Complex Structure of Molten NaCl-CrCl_x Salts: Octahedra Network and Intermediate-Range Order

In the last decade, there has been a sustained resurgence of the Molten-Salt Nuclear Reactor (MSR) concept. As the MSRs operate, chemical compositions and physical properties of salts change because of the creation of fission products, and the effects of corrosion and radiolysis. Therefore, computer simulations of the microscopic structure of multi-component molten salts are necessary to predict changing thermophysical properties. However, first-principles simulations inevitably involve simplifying approximations and therefore require careful experimental validation. Here we present detailed experimental and simulation studies of the structure of molten NaCl-CrCl₃ and NaCl-CrCl₂ near-eutectic mixtures. These compositions have been chosen to elucidate the behavior of Cr ions in the melts since Cr is the most important corrosion product. Although the corrosion will result in relatively small quantities of Cr in the melt, we used much larger Cr concentration to measure the structure around Cr ions with high resolution. The results of X-ray and neutron diffraction with isotopic substitution, X-ray absorption spectroscopy, and ab initio molecular-dynamic simulations are in agreement. In NaCl-CrCl₃ we found CrCl₆³⁺ octahedra and their networks, as well as intermediate-range order, which manifests itself in the prepeak at ~ 1 Å^-1 with non-monotonic temperature behavior.

Figure 1 shows one of the main results of this work, namely the comparison of measured and calculated total and partial PDFs of the molten NaCl-CrCl₃ near-eutectic mixture. The consistency between the measured and calculated PDFs is remarkable. Neutron diffraction with isotopic substitution allows for a high-fidelity determination of the distances between Cr ions and other species in the melt. By utilizing a combination of neutron diffraction with isotopic substitution of ⁵³Cr, X-ray diffraction, and ab-initio molecular dynamic simulations, we discovered the formation of chains of CrCl₆^3- corner-, edge-, and face-sharing octahedra, with Cr-Cl distances of 2.4 Å, and Cl-Cl distance of 3.3 Å. We also found evidence for intermediate-range order that is likely related to inter-chain correlations. In this regard, these ternary salt shows a structure that is reminiscent of that of KCl-MgCl₂, LiF-BeF₂, and NaCl-UCl_n.

Neutron and X-ray diffraction augment each other to determine partial PDFs Neutrons are particularly useful for studies involving, for example, Cr and Ni, where isotopes with a large difference in neutron cross-section are available. But when isotopic substitution is not achievable, the combination of X-rays and neutrons can extract the information about partial PDFs not otherwise available. The combination of neutrons and X-rays is also useful to overcome experimental constraints, such as compatibility between the salts and crucibles, potential radiation damage to crucibles, and sometimes large background from the sample environment. For example, quartz crucibles are suitable for both neutrons and X-rays and can be used with chloride salts but not with fluoride salts. The latter can be contained in vanadium crucibles for neutrons experiments. Our measurements demonstrate the relative strengths and weaknesses of both techniques for understanding the structure of multi-component salts. In conclusion, the understanding of the structure around Cr ions will be used to calculate properties of molten salt components (e.g., diffusivity) in the presence of Cr impurities. These properties will affect macroscopic thermo-physical properties such as thermal conductivity and viscosity of melts containing that, in turn, are necessary for designing safe and efficient MSRs. The extension of these methods to study melts containing up to 10 components will be discussed.

We acknowledge useful discussions with Richard Mayes, Stephen Raiman, Jake Mcmurray (ORNL), and Raluca Scarlat (UC Berkeley). This material is based upon work supported by the Department of Energy under Award Number DE-NE0008751. This research used resources at the Spallation Neutron Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated by the Oak Ridge National Laboratory, and the beamline 28-ID-1 (PDF) of the National Synchrotron Light Source II, a DOE Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

Figure 1