Ionic conductivity of molten alkali-metal carbonates A₂CO₃ (A = Li, Na, K, Rb, and Cs) and binary mixtures...

Based on experimental data, we optimized the potential parameters for the classical molecular dynamics simulation to reproduce the volume and ionic conductivity of the molten alkali-metal carbonates A₂CO₃ where A = Li, Na, K, Rb, and Cs at T/K = 1223 and ambient pressure. The force field was then applied to the binary mixtures (Li_1-xCs_x)₂CO₃ and (Li_1-xK_x)₂CO₃. In (Li_1-xCs_x)₂CO₃, the diffusion coefficient D_Cs exceeds D_Li at x > 0.6, testifying to the Chemla effect. The net ionic conductivity was broken down into the contributions from the velocity auto- and cross-correlations of each ionic species. The significant negative deviation of the real conductivity of (Li_1-xCs_x)₂CO₃ from the one estimated by the Nernst-Einstein (NE) relation is clearly explained by the contribution from the cross correlations; specifically, the cross term between Li⁺and CO₃ ^2-, which is negative at x = 0, significantly shifts to the positive side when x increases, which is dominantly responsible for dampening the conductivity from the NE conductivity. A similar behavior was observed in (Li_1-xK_x)₂CO₃ with a less pronounced manner than in (Li_1-xCs_x)₂CO₃. These observations corroborate the precedent studies pointing to the trapping of Li⁺ by the anion when a lithium salt is mixed with another salt of which the cation size is greater than that of Li⁺.