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 ∼210 K?T?∼450 K. The Fe-phosphates are semiconducting with activation energy of DC conductivity σ_DC of E_A∼0.35-0.45 eV. For α-Fe₂PO₄O, the AC conductivity σ_AC in the low-temperature range is considerably enhanced relative to σ_DC. The frequency dependence of σ_AC can be described by an approximate power law. For the same compound, the thermopower Θ (Seebeck effect) was found to be positive in the range ∼330 K?T?∼700 K, i.e. seemingly p-type conduction occurs; at T>700 K, Θ 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 Θ can be described suggesting equal concentrations of Fe²⁺ and Fe³⁺ to take part in conduction.