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

The local atomic structure of NaF-ZrF_4 (53-47 mol evolution with temperature are examined with x-ray scattering measurements and compared with ab-initio and Neural Network-based molecular dynamics (NNMD) simulations 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 co-existence of three different fluorozirconate complexes: [ZrF_6]^2-, [ZrF_7]^3-, and [ZrF_8]^4-, with a temperature-dependent shift in the dominant coordination state towards a 6-coordinated Zr ion at 700C. The study also highlights the metastability of different coordination structures, with frequent interconversions between 6 and 7 coordinate states for the fluorozirconate complex from 525 C to 700 C. 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 non-random 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.