Electrical conductivity and thermopower of Fe-phosphate compounds with the lazulite-type structure

Measurements of the electrical DC and AC conductivities were performed on three Fe-phosphate compounds in the temperature range similar to210K less than or equal to T less than or equal to similar to450K. The Fe-phosphates are semiconducting with activation energy of DC conductivity sigma(DC) Of E(A)similar to0.35-0.45 eV. For alpha-Fe2PO4O, the AC conductivity sigma(AC) in the low-temperature range is considerably enhanced relative to sigma(DC). The frequency dependence Of sigma(AC) can be described by an approximate power law. For the same compound, the thermopower Theta (Seebeck effect) was found to be positive in the range similar to 330 K less than or equal to T less than or equal to similar to 700 K, i.e. seemingly p-type conduction occurs; at T > 700 K, Theta appears to become negative. The results are described in terms of a small polaron hopping model between localized states with electron transfer of the type Fe2+ --> Fe3+ with Fe2+ as donors. In this model Theta can be described suggesting equal concentrations of Fe2+ and Fe3+ to take part in conduction. (C) 2003 Published by Elsevier Inc.