Electrical conductivity and vibrational studies induced phase transitions in [(C2H5)4N]FeCl4

Abstract

The compound, tetraethylammonium tetrachloroferrate [(C₂H₅)₄N]FeCl₄, was prepared by slow evaporation at room temperature. It was characterized by X-ray powder diffraction, thermal analysis, and impedance and vibrational spectroscopy. X-ray diffraction data confirmed formation of a single phase material which crystallized at room temperature in the hexagonal system (P6₃ mc space group). DSC showed the existence of two phase transitions at 413 K and 430 K. Electrical conductivity was measured in the temperature and frequency ranges of 390 K to 440 K and 40 Hz to 110 MHz, respectively. Nyquist plots revealed the existence of grains and grain boundaries that were fitted to an equivalent circuit. AC conductivity plots were analyzed by Jonscher's power law. Variations in the "s" values indicated that CBH models describe the conduction mechanism in regions I and II. Temperature dependence of Raman spectra showed that the most important changes were observed in the cationic parts ([(C₂H₅)₄N]⁺). The activation energy value obtained from the line width decreased which indicated an order-disorder model.

Fig. 1. X-ray diffractogram of [(C₂H₅)₄N]FeCl₄ in the 2θ range 10–60°. The red circle indicates the experimental data and calculated data are represented by black continuous lines. The lowest curve in blue shows the difference between experimental and calculated patterns. The green vertical bars describe the Bragg position.

Fig. 2. Differential scattering calorimetric trace of [(C₂H₅)₄N]FeCl₄.

Fig. 3. TGA curve of [(C₂H₅)₄N]FeCl₄.

Fig. 4. Complex impedance spectra of [(C₂H₅)₄N]FeCl₄ at different temperatures.

Fig. 5. Variation of the real part of impedance as a function of frequency and temperature.

Fig. 6. Variation of the imaginary part of impedance as a function of frequency at various temperatures.

Fig. 7. Plot of ln(σ_gT) versus 1000/T for the [(C₂H₅)₄N]FeCl₄ crystal.

Fig. 8. Variation of σ_acversus 1000/T at different frequencies.

Fig. 9. Temperature dependence of the frequency exponent s.

Fig. 10. Temperature dependence of ln(σ_ac) at different frequencies.

Fig. 11. Experimental IR and Raman spectra of [(C₂H₅)₄N]FeCl₄ at room temperature.

Fig. 12. Evolution of the Raman spectrum as a function of temperature.

Fig. 13. Temperature dependence of some Raman position (a) and half-maximum (b) associated with the inorganic part.

Fig. 14. Temperature dependence of some Raman position (a) and half-maximum (b) associated with the organic part.

Fig. 15. Temperature dependence of the band position at 1440 cm⁻¹.

Fig. 16. Temperature dependence of the band half-widths at 1440 cm⁻¹.