Study of the electrical charge transport in ilvaite using impedance spectroscopy and thermopower data

Abstract 

The electrical conduction in the mineral ilvaite was studied between ≈170 and 450 K. A natural ilvaite from Elba (Italy) was found to be semiconducting with a DC conductivity 1.8×10^–3 (Ωcm)^–1 at 300 K, measured parallel (∥) to the [001] direction; perpendicular (⊥) to [001] it was 1.4×10^–5 (Ωcm)^–1, i.e. the conductivity is highly anisotropic. The conduction is effected by a hopping charge transport between localized levels in the energy gap associated with activation energies E _A =0.3–0.5 eV. It is concluded that impurities (Mg,Al,Mn) may play a decisive role in the charge hopping transport ∥ [001] that is basically governed by Fe²⁺-Fe³⁺ pairs on A-sites of the lattice as the source of electrons. Although the E_A-values were similar for both measured directions, the sign of thermopower is different which points to different charge transfer mechanisms. The bulk DC conductivity σ_DC ^AC for measurements ∥ [001], obtained by extrapolation of AC data using impedance spectroscopy, could only be determined at T<300 K owing to sample–electrode interfacial effects. In contrast, the bulk σ_DC ^AC⊥ [001] showed a slight break at ≈380 K that may reflect the structural phase transition monoclinic→orthorhombic at ≈345 K. From AC conductivity measurements in the frequency range 20 Hz–1 MHz at T<300 K, a dispersive character of electronic relaxation was found, resembling that of amorphous semiconductors and of impurity conduction in crystalline semiconductors where it was ascribed to charge hopping processes of electrons between localized levels of cation pairs or clusters of limited lengths.