Nonstoichiometric oxides as a continuous homologous series: linear free-energy relationship in oxygen exchange

Abstract

A novel methodology for the analysis of oxygen exchange in practically important nonstoichiometric oxides with mixed ionic electronic conductivity (MIEC) is suggested. It is based on the fact that the kinetic and thermodynamic properties of such oxides vary continuously with oxygen stoichiometry. This allows MIEC oxides to be considered as a homologous series, with the difference that traditional series are discrete in their chemical composition whereas MIEC oxides are continuous in oxygen stoichiometry. Analysis of the relations between Gibbs energies of reactions and activation barriers traditionally performed for homologous series can be useful in studies of oxygen exchange in MIEC oxides. To demonstrate the approach, thermodynamic and oxygen-exchange kinetics parameters are measured as functions of oxygen nonstoichiometry δ for two perovskites, SrCo0.8Fe0.2O3-δ and SrFeO3-δ, having metal-like and p-type semiconducting conductivities, respectively. Both oxides are shown to obey linear free energy relationships of the Brønsted-Evans-Polanyi form in spite of their different types of electronic structures. The results open up new possibilities for understanding the mechanism of the rate determining step of oxygen exchange in MIEC oxides.

Electronic Structure and High-Temperature Thermochemistry of Oxygen-Deficient BaMO3 (M = Ti – Cu) Perovskites

“…Reduction free energies of BaMO 3−δ were calculated here in the manner of Deml et al , and Ezbiri et al Single neutral oxygen vacancies were created by randomly removing one oxygen atom from the 2 × 2 × 2 supercell to afford δ values of 0.125. A number of prior reports have demonstrated that neutral oxygen vacancies, with two trapped electrons, are favored in undoped oxides under reducing conditions. − Therefore, this investigation is limited to the formation of neutral oxygen vacancies as the stoichiometry of the perovskite reduction reaction requires the formation of O 2 from two neutral oxygen atoms . Here, the structures of BaMO 3−δ were fully optimized with cubic symmetry and constant volume conditions in the manner of Deml et al and Sundell et al Full details of this method are provided in the Supporting Information.…”

“…28−31 Therefore, this investigation is limited to the formation of neutral oxygen vacancies as the stoichiometry of the perovskite reduction reaction requires the formation of O 2 from two neutral oxygen atoms. 32 Here, the structures of BaMO 3−δ were fully optimized with cubic symmetry and constant volume conditions in the manner of Deml et al 27 and Sundell et al 24 Full details of this method are provided in the Supporting Information.…”

Oxygen Exchange in MIEC Perovskite Oxide La0.6Sr0.4Co0.2Fe0.8O3−δ: Kinetic and Equilibrium Parameters and Their Interrelation

“…where the Brønsted coefficient β is redesignated as n and R 0 is the equilibrium exchange rate in the eq 1. 33 A direct consequence of eq 3 is the power-law dependence of R 0 on the equilibrium partial pressure of oxygen pO 2 eq often observed in experiments:…”

“…This result is in a good agreement with the literature data, 45,57 in which the change in the Gibbs energy (μO 2 oxide (δ)) in ferrites is almost completely determined by the entropy coefficient, which is caused by the Fe 3+ /Fe 4+ disorder in the oxide lattice. At the same time, in the cobaltites considered in detail in the papers, 24,28,29,33 the dependences of the activation enthalpy R 0 on δ are completely determined by the component of the partial molar enthalpy in μO 2 oxide (δ). It should be understood that in the formal definition of the effective Gibbs activation energy R 0 , defined as −RT ln R 0 , there are summands having an effective entropic form arising due to constant coefficients in R 0 .…”

“…Therefore, required LFER formulation, strictly speaking, should not refer to the rate of the transformation but to the rate of equilibrium exchange in the reaction between a particular δ-homologue and oxygen. Taking this into account, the LFER formulation can be defined by the expression: R T ⁡ ln ⁡ R 0 false( δ false) = − Δ G a 0 + μ normalO 2 oxide n false( δ false) where the Brønsted coefficient β is redesignated as n and R 0 is the equilibrium exchange rate in the eq . A direct consequence of eq is the power-law dependence of R 0 on the equilibrium partial pressure of oxygen p O 2 eq often observed in experiments: R 0 = R 00 false( p normalO 2 eq false) n where R 00 is a coefficient independent of p O 2 eq .…”