Complex Structure of Molten NaCl-CrCl₃Salt: Cr-Cl Octahedral Network and Intermediate-Range Order

The resurgence of the molten-salt nuclear reactors (MSRs) creates interesting problems in molten-salt chemistry. As MSRs operate, the composition and physical properties of salts change because of fission and corrosion. Since Cr is the principal corrosion product and NaCl is a common constituent, we studied the atomic structure of molten NaCl-CrCl3. We found networks of CrCl63- octahedra and an intermediate-range order with a nonmonotonic temperature behavior with a remarkable agreement between measurements and ab initio simulations. Even though the corrosion results in minute quantities of dissolved Cr, the speciation of Cr could lead to changes in molten-salt properties in nuclear and solar salts. In particular, we found a much lower melting temperature than expected and a broad metastable liquid-solid coexistence phase. The availability of Cr isotopes with very different neutron scattering properties makes Cr an ideal model multivalent ion for experimental validation of new atomistic models such as neural network interatomic potentials.