Correlative Analysis of Ni(II) Coordination States in Molten Salts Using X-ray and Optical Spectroscopies and Simulations

December 1, 2025

The Science 

In nuclear systems, dissolved metal ions influence corrosion and salt chemistry, but their local coordination environments are difficult to resolve due to structural disorder. Researchers combined X-ray absorption spectroscopy, optical spectroscopy, and molecular simulations to study Ni²⁺speciation in molten chloride salts. They found that Ni²⁺ exists in multiple dynamically interconverting coordination states whose populations depend on temperature and salt composition. These results show that the coordination chemistry of metal ions in molten salts is governed by the electrostatic (polarizing) properties of the surrounding melt.  

The Impact 

Understanding metal ion speciation is essential for interpreting spectroscopy and predicting corrosion behavior in molten salt systems. This work provides a framework for resolving multiple coexisting coordination states directly from experimental data. The results improve the ability to connect spectroscopic measurements to atomic-scale structure in complex molten salt environments. 

Summary 

This study used X-ray absorption spectroscopy, optical spectroscopy, and ab initio molecular dynamics simulations to determine the coordination environments of Ni(II) ions in multiple molten chloride salts. 

Analysis using multivariate curve resolution and principal component methods revealed that Ni(II) exists in a mixture of four-, five-, and distorted six-coordinate states. The relative populations of these states vary with temperature and salt composition. 

Optical spectroscopy confirmed these coordination changes through distinct electronic transitions associated with different geometries. Simulations showed that these coordination environments are highly dynamic, with rapid interconversion between states driven by thermal fluctuations. 

The results further demonstrate that the polarizing power of the melt cations controls coordination behavior by modifying chloride ion availability for bonding. 

Overall, the study shows that metal ion speciation in molten salts reflects a balance between local electronic structure and dynamic coordination, linking melt composition to atomic-scale structure. 

Contact 

Simerjeet K. Gill, Brookhaven National Laboratory, gills@bnl.gov 
Anatoly I. Frenkel, Stony Brook University / Brookhaven National Laboratory, anatoly.frenkel@stonybrook.edu 
Ruchi Gakhar, Idaho National Laboratory, ruchi.gakhar@inl.gov 
James F. Wishart, Brookhaven National Laboratory, wishart@bnl.gov 

Funding 

This work was supported by the Molten Salts in Extreme Environments Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. This research used resources of the National Synchrotron Light Source II at Brookhaven National Laboratory and computational resources at Oak Ridge National Laboratory and NERSC. 

Citation 

Liu, Y., M. Topsakal, K. Zheng, L. E. Betancourt, M. Woods, S. Roy, N. Patra, D. Leshchev, P. Halstenberg, D. S. Maltsev, S. Dai, A. S. Ivanov, V. S. Bryantsev, J. F. Wishart, R. Gakhar, A. I. Frenkel, and S. K. Gill. “Correlative Analysis of Ni(II) Coordination States in Molten Salts Using a Combination of X-ray and Optical Spectroscopies and Simulations.” Chemical Science 16 (2025): 10414–10423. https://doi.org/10.1039/D5SC01059D 

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