Fitting of Site Occupation and Diffusion in α-RbCu ₄ Cl ₃ I ₂ by Tuning Effective Charge and Ionic Radius

Abstract Considering the inability of the existing molecule simulation to reproduce the Ag site occupation rates in α-RbAg 4 I 5 crystals, we have explored an effective approach to simultaneously fit both the site occupation rates and the diffusion coefficients of Cu in α-RbCu 4 Cl 3 I 2 . This is achieved by tuning only the effective charge difference between Rb and Cu, and the ionic radius of Cu. We also provide an effective method that uses the distance of the first peak in the radial distribution function (RDF) of Rb-Cu pair to qualitatively determine the major occupation sites. With the Rb charge fixed at +1, the Cu charge set to +0.37, and the Cu radius set to be 0.5 Å, the simulated occupation rates of Cu1 and Cu2 sites are found to be 49.40% and 49.42%, respectively, closely matching the XRD results 50.06% and 43.42%. The fitted Cu diffusion coefficient is (2.365 ± 0.304) × 10 −6 cm 2 /s at room temperature, in excellent agreement with the value of (2.352 ± 0.175) × 10 −6 cm 2 /s estimated from α-RbAg 4 I 5 via the Einstein relation. Furthermore, we have discussed about the fitted system, finding a diffusion activation energy of 0.0823 eV, which is quite close to the value of 0.0861 eV reported for the most similar material Rb 4 Cu 16 I 7 Cl 13 . The jump rate between Cu1 and Cu2 is 89.42%, indicating the dominant pathway for ion conduction as expected. Cu4 shows almost no exchange with its neighbor, with an average residence time of 319 ps, while the residence times for Cu1 to Cu3 are 1.83 ps, 2.18 ps and 0.76 ps, respectively. These results significantly advance the understanding of structure-property relationships in solid electrolytes and provide a refined methodology for parametrizing empirical potentials in molecular dynamics simulations.