High temperature X-ray diffraction and thermo-gravimetrical analysis of the cubic perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ under different atmospheres

Abstract

ARTICLE This journal is © The Royal Society of Chemistry 2013Dalton Trans., 2013, 00, 1-3 | 1 (BSCF) with the cubic perovskite structure is known to be metastable at low temperature in oxidizing atmosphere. Here, the thermal and chemical expansion of BSCF were studied by in situ high temperature powder X-ray diffraction and thermo-gravimetrical analysis (TGA) in partial pressure of oxygen ranging from inert atmosphere (~10 -4 bar) to 10 bar O 2 . The BSCF powder, heat treated at 1000 ºC and quenched to ambient prior to the analysis, was shown to oxidize in oxidizing atmosphere before thermal reduction took place. With decreasing partial pressure of oxygen the initial oxidation was suppressed and only reduction of Co/Fe and loss of oxygen were observed in inert atmosphere. The thermal expansion of BSCF in different atmospheres was determined from the thermal evolution of the cubic unit cell parameter, demonstrating that the thermal expansion of BSCF depends on the atmosphere. Chemical expansion of BSCF was also estimated based on the diffraction data and thermo-gravimetrical analysis. A hexagonal polymorph BSCF, coexisting with the cubic polymorph, was observed to form above 600 ºC during heating. The formation of the hexagonal polymorph was driven by oxidation, and the unit cell of cubic BSCF was shown to decrease with increasing amount of hexagonal BSCF formed. The hexagonal BSCF polymorph disappeared upon further heating, accompanied with an expansion of the unit cell of the cubic BSCF.

Understanding the effect of water transport on the thermal expansion properties of the perovskites BaFe0.6Co0.3Nb0.1O3−δ and BaCo0.7Yb0.2Bi0.1O3−δ

“…The BCYB sample showed a higher mass loss associated with water release, indicating greater water content for this phase; the mass change was -1.4% compared to -0.35% for BFCN. At higher temperatures (onset temperature around 350°C for both samples), further mass loss was observed, which can be attributed to oxygen loss and thus a reduction in Fe/ Co oxidation state, as observed for other Fe/Cobased perovskite electrode materials [50,51]. Only oxygen and water were detected by mass spectrometer with the increase in temperature (test in N 2 ).…”

Tuning the thermoelectric properties of A-site deficient SrTiO3 ceramics by vacancies and carrier concentration

“…The carrier concentrations reported here are in broad agreement with values reported for heavily doped SrTiO 3 ceramics, while the mobility values are almost an order lower than for similar oxides. 23,56 Based on the thermoelectric parameters presented in Fig. 7, the thermoelectric figure of merit (ZT) was determined (Fig.…”

Thermodynamic Stability and Microscopic Behavior of BaxSr1-xCo1-yFeyO3-δ Perovskites

“…This extraordinary ability to host oxygen vacancies and to transport a significant amount of oxygen anions via oxygen vacancies (with an oxygen diffusion coefficient of about 10 À6 cm 2 /s and ionic conductivity of 0.018 S/cm at 973 K) has been reported for Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 À δ [4][5][6][7][8]. However, significant differences were observed in the structural characteristics and the electrochemical performance of the BSCF solid solutions as a function of temperature (T), atmosphere (inert [9], oxidizing [10][11][12] and reducing [13,14]), as well as of the nature and concentration of A-and B-sites substituents [15][16][17][18]. As consequence, there are questions regarding the long-term stability and performance of such highly defective material under the operating conditions of an intermediate temperature solid oxide fuel cell (IT-SOFC) of 873-1073 K. At low oxygen partial pressures (under 10 À5 atm) and in reducing conditions the structural changes through different pathways was shown [16,17,19]; the variation of the oxidation state of the Co ion was suggested as the driving force for these changes [14].…”