Fehr, Th.; Hochleitner, Rupert; Laumann, A.; Schmidbauer, E.; Schneider, J.
Mineralogy, Mössbauer spectroscopy and electrical conductivity of heterosite (Fe3+, Mn3+)PO4.
In: Physics and Chemistry of Minerals, Vol. 37, No. 3: pp. 179-189
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Mineralogical analysis, electrical conductivity and thermopower are reported for monocrystalline heterosite (Fe3+, Mn3+)PO4 with the orthorhombic olivine-type structure. The 57Fe Mössbauer spectrum could be adequately described using two Fe3+ doublets. By impedance spectroscopy (20 Hz-1 MHz) the electrical DC conductivity σDC and AC conductivity σAC were determined parallel (double pipe) and perpendicular to the  direction (space group Pnma) in the range 160-440 K. The graph log σDC-1/T shows a slightly bent curve in both directions with activation energies of EA 0.30 and 0.15 eV in the high and low temperature ranges, respectively. The reduced EA is associated with electronic conduction; σDC double pipe  follows Mott’s T1/4 variable range hopping law at lower temperatures with hopping between localized levels. The values of σAC are increased relative to σDC at high frequencies and low temperatures, obeying Jonscher’s universal dynamic response law; for σAC double pipe , the variation with temperature of the frequency exponent is in fair agreement with the model of small polaron hopping. The absolute thermopower Θ is negative and low between 295 and 440 K, Θ does hardly vary with temperatures in both directions; the temperature independency of Θ double pipe  is consistent with the small polaron hopping model.