Correlative analysis of Ni(ii) coordination states in molten salts using a combination of X-ray and optical spectroscopies and simulations

Understanding the factors that control the speciation of metal ions in molten salts is crucial for the successful deployment of molten salts in both concentrated solar power and nuclear energy applications. The speciation of the Ni(ii) ion is of interest because it is a common corrosion product, and the distribution of coordination states it occupies is highly sensitive to the molten salt matrix. We employ in situ X-ray absorption spectroscopy (XAS), optical spectroscopy, and ab initio molecular dynamics (AIMD) simulations to investigate and understand the heterogeneities of Ni(ii) coordination in LiCl-KCl, NaCl-MgCl_2, and LiCl-ZnCl₂ molten salt systems. The main challenge lies in identifying the population distribution of Ni(ii) coordination states as a function of temperature and melt composition. We combined the multivariate curve resolution - alternating least squares (MCR-ALS) analysis of the XAS data and principal component analysis (PCA) of the optical spectra to determine the number of unique coordination states coexisting in the molten state, extract X-ray spectra for each state, and obtain their mixing fractions at different temperatures and for different salt mixtures. AIMD simulations were essential in identifying the coordination states corresponding to the deconvoluted spectra. The differences in the coordination states of Ni(ii) in different salt systems are discussed in terms of the effects of the varying polarizing powers of the cations in the host salt matrix on chloride ion coordination to Ni(ii). Such elucidation of the local structure adopted by metal ions enables a better understanding of the factors controlling the speciation of ions and their effect on molten salt properties.