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

Journal Article · 22 July 2024 · Physical Review Materials

DOI: https://doi.org/10.1103/physrevmaterials.8.075402 · OSTI ID:2446652

Wadehra, Anubhav  ^[1]; Chahal, Rajni  ^[2]; Banerjee, Shubhojit  ^[3]; Levy, Alexander  ^[1]; Zhang, Yifan  ^[4]; Yan, Haoxuan  ^[1]; Olds, Daniel ^[5]; Zhong, Yu  ^[4]; Pal, Uday  ^[1]; Lam, Stephen  ^[3]; Ludwig, Karl  ^[1]

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1. Boston Univ., MA (United States)
2. Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
3. Univ. of Massachusetts, Lowell, MA (United States)
4. Worcester Polytechnic Institute, MA (United States)
5. Brookhaven National Laboratory (BNL), Upton, NY (United States). National Synchrotron Light Source II (NSLS-II)

The local atomic structure of NaF-Zr⁢F₄ (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: [Zr⁢F₆]^2–, [Zr⁢F₇]^3–, and [Zr⁢F₈]^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. Finally, 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.

• Research Organization: Brookhaven National Laboratory (BNL), Upton, NY (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); USDOE Office of Nuclear Energy (NE), Nuclear Energy University Program (NEUP)
• Grant/Contract Number: SC0012704; NE0009204
• OSTI ID: 2446652
• Report Number(s): BNL--226129-2024-JAAM
• Journal Information: Physical Review Materials, Journal Name: Physical Review Materials Journal Issue: 7 Vol. 8; ISSN 2475-9953
• Publisher: American Physical Society (APS)Copyright Statement
• Country of Publication: United States
• Language: English