INFLUENCE OF NEGATIVE CHARGE DISTRIBUTION AND LOCATIONS OF Na⁺ IONS ON IONIC CONDUCTIVITY OF Na-MICAS

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In this study, the influence of the negative charge distribution and the locations of Na⁺ ions on ionic conductivity of Na-micas was investigated. The locations of Na⁺ ions in as-prepared Na-micas and heated Na-micas at 700°C were determined in details using X-ray diffraction analyzer, Fourier transform infrared spectrophotometer, ²³Na magic angle spinning nuclear magnetic resonance and X-ray photoelectron spectroscopy. As the results, four kinds of Na⁺ ions, such as Na⁺ ions induced into vacant sites in the octahedral sheets, hydrated Na⁺ ions in the interlayer, dehydrated Na⁺ ions surrounded by six basal oxygens and Na⁺ ions drawn into the inside of the ditrigonal hole, were found in Na-micas. Ionic conductivities of the heated Na-micas were measured at 400 to 600°C by an alternating current four-probe method. Among the Na-micas, Na-taeniolite showed the maximum ionic conductivity, which was 6.61×10⁻⁴ S/cm at 600°C. Following relationships were obtained from the results. The hydrated Na⁺ ions in the interlayer and the dehydrated Na⁺ ions surrounded by six basal oxygens contributed to the ionic conductivity of Na-mica, while the octahedral Na⁺ ions and the Na⁺ ions drawn into the inside of the ditrigonal hole hardly contribute to the ionic conductivity. In addition, because tetrasilisic type mica had weaker electrostatic bonding force between layer and interlayer cation than trisilisic type mica, the Na⁺ ions of tetrasilisic type mica were migrated more easily than that of trisilisic type mica.