Complex Structure of Molten NaCl–CrCl₃ Salt: Cr–Cl Octahedral Network and Intermediate-Range Order

Abstract

The resurgence of the Molten-Salt Nuclear Reactors (MSR) 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-CrCl 3 . We found networks of CrCl 3− 6 octahedra and an intermediate-range order with nonmonotonic temperature behavior in 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 than expected melting temperature and a broad metastable liquid-solid coexistence phase. The availability of Cr isotopes with very different neutron-scattering properties makes Cr an ideal model multi-valent ion for experimental validation of new atomistic models such as neural network interatomic potentials.

X-ray scattering reveals ion clustering of dilute chromium species in molten chloride medium

“…This behavior is consistent with the Cl–Cl contraction observed recently in neutron diffraction experiments for the binary CrCl 3 –NaCl melt with much higher CrCl 3 content (22 mol%). 31 …”

Short- to Intermediate-Range Structure, Transport, and Thermophysical Properties of LiF–NaF–ZrF4 Molten Salts

“…C p, composition C (8.285 ± 6 ) × 10 −4 T( °C) − 0.4823 ± 0.477 (700 − 900 °C) (5) The specific heat capacities values of 0.256 ± 0.002 and 0.24 ± 0.006 cal/gm-°C were obtained for compositions A and B, respectively. It should be noted that the constant values of specific heat capacity only represent the approximate C p vs. T trend in the considered temperature ranges.…”

“…The precise experimental interrogation of salt structure and properties is challenged by high-temperature conditions, cost, material handling, and difficulties in interpreting experimental data. Particularly, it is difficult to access the structure of multivalent cations using techniques such as Raman spectra and EXAFS alone due to their existence in multiple coordination states and intermediate-range ordering [5][6][7]. Therefore, ab-initio and classical molecular dynamics simulations can be used to interpret experimental data and predict temperaturedependent structures, transport, and thermophysical properties of LiF-NaF-ZrF 4 molten salts.…”

Constrained non-negative matrix factorization enabling real-time insights of $\textit{in situ}$ and high-throughput experiments

“…These molten salts are of interest for the development of nuclear reactors. 35 In this experiment there was a residual solid phase after the melting transition. Without any additional analysis, it remained unclear to the research team whether that solid was a distinct high-temperature phase or an experimental artefact.…”

“…Fortunately, the nature of these in situ experiments typically follows a standard trajectory across a set of state variables, under which the sample begins read-ily known phase to the researcher that may undergo a transition at some future point into known or unknown phases. 35 Various combinations of known and unknown components or weights can be occur in a data series, such as knowledge of two components at the edges of a dataset with curiosity about a third, or no knowledge of the components, but expectations for their concentrations. 34 Thus, through use of constraints we can employ NMF in highly productive and insightful ways for diffraction experiments.…”