## 4 Discussion  
Our calculation of the self-diffusion coefficients of ions in the molten salt mixture FLiNaK+CeF\({}_{3}\) showed that in the entire temperature range under study, the F- and Li+ ions are more mobile than the Na and K ions. Such a ratio between the mobility values is primarily determined by the composition of the liquid-salt mixture and only then by the atomic weights of the components. The low self-diffusion coefficient of the Ce ion is mainly related to its binding by F ions [33]. The slowing down of dynamics by a local charge also exists in ionic liquids [34]. The partial radial distribution functions \(g_{\text{Ce-Ce}}(r)\) and \(g_{\text{Li-Li}}(r)\) have the most pronounced first and second peaks, which reflects the greater structural stability in these subsystems. The subsystems formed by ions, whose representation clearly dominates in the salt melt, are characterized by the largest coordination number (as is the case with FF indexing). However, when the concentrations of the components differ slightly, the size of the ion may be more important. Thus, the coordination number with the KK index slightly exceeds the corresponding characteristic with the LiLi index, although the molar concentration of Li ions is slightly higher than that of K ions. The form of the calculated partial radial distribution function \(g_{\text{Ce-F}}(r)\), which has a high, well-resolved first peak, indicates the presence of a stable short-range order in the arrangement of F ions around Ce ions. The value of the coordination number, determined from the cation-anion radial  
Figure 13: Distributions for Ce subsystem of molten salt mixture FLiNaK+CeF\({}_{3}\) at a temperature of 950 K in form of: (**a**) VP by number of faces and VP faces by number of sides (in the insert), (**b**) angles \(\theta\). distribution function, is mainly determined by the size of the cation. For the molten salt studied here, the value of this coordination number increases following the sequence LiF, NaF, KF, CeF. For almost all ions present in the system, with the exception of K ions, the addition of CeF\({}_{3}\) to the FLiNaK salt melt slows down the movement of these charge carriers and increases the cation-cation, anion-anion, and cation-anion coordination numbers. The amount of increase in each coordination number depends on such factors as the size of the ion, the concentration of the corresponding component in the salt melt, and the temperature of this melt. The combination of these factors makes the increase in the coordination number \(n_{\text{Li-Li}}\) the most noticeable, which, depending on temperature, can vary from 29 to 40%. Topological characteristics (\(n\), \(m\) distributions) point to the irregular nature of the distribution of particles in the subsystems of F and Li ions and the presence of a more stable short-range order (with a narrower \(n\), \(m\) distribution) for the subsystems of Na and K ions. In addition, the fifth-order rotational symmetry inherent in a simple liquid dominates in the representation of the structure of subsystems with fluorine and lithium ions. At the same time, when describing the structure of subsystems of Na and K ions, the most important is the third and fourth-order rotational symmetry more inherent in the periodic crystal [35]. The breakdown of the system into subsystems of the same type of ions showed that the structures of these subsystems differ significantly. In particular, the shapes of the angular distributions of the nearest geometric neighbors for the subsystems of Li, Na, K, F, and Ce ions differ dramatically, and the maxima of these distributions are set at 66\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), 99\({}^{\circ}\), and 60\({}^{\circ}\), respectively. Only in the case of the subsystem of cerium ions, the \(\theta\) spectrum is represented by well-resolved peaks, which may indicate the presence of a certain regularity in the mutual arrangement of Ce ions.