Use of the Rigid Band Formalism to Interpret the Relationship between O Chemical Potential and Electron Concentration inLa1−xSrx

Abstract

The equilibrium oxygen chemical potential of mixed oxygen ion and electron conducting perovskites La 12x Sr x CoO 32d was measured as a function of d and x by high temperature oxygen coulometric titration. An almost linear decrease in the chemical potential is observed with increasing net electron concentration. The observed behavior is interpreted to reflect the corresponding change in the Fermi level upon gradually filling up states in a broad electron band with electrons induced by vacancy formation and Sr doping. Assuming a rigid band formalism, an apparent density of states at the Fermi level was calculated, the value of which is in good agreement with that obtained from XPS.[ S0031-9007(96) PACS numbers: 71.20. Eh, 61.72.Ji, 66.30.Fq Many of the 3d transition metal perovskite-type oxides are characterized by increased lattice oxygen vacancy formation at high temperatures, which results in substantial departure from oxygen stoichiometry. Vacancy formation is accompanied by a proportional change in the average valency of either the transition metal or oxygen ions. The relatively high vacancy mobility in conjunction with the metalliclike electronic conductivity causes these materials to be permeable for oxygen gas [1], which is of interest for various applications. In addition, many of the 3d transition metal perovskites are studied for special electronic and magnetic transitions which may occur either as a function of temperature or composition [2][3][4]. In the ZSA theory [5], the electronic structure of these oxides is interpreted in terms of the competition between charge disproportionation of the Mott Hubbart type,L, where d n denotes the n-electron 3d state of the transition metal and L denotes a hole in the wide anion valence band. Although charge disproportionation was not observed in LaCoO 3 by XAS [3], Sarma et al. [4] state that for this material the influence of both types of charge fluctuation is of the same order of magnitude, leading to a very mixed character of the ground state. Charge disproportionation was used to model oxygen nonstoichiometry [6] and Seebeck data [7] of La 12x Sr x CoO 32d . In both cases, however, the best fit implied the simultaneous presence of unrealistically high concentrations of Co 21 and Co 41 . In Sr-doped LaCoO 3 , significant spectral intensities at the Fermi energy have been observed by Sarma et al.[4] using UPS and bremsstrahlung isochromat spectroscopy. They concluded that electron hole states, created by substitution of La 31 by Sr 21 , overlap the top of the wide oxygen 2p band giving rise to a mixed-valency metallic compound. Furthermore, conductivity and Seebeck measurements [7] at high temperatures also indicate metalliclike properties for Sr-doped LaCoO 3 . These observations favor the interpretation in terms of itinerant electron behavior in these solids.The aim of this study is, on the one hand, to improve existing oxygen nonstoichiometry models for the compounds La 12x Sr x CoO 32d by incorporating the delocalized nature of the conduction electron...

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

“…2, which was described previously in Refs. (13,24). A cylindrically shaped disk of La \V Sr V CoO \ (x"0.4 or x"0.7) was enclosed in an electrochemical cell with an internal volume of approximately 250 mm .…”

Nonlinear Impedance Analysis of La0.4Sr0.6Co0.2Fe0.8O3-δThin Film Oxygen Electrodes

“…17,28 In the case of LSC, one possible cause of modified properties is maldistribution of the Sr dopant, which has a proportional influence on the enthalpy of oxygen incorporation, and thus an exponential impact on oxygen vacancy concentration. 16,29 A number of workers have reported that the Sr dopant concentration near the surface of as-sintered LSCF is substantially increased, either by stratification or segregation, which might be expected to lead to an overall average increase in oxygen nonstoichiometry that depends on surface area. [19][20][21] Indeed, this possibility is consistent with published TGA measurements, which reveal an increase in the overall nonstoichiometry in LSCF with decreasing sintering temperature (i.e.…”

Electrochemical Performance of Nanoscaled La0.6Sr0.4CoO3-δ as Intermediate Temperature SOFC Cathode

“…The high performance of the nanoscaled cathodes can thus not solely be explained by an enlargement of the inner surface area, resulting from a nanoscaled microstructure, as suggested in (7). Therefore we conclude, that LSC prepared by MOD must exhibit enhanced catalytic properties for the oxygen surface exchange reaction, significantly higher than those of LSC prepared by conventional methods such as solid state reaction, glycine-nitrate route, etc.. where f = 0.69 is the correlation factor for a vacancy diffusion mechanism determined for perovskites (26) and γ O = 180 is the thermodynamic factor for LSC at 600 °C, calculated applying the rigid-band formalism by Lankhorst et al (27) with the data from (21,22). Figure 7 summarizes the output for T = 600 °C, porosity ε = 30 %, k δ = 1.12·10 -4 cm/s and D δ = 1.27·10 -7 cm²/s.…”