X-ray and molecular dynamics study of the temperature-dependent structure of molten NaF-ZrF4

A Wadehra and R Chahal and S Banerjee and A Levy and YF Zhang and HX Yan and D Olds and Y Zhong and U Pal and S Lam and K Ludwig, PHYSICAL REVIEW MATERIALS, 8, 075402 (2024).

DOI: 10.1103/PhysRevMaterials.8.075402

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 degrees 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: ZrF62-, ZrF73-, and ZrF84-, 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.

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