Oxygen release technique as a method for the determination of “δ–pO₂–T” diagrams for MIEC oxides

Abstract

A new approach to the determination of oxygen nonstoichiometry (δ) of MIEC oxides as a continuous function of pO2 at high temperatures was developed. The description of the model allowing one to distinguish the contribution of oxygen released from the samples to the partial pressure of oxygen at the outlet of the continuous-flow fixed-bed reactor after the stepwise change of the oxygen partial pressure of inlet gas from 0.2 to 10(-5) atm and to calculate the dependence of δ on pO2 is presented. The criterion for assessing the achievement of quasi equilibrium release of oxygen from the MIEC oxides is proposed. The adequacy of the method was confirmed by comparing the obtained and published data for well-studied SrCo0.8Fe0.2O3-δ and SrFeO3-δ MIEC oxides.

Determination of oxygen nonstoichiometry in SrFeO3−δ by solid‐state Coulometric titration

“…Mizusaki et al. considered the plateau in a δ vs log( p O 2 ) plot as corresponding to δ=0.5, and Starkov determined this by using iodometric titration . In addition, the present reference δ values differ somewhat from the previous data, since the absolute δ values that are determined by the chemical titrations tend to be scattered …”

“…In Figure A, lines of constant log( p O 2 ) are shown in a plot of temperature vs δ, and Figure B shows lines of constant δ in a plot of log( p O 2 ) vs reciprocal temperature. The previous data, measured by TGA and oxygen release techniques are also provided for comparison . Minor differences are observed in part because the absolute δ values were determined in different ways.…”

“…The lines correspond to p O 2 isobars and the negative numbers are log( p O 2 ), p O 2 in atm. The dotted lines are from previously published data, and B, the equilibrium δ of SrFeO 3−δ . The lines correspond to constant δ and the numbers correspond to the δ‐values.…”

“…In Figure , different values of δ are observed on increasing and decreasing p O 2 at T ≤850°C. The difference is due to the separation into two phases (perovskite and brownmillerite) which is known to occur with the formation of immiscible coexisting nanostructured domains of both compositions . On increasing the temperature, the perovskite phase width increases and the miscibility gap narrows.…”

“…The phase behavior of oxygen‐deficient SrFeO 3−δ has been extensively studied because of its interesting order‐disorder transitions . The phase behavior is closely related to the oxygen nonstoichiometric data, which are only available in a limited oxygen partial pressure ( p O 2 ) region . According to a phase diagram of SrFeO 3−δ drawn by Takeda et al., a cubic perovskite phase occurs at very high pO 2 region (δ<0.03), and at δ<0.3 when T >500°C .…”