Electronic Transport Properties of [Ca2CoO3-δ]q[CoO2]

The oxygen stoichiometry and the electronic transport properties of [Ca2CoO3-delta](q)[CoO2] (q = 0.62) have been investigated in the temperature range 300 to 875 degrees C as a function of oxygen partial pressure by means of thermogravimetry, conductivity, and Seebeck coefficient measurements. A defect chemical model has been established to assess the concentrations of the different relevant defects in [Ca2CoO3-delta](q)[CoO2] , including the concentration of mobile holes. By controlling the oxygen content, the hole concentration changes from 0.32 to 0.42 per formula unit, leading to a significant variation of both the conductivity and the Seebeck coefficient. The transport properties of [Ca2CoO3-delta](q)[CoO2] at high temperatures are discussed, and it is concluded that the electronic conduction is by itinerant holes-rather than by localized small polarons. Furthermore, it is shown that the concentration-dependent Seebeck coefficient cannot be described by Heikes' formula in the tested temperature range despite its widespread use. It is suggested that a significant contribution to the concentration dependency of the Seebeck coefficient is due to the mobility varying with the hole concentration (or equivalently the Fermi level).