Oxygen nonstoichiometry and defect structure analysis of B-site mixed perovskite-type oxide (La, Sr)(Cr, M)O3−δ (M=Ti, Mn and Fe)

The defect chemical relationships in various B-site mixed LaCrO3-based ceramics were investigated by means of high-temperature gravimetry. The nonstoichiometric deviation, δ, in (La0.7Sr0.3)(Cr1−yTiy)O3−δ (y=0.1, 0.2 and 0.3) (LSCT), (La0.75Sr0.25)(Cr0.5Mn0.5)O3−δ (LSCM) and (La0.75Sr0.25)(Cr0.5Fe0.5)O3−δ   (LSCF) were measured as a function of oxygen partial pressure, PO2PO2, at temperatures between 973 and 1373 K.The effects of partial replacement of the donor on Cr-sites were examined in LSCT. In LSCM and LSCF, effects of the partial substitution of isovalent transition metals on Cr-sites are discussed. Oxygen nonstoichiometries of various B-site mixed LaCrO3-based ceramics were compared with those of A-site substituted perovskite-type oxides, (La1−xSrx)MO3−δ (where x=0–0.3, M  =Cr, Mn and Fe). The partial substitution of the different elements on Cr-sites drastically changed the PO2PO2 and temperature dependence of oxygen vacancy formation in LaCrO3-based ceramics. The defect equilibrium relationships of the localized electron well explained the oxygen vacancy formation in B-site mixed LaCrO3-based ceramics. Oxygen vacancy formation in (La0.7Sr0.3)(Cr1−yTiy)O3−δ (y=0.1 and 0.2) and (La0.7Sr0.3)(Cr0.7Ti0.3)O3−δ was explained by redox reaction of Cr and Ti ions, respectively. The defect equilibrium relationships of LSCM and LSCF were interpreted by redox reaction of Mn ions and Fe ions, respectively. No significant change in valence state of Cr3+ ions in LSCM and LSCF was confirmed under the experimental conditions.

Oxygen nonstoichiometry of (La0.75Sr0.25)(Cr0.5Fe0.5)O3−δ was plotted as the functions of partial oxygen pressure and temperature. The results were well explained by the localized electron on the Fe-sites and the equilibrium constants of the defect chemical equation were determined. A hysteresis was observed under the reducing atmospheres above 1173 K due to decomposition of Fe ions.Figure optionsDownload as PowerPoint slide