Mineralogy, Mossbauer spectroscopy and electrical conductivity of heterosite (Fe3+, Mn3+)PO4
Physics and Chemistry of Minerals 37(3): 179-189
Mineralogical analysis, electrical conductivity and thermopower are reported for monocrystalline heterosite (Fe3+, Mn3+)PO4 with the orthorhombic olivine-type structure. The Fe-57 Mossbauer spectrum could be adequately described using two Fe3+ doublets. By impedance spectroscopy (20 Hz-1 MHz) the electrical DC conductivity sigma(DC) and AC conductivity sigma(AC) were determined parallel (ayen) and perpendicular to the  direction (space group Pnma) in the range similar to 160-440 K. The graph log sigma(DC)-1/T shows a slightly bent curve in both directions with activation energies of E (A) similar to 0.30 and similar to 0.15 eV in the high and low temperature ranges, respectively. The reduced E (A) is associated with electronic conduction; sigma(DC) ayen  follows Mott's T (1/4) variable range hopping law at lower temperatures with hopping between localized levels. The values of sigma(AC) are increased relative to sigma(DC) at high frequencies and low temperatures, obeying Jonscher's universal dynamic response law; for sigma(AC) ayen , the variation with temperature of the frequency exponent is in fair agreement with the model of small polaron hopping. The absolute thermopower I similar to is negative and low between similar to 295 and similar to 440 K, I similar to does hardly vary with temperatures in both directions; the temperature independency of I similar to ayen  is consistent with the small polaron hopping model.