Ionic conductivity of molten alkali-metal carbonates A(2)CO(3) (A = Li, Na, K, Rb, and Cs) and binary mixtures (Li1-xCsx)(2)CO3 and (Li1-xKx)(2)CO3: A molecular dynamics simulation
T Kiyobayashi and T Kojima and H Ozaki and K Kiyohara, JOURNAL OF CHEMICAL PHYSICS, 151, 074503 (2019).
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 A(2)CO(3) 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 (Li1-xCsx)(2)CO3 and (Li1-xKx)(2)CO3. In (Li1-xCsx)(2)CO3, 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 (Li1-xCsx)(2)CO3 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 CO32-, 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 (Li1-xKx)(2)CO3 with a less pronounced manner than in (Li1-xCsx)(2)CO3. 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+.
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