In Situ Determination of Speciation and Local Structure of NaCl–SrCl₂ and LiF–ZrF₄ Molten Salts

Abstract

Understanding the local environment of the metal atoms in salt melts is important for modeling the properties of melts and predicting their behavior and thus helping enable the development of technologies such as molten salt reactors and solar-thermal power systems and new approaches to recycling rare-earth metals. Toward that end, we have developed an in situ approach for measuring the coordination of metals in molten salt coupling X-ray absorption spectroscopy (XAS) and Raman spectroscopy. Our approach was demonstrated for two salt mixtures (1.9 and 5 mol % SrCl2 in NaCl, 0.8 and 5 mol % ZrF4 in LiF) at up to 1100 °C. Near-edge (X-ray absorption near-edge structure, XANES) and extended X-ray absorption fine structure (EXAFS) spectra were measured. The EXAFS response was modeled using ab initio FEFF calculations. Strontium’s first shell is observed to be coordinated with chlorine (Sr2+–Cl–) and zirconium’s first shell is coordinated by fluorine (Zr4+–F–), both having coordination numbers that decrease with increasing temperature. Multiple zirconium complexes are believed to be present in the melt, which may interfere and distort the EXAFS spectra and result in an anomalously low zirconium first shell coordination number. The use of boron nitride (BN) powder as a salt diluent for XAFS measurements was found to not interfere with measurements and thus can be used for investigations of such systems.

Modeling Metallic Halide Local Structures in Salt Melts Using a Genetic Algorithm

“…We present here a computational study of CNs in two salt binary and ternary systems using the USPEX structure predictor algorithm coupled with the VASP electronic structure program. When compared to EXAFS experiments, 28 where appropriate, results for our ternary systems were found to closely model experimental observations and physical trends.…”

“…Our predicted roomtemperature CN of 6.2 for Sr−Na−Cl is within the calculated error limits for our companion EXAFS measurements that determined a value of 7.5 ± 1.3 for 5% SrCl 2 −NaCl. 28 57 The solidus of this composition is at 585 ± 5 °C and the liquidus is at 824 ± 5 °C. 58 Consequently, the expected CN for Zr is 8, which is in good agreement with our EXAFS measurement; however, the predicted CN of 6.8 is relatively low.…”

“…However, many Sr−Na−Cl crystal structures with CN = 0−3 are only slightly less stable than those with CN = 4−8, having a formation enthalpy between −2.0 and −2.5 eV. Formation enthalpies for Zr−Li−F crystal structures are the greatest for CN = 6−8, but there is 28 In order to directly compare our calculations to these experiments, the data set was narrowed down to only contain crystal structures that contained <10% atom fraction of Sr and Zr. The CN distribution of this narrowed data set, with arrows indicating experimental XAS measured CNs, are shown in Figure 3.…”

“…where T is the temperature in Kelvin, k B is the Boltzmann constant, and E j is the per-atom formation enthalpy for the set of the metal center structure. The total probability of a metal having a given CN, P(CN), is obtained by summing the probability of all CN states, P i (CN) investigations carried out by Lynch et al, 28 who investigated melts containing 5 mol % SrCl 2 in NaCl and 5 mol % ZrF 4 in LiF. The most stable structures identified using USPEX were used as input for structural fits to EXAFS measurements.…”

“…21 Though these applications are for solids, USPEX has been used for analyzing atom clusters in various applications, including liquids. 22−27 This work uses the USPEX algorithm to analyze the coordination of strontium in a SrCl 2 −NaCl melt and zirconium in a ZrF 4 −LiF melt that have been previously experimentally investigated using both Raman spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy, 28 and Boltzmann weighted probability distributions for coordination numbers (CNs) are generated from the USPEX data and compared to the experimental values. This approach assumes that the local environment in USPEX's generated crystal structures is the same as or similar to the local structure of the metals in the salt melt.…”