Nonstoichiometry, Defect Chemistry and Oxygen Transport in Fe-Doped Layered Double Perovskite Cobaltite PrBaCo2-xFexO6-δ (x = 0-0.6) Membrane Materials.

Mixed conducting cobaltites PrBaCo2-xFexO6-δ (x = 0-0.6) with a double perovskite structure are promising materials for ceramic semi-permeable membranes for oxygen separation and purification due to their fast oxygen exchange and diffusion capability. Here, we report the results of the detailed study of an interplay between the defect chemistry, oxygen nonstoichiometry and oxygen transport in these materials as a function of iron doping. We show that doping leads to a systematic variation of both the thermodynamics of defect formation reactions and oxygen transport properties. Thus, iron doping can be used to optimize the performance of mixed conducting oxygen-permeable double perovskite membrane materials.

Thermodynamics of Formation and Disordering of YBaCo2O6-δ Double Perovskite as a Base for Novel Dense Ceramic Membrane Materials.

Differential scanning calorimetry studies of the complex oxide YBaCo2O6-δ (YBC), combined with the literature data, allowed outlining the phase behavior of YBC depending on the oxygen content and temperature between 298 K and 773 K. The oxygen nonstoichiometry of single-phase tetragonal YBC was measured at different temperatures and oxygen partial pressures by both thermogravimetric and flow reactor methods. The defect structure of YBC was analyzed. As a result, the thermodynamic functions (∆Hi○, ∆Si○) of the defect reactions in YBC were determined. Experimental data on the oxygen content and those calculated based on the theoretical model were shown to be in good agreement. Standard enthalpies of formation at 298.15 K (∆Hf○) were obtained for YBC depending on its oxygen content using solution calorimetry. It was found that ∆Hf○ = f(6-δ) function is linear in the range of (6-δ) from 5.018 to 5.406 and that its slope is close to the value of the enthalpy of the quasichemical reaction describing oxygen exchange between the oxide and ambient atmosphere, which confirms the reliability of the suggested defect structure model.

Redox Thermochemistry, Thermodynamics, and Solar Energy Conversion and Storage Capability of Some Double Perovskite Cobaltites.

The oxygen nonstoichiometry, δ, and oxidation enthalpy, ΔHox, of double perovskites RBaCo2O6-δ (R = Sm or Eu) were simultaneously measured depending on the temperature and oxygen partial pressure, pO2. Theoretical equations for ΔHox(T, δ) and pO2(T, δ) were derived from the defect structure model based on the oxygen exchange and cobalt disproportionation reactions. These equations were fitted independently to each of the experimental ΔHox(T, δ) and pO2(T, δ) data sets. The resulting enthalpies of defect reactions were found to be almost the same irrespective of the calculation method. In other words, the models, describing satisfactorily the data, can be used to calculate both compositional dependences and redox thermodynamics of RBaCo2O6-δ (R = Sm or Eu). In addition, from the previously published data and the data presented here, trends were determined in the defect reaction thermodynamics of RBaCo2O6-δ (R = La, Pr, Nd, Sm, Eu, Gd, or Y). Drop calorimetric measurements were performed in air to obtain enthalpy increments for RBaCo2O6-δ (R = Sm or Eu) with variable oxygen content because the samples lost oxygen upon being heated in the calorimetric cell. As-obtained data were used to calculate the functional dependences of enthalpy increments of EuBaCo2O5.56 and SmBaCo2O5.6 with a constant oxygen content. In addition, as an example of practical application-oriented calculations for solar energy conversion and oxygen storage, the performances at equilibrium of RBaCo2O6-δ (R = Pr, Sm, Eu, or Gd) were evaluated and compared to those of SrFeO3-δ as a reference material.

In Situ and ex Situ Study of Cubic La0.5Ba0.5CoO3-δ to Double Perovskite LaBaCo2O6-δ Transition and Formation of Domain Textured Phases with Fast Oxygen Exchange Capability.

The disordered La0.5Ba0.5CoO3-δ ↔ ordered LaBaCo2O6-δ transition was studied in detail using several complementary in situ (X-ray diffraction, thermogravimetry, and coulometric titration) and ex situ (transmission electron microscopy) techniques. This transition was found to proceed through the formation of complex domain textured intermediate products. They were shown to have strong affinity to oxygen and exhibit its fast absorption from ambient atmosphere (oxygen partial pressure ( pO2) 0.21 atm) at a temperature as low as 70 °C. The thermodynamic stability limits of the cubic and double perovskites were determined by coulometric titration. The stability diagram of the LaBaCo2O6-δ - La0.5Ba0.5CoO3-δ system was plotted as a result. Oxygen nonstoichiometry of the thermodynamically stable cubic perovskite La0.5Ba0.5CoO3-δ was measured as a function of pO2 in the temperature range between 1000 and 1100 °C using a coulometric titration technique.