Conduction in LaCoO3 by small-polaron hopping below room temperature

In order to study the electrical transport of holes in LaCoO3, the bulk conductivities and dielectric properties in a polycrystalline specimen were investigated. The bulk conduction was separated from others by complex-plane impedance analysis. At high temperatures, i.e., T>170 K, the activation energy required for bulk conduction is 0.34 eV, while that required for dielectric relaxation is 0.25 eV, and the spectral intensity of the relaxation peak is thermally activated with an energy of 0.097 eV. This is consistent with a polaronic picture in which the bulk conduction involves the excitation of carriers through the band gap, E(g), between the two energy states participating in the electrical transport; in such a picture the hopping process of the holes in the lower-energy states has a hopping energy WH, and the temperature dependence of the bulk conduction is proportional to exp[ - (W-H + E(g)/2)/k(B)T]/T. The present experiment obtained these energies independently, i.e., (W-H + E(g)/2) congruent to 0.35 eV, E(g)/2 congruent to 0.097 eV, and W-H congruent to 0.25 eV. At low temperatures, i.e., T<170 K, the - non-Arrhenius regime is observed, which might be interpreted as a usual crossover from a thermally activated process to a tunnel-hopping process.