Empirical Electronic Polarizabilities: Deviations from the Additivity Rule. II. Structures Exhibiting Ion Conductivity

Comprehensive study of the synthetic tychite, Na6Mg2(CO3)4SO4: synthesis, structure and conductive properties

The structural and conductive properties of the synthetic tychite Na6Mg2(CO3)4SO4 (space group: Fd3̄) were explored. Optimal synthesis parameters were selected by varying the molar ratio of reagents, synthesis temperature and time. For structural descriptions, we carried out scanning electron microscopy, thermal analysis, and Raman and Fourier-transform infrared spectroscopy. We carried out theoretical and experimental studies of conductivity. Theoretical calculations were obtained using geometrical-topological analysis based on Voronoi partitioning (the evaluation of geometrical sizes of voids and channels), the bond valence site energy (BVSE) method (the fast calculation of migration energies, Em), kinetic Monte Carlo simulation (the estimation of ionic conductivity at different temperatures), and density functional theory (the precise assessment of Em and vacancy formation energies). The experimental techniques included electrochemical impedance spectroscopy EIS (total conductivity determination) and direct current DC polarization (electronic conductivity assessment) measurements. Na6Mg2(CO3)4SO4 was found to have a three-periodic Na+-ion migration map with an Em of 0.37 eV and 0.18 eV from BVSE and DFT results, respectively. The theoretical value of cationic conductivity was 2.6 × 10-7 S cm-1 at 100 °C. The experimental total conductivity was equal to 6.0 × 10-7 S cm-1 at 100 °C, which is close to theoretical results. Using additional DC polarization, the electronic contribution to the total conductivity was characterized as insignificant.