Connections between the Speciation and Solubility of Ni(II) and Co(II) in Molten ZnCl₂

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 ZnCl₂, where it is poorly soluble, and contrast it with highly soluble Co(II) over a wide temperature range. In solid NiCl₂, the Ni ion is octahedrally coordinated, whereas the ZnCl₂ host matrix favors tetrahedral coordination. Our experimental and computational results show that the coordination environment of Ni(II) in ZnCl₂ is disordered among tetra- and pentacoordinate states. In contrast, the local structure of dissolved Co(II) is tetrahedral and commensurate with the ZnCl₂ 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 ZnCl₂.