High-temperature electrical conductivity and thermopower in nonstoichiometric La1-xSrxCoO3-delta (x=0.6)

From 650 to 900 degrees C in situ conductivity and thermopower measurements it was determined that La1-xSrCoO3-delta (x = 0.6) is a small polaron cond uctor over the range 0.12 < delta < 0.38 with a hopping energy of approxima tely 0.08 eV. Cobalt's valence distribution was calculated from the thermop ower data complemented with the site balance and electroneutrality relation s. Three valence states of cobalt appear to be present with the equilibrium constant of disproportionation, 2Co(3+) = Co2+ + CO4+ independent of tempe rature and dependent of oxygen content. As delta decreases, a p-to-n transi tion occurs indicating a near half-filling of the conductance band. It is c oncluded also that the simple polaronic description of conductance becomes insufficient when approaching the stoichiometric oxygen composition. (C) 20 00 Elsevier Science B.V. All rights reserved.