Local Coordination Environment of 3d and 4d Transition Metal Ions in LiCl-KCl Eutectic Mixture.

Jon Fuller,Qi An,Ruchi Gakhar,William Phillips

doi:10.3390/ma15041478

Abstract

In this study, the structure and coordination environment of two 3d transition elements (Ni and Cr) is investigated in a molten chloride salt system. Electronic absorption spectroscopy was employed to elucidate their coordination environment in 3LiCl-2KCl eutectic salt, as a function of temperature. Density functional theory (DFT) modeling was used to determine the coordination environment of the transition metal species in the eutectic composition as well as the optical spectra computationally. The Ni2+and Cr3+ exist in a tetrahedral and octahedral coordination environment, respectively, in eutectic salt. The spectra thus obtained were compared with the experimental data; a reasonable qualitative agreement was obtained between experimental and computational Ni2+ and Cr3+spectra, and the coordination of both elements in the eutectic composition were in excellent agreement with the experimentally determined results. Computational results were also obtained for two 4d elements, Mo3+ and Nb3+, with both quantum molecular dynamics (QMD) and hybrid functional optical spectra indicating octahedral coordination.

Highlights

The coordination chemistry of various metal species in different molten salt compositions has become a very important focus for research development in molten salts, in order to discover the nature of their chlorocomplex formation mechanisms and their relationships with chemical processes, which are relevant to the functionality of molten salts applications, such as corrosion, electrolysis effects and other chemical processes [1,2]
We report the experimental and computational optical absorption spectra for four transition metal chlorocomplexes important to pyroprocessing and corrosion processes in the LiCl-KCl binary eutectic salt composition, including those formed by Ni2+, Cr3+, Mo3+ and Nb3+ as well as their predicted coordinations using
We find from the quantum molecular dynamics (QMD) that Ni2+ forms the tetrahedral NiCl4 2− chlorocomplex, while Cr3+, Mo3+ and Nb3+ form the octahedral CrCl6 3−, MoCl6 3− and NbCl6 3− chlorocomplexes, respectively

Summary

Introduction

The coordination chemistry of various metal species in different molten salt compositions has become a very important focus for research development in molten salts, in order to discover the nature of their chlorocomplex formation mechanisms and their relationships with chemical processes, which are relevant to the functionality of molten salts applications, such as corrosion, electrolysis effects and other chemical processes [1,2]. Molten salt-based pyroprocessing technology is the key process for treating the used nuclear fuels to close the nuclear fuel cycle [3]. The coordination chemistry of these metal complexes in the melt are intrinsically linked to several important properties for the molten salt, including reactivity, diffusion rates and thermophysical behavior with other species, actinides, encountered during the pyroprocessing operation [5]. For investigation of the structural symmetry and chemical constitution of molten salts for application to electrorefining and molten salt reactor (MSR) applications, a versatile high-temperature apparatus for in situ spectroscopic studies of molten salts has been designed at the Idaho National Laboratory

Methods

Results

Conclusion