Absence of the Coulomb gap at the Fermi level in the variable-range hopping regime of zinc stannate polycrystalline systems

Measurements of the direct current resistivity, , on ceramic zinc stannate polycrystalline samples that had been annealed in a reducing atmosphere at temperatures of C, C, C and C were made as a function of temperature T from room temperature down to the liquid nitrogen temperature. In this temperature range, the data showed that the main contribution to the conductivity comes from carriers that hop directly between localized states executing variable-range hopping processes. Analysis of the data showed that the resistivity - temperature characteristics were well described by the Mott law, confirming the absence of a Coulomb gap at the Fermi level and suggesting that the density of states at the Fermi level is constant. The variation in the characteristic hopping temperature with the annealing temperature showed that was thermally activated with an activation temperature of .