S. Department of Energy (DOE) NEUP program (Grant No. 20\u201319373) and Award No. DE-NE0009204, and the U.S. National Science Foundation (NSF) (Awards No. CMMI-1937818 and No. CMMI 1937829). This research used resources of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science user facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

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