High-temperature electrical conductivity and thermopower in nonstoichiometric La_1-xSr_xCoO_3-δ (x = 0.6)

From 650 to 900°C in situ conductivity and thermopower measurements it was determined that La_1-xSr_xCoO_3-δ (x = 0.6) is a small polaron conductor over the range 0.12 < δ < 0.38 with a hopping energy of approximately 0.08 eV. Cobalt's valence distribution was calculated from the thermopower data complemented with the site balance and electroneutrality relations. Three valence states of cobalt appear to be present with the equilibrium constant of disproportionation, 2Co³⁺ = Co²⁺ + Co⁴⁺ independent of temperature and dependent of oxygen content. As δ decreases, a p-to-n transition occurs indicating a near half-filling of the conductance band. It is concluded also that the simple polaronic description of conductance becomes insufficient when approaching the stoichiometric oxygen composition.