Determination of Oxygen Nonstoichiometry and Diffusivily in Mixed Conducting Oxides by Oxygen Coulometric Titration II. Oxygen Nonstoichiometry and Defect Model for 1a085r02CoO3_8

The oxygen nonstoichiometry of La0 8Sr0 3CoO3 has been determined as a function of oxygen partial pressure and temperature using a high-temperature coulometric titration cell. For each measured value of the oxygen chemical potential, the oxygen nonstoichiometry is found to be nearly independent of temperature. The equilibrium partial energy and entropy associated with oxygen incorporation have been determined as a function of oxygen nonstoichiometry and temperature. The results are interpreted in terms of a model in which it is assumed that conduction electrons, created during vacancy formation, gradually fill electron states in a wide electron band. A new relation between vacancy concentration, temperature, and oxygen partial pressure has been formulated which does not have the familiar appearance of a mass action type of equation. EMF = p.g, — oxide [1] 4F = 1(t) — 'leak dt msample .in.o 2F La1_0SrCoO3_0. Data of oxygen nonstoichiometry and partial energy and entropy of oxygen are measured for the compound La0 8Sr0 3CoO33 using high temperature oxygen coulometric titration. Data of chemical diffusion coefficient for La03Sr02CoO3_8 obtained with this technique have been presented in Part I of this paper.7 Theory Oxygen coulometric titration.—In a high-temperature oxygen coulometric titration experiment the oxide sample is enclosed in a sealed electrochemical cell. The oxygen stoichiometry of the sample is adjusted by electrochemically pumping oxygen into or out of the cell using a solid electrolyte. The oxygen chemical potential of the sample inside the sealed compartment is determined from the EMF measured across an auxiliary electrolyte using identical metallic probes Introduction Acceptor-doped cobaltite perovskite-type oxides are characterized by enhanced lattice oxygen vacancy formation, which results in substantial departures from oxygen stoichiometry at increased temperatures. A change in the vacancy concentration is accompanied by a corresponding change in the average valence of either the cobalt or oxygen ions. The high concentration of oxygen vacancies in conjunction with their relatively high mobility cause these materials to exhibit high oxygen ion conductivity. The electronic conductivity is even higher and becomes metallic at high temperatures. Due to the mixed conductivity, these materials are permeable to oxygen gas. Advantageous use can be made as a ceramic membrane for the separation of oxygen from air, e.g., for the production of oxygen and nitrogen or for selectively feeding of oxygen into chemical reactors. The defect equilibria for the formation and annihilation of ionic and electronic defects by the reaction of the oxide with the gas phase are commonly analyzed using a traditional formalism12 in which the relationships among defect concentrations and environmental parameters can be expressed by simple mass action type of equations. The latter are based on the assumption that all involved point defects can be treated as noninteracting and ideally diluted species for which corresponding chemical potentials can be defined. Such a formalism has been applied successfully to model experimental data of oxygen nonstoichiometry and electrical conductivity of La1_Sr1FeO31 and La1.0Sr0CrO3_1. l In these compounds the transition metal ions are assumed to be charge disproportionated among three different valence states. Electronic charge carriers introduced either by acceptor doping or by oxygen vacancy formation upon decreasing the oxygen partial pressure occupy localized transition-metal 3d states. The validity of the charge disproportionation model cannot be assumed a priori for compounds La1_SrCoO30. There is substantial evidence for itinerant electron behavior in these compounds. Using UPS and Bremsstrahlung isochromat spectroscopy, Sarma et al.4 observed significant spectral intensities at the Fermi energy. They concluded that electron hole states, created by substitution of La by Sr, overlap the top of the wide oxygen 2p band giving rise to a mixed-valence metallic compound. The relatively large electronic conductivity and low Seebeck coefficients observed for the lanthanum-based cobaltite perovskites5'6 also favor the interpretation in terms of itinerant electrons. The general aim of this paper is to obtain insight in the thermodynamics of oxygen incorporation into perovskites where F is Faraday's constant and the oxygen chemical potential of the reference gas (e.g., air), the value of which can be calculated by substituting the values of the oxygen partial pressure and temperature into Eq. A-i given in the Appendix. When the above procedure to determine is performed as a function of temperature at constant oxygen stoichiometry of the sample, the entropy part of gxide, 5gxide defined by oiode = [oade] [2] may be evaluated. The energy part €gmay be calculated from = oxide — As was discussed in Part I, electrochemical pumping of oxygen is generally performed using either galvanostatic or potentiostatic methods. Both methods allow evaluation of the change in oxygen nonstoichiometry of the sample AS by integration of the current I over time __________ ___________ [3] where Il is the unavoidable leakage current, maample the mass of the sample, and mmoja. moss the molar mass. The potentiostatic method allows easy determination of 'leak by measuring the value to which the current decays after a potentiostatic step. In the galvananostatic method, 'leak can 1268 J. Electrochem. Soc., Vol. 144, No. 4, April 1997 The Electrochemical Society, Inc. Downloaded 22 Jun 2009 to 130.89.112.86. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp J Electrochem. Soc., Vol. 144, No. 4, April 1997 The Electrochemical Society, Inc. 1269 be calculated from the change in EMF with time when the sample is in equilibrium with the surrounding gas. Nonstoichiometry models: charge disproportionation model.—Mizusaki et at. were able to describe data of nonstoichiometry of La2_SrFe03_9 and La5SrCr03_9 from thermogravimetric measurements, using a model based on randomly distributed and noninteracting point defects. Applying the model to La1_SrCo03, Co-ions are charge disproportionated among oxidation states Co2, Co3, and Co4t Using KrUger-Vink notation,'° this reaction can be represented as