High-Temperature Coulometric Titration of La1–xSrxCoO3–d : Evidence for the Effect of Electronic Band Structure on Nonstoichiometry Behavior

The equilibrium oxygen chemical potential and the partial energy and entropy of O2 in perovskites La12xSrxCoO32d were measured as a function of d and x by high-temperature oxygen coulometric titration. An almost linear decrease in the oxygen chemical potential is observed with increasing net electron concentration 2d2x. The observed behavior is interpreted to reflect the corresponding increase in the Fermi level on filling up states in a broad electron band with electrons induced by vacancy formation or reducing the Sr-dopant level. The results enable calculation of the apparent density of states at the Fermi level. For low values of d and high temperatures, the oxygen vacancies in La12xSrxCoO32d are randomly distributed among equivalent oxygen sites. By increasing d or by lowering the temperature, additional ionic contributions to the partial energy and entropy arise, but these cancel in the chemical potential of oxygen vacancies. These results are possibly due to ordering of oxygen vacancies into microdomains. ( 1997 Academic Press