Understanding the factors that control solubility and speciation of metal ions in molten salts is key for their successful use in molten salt reactors and electrorefining. Here, we employ X-ray and optical absorption spectroscopies and molecular dynamics simulations to investigate the coordination environment of Ni(II) in molten ZnCl 2 , where it is poorly soluble, and contrast it with highly soluble Co(II) over a wide temperature range. In solid NiCl 2 , the Ni ion is octahedrally coordinated, whereas the ZnCl 2 host matrix favors tetrahedral coordination. Our experimental and computational results show that the coordination environment of Ni(II) in ZnCl 2 is disordered among tetraand pentacoordinate states. In contrast, the local structure of dissolved Co(II) is tetrahedral and commensurate with the ZnCl 2 host's structure. The heterogeneity and concomitant large bond length disorder in the Ni case constitute a plausible explanation for its lower solubility in molten ZnCl 2 .
Knowledge of structural and thermal properties of molten salts is crucial for understanding and predicting their stability in many applications such as thermal energy storage and nuclear energy systems. Probing the behavior of metal contaminants in molten salts is presently limited to either foreign ionic species or metal nanocrystals added to the melt. To bridge the gap between these two end states and follow the nucleation and growth of metal species in molten salt environment in situ, we use synchrotron X-rays as both a source of solvated electrons for reducing Ni 2+ ions added to ZnCl 2 melt and as an atomic-level probe for detecting formation of zerovalent Ni nanoparticles. By combining extended X-ray absorption fine structure analysis with Xray absorption near edge structure modeling, we obtained the average size and structure of the nanoparticles and proposed a radiation-induced reduction mechanism of metal ions in molten salts.
Ion clustering of dilute chromium species was unexpectedly revealed in a high-temperature molten chloride salt, challenging several long-held assumptions regarding specific ionic interactions and transport in molten ionic media.
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