Schmidbauer, E.; Fehr, Th.; Hochleitner, Rupert; Schneider, J. (2005): Structure and physical property relations of Mn ilvaite Part II: Electrical conductivity and thermopower. In: Physics and Chemistry of Minerals, Vol. 32, Nr. 5-6: S. 400-411
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DC and AC electrical conductivities were measured on a monocrystalline Mn-rich ilvaite sample in the temperature range ∼130 < T < ∼300 K. Impedance spectroscopy was applied to determine the DC conductivity σDC, extrapolating AC data to zero frequency. The conductivity σDC, measured parallel (∥) and perpendicular (⊥) to the [001] direction, exhibits semiconducting behaviour with a non linear log σDC-1/T relationship. A distinct anisotropy was found with σDC(300 K) ∼4×10-2 Ω-1 cm-1 ∥ [001] and ∼1×10-4 Ω-1cm-1 ⊥ [001]. The AC conductivity σ′(ω) (ω/2 π=frequency) is enhanced in both directions relative to σDC at low temperatures, with a marked effect for high frequencies within the range 20 Hz-1 MHz. For the frequency dependence ∥ [001], an approximate power law is valid σ′(ω) ∝ ωs, with s<1. This behaviour is typical for hopping charge transport between localized states. Charge transfer is suggested to occur by electron hopping between mixed-valence Fe cations according to Fe2+ → Fe3+ preferentially along chains ∥ [001]. The absolute thermopower Θ (Seebeck-effect) ∥ [001] has a positive sign and it is Θ ∝ 1/T, while Θ ⊥ [001] is negative and shows hardly a temperature variation within the range ∼300 K and ∼430 K. The results are discussed in terms of one-dimensional models of electron hopping between localized states.