A determination of the oxygen non-stoichiometry of the oxygen storage materials LnBaMn(2)O(5+delta) (Ln = Gd, Pr)

摘要

The A-site ordered double-perovskite oxides, LnBaMn(2)O(5+delta) (Ln=Gd, Pr), were synthesized and investigated to understand the effect of A site cation substitution on the oxygen storage properties of these materials. The present results are compared with our previous data for YBaMn2O5+delta. The results clearly reveal that changing the Ln cation strongly influences the oxidation/reduction behavior of LnBaMn(2)O(5+delta). Based on thermogravimetric analysis data, oxygen uptake begins at lower temperatures in both air and oxygen in compounds with Ln(3+) ions larger than Y3+. These oxides exhibit almost complete and reversible oxygen uptake/release between fully-reduced LnBaMn(2)O(5) and fully-oxidized LnBaMn(2)O(6) during changes of the oxygen partial pressure between air and 1.99% H-2/Ar. In addition, the oxygen non-stoichiometries of GdBaMn2O5+delta and PrBaMn2O5+delta were determined as a function of pO(2) at 600, 650, 700 and 750 degrees C by Coulometric titration at near-equilibrium conditions. The results confirm that these materials have two distinct phases on oxidation/reduction with delta approximate to 0, 0.5 and a third phase with a range of composition with an oxygen content (5+delta) approaching similar to 6. The stabilities of the LnBaMn(2)O(5+delta) phases extend over a wide range of oxygen partial pressures (similar to 10(-25) <= pO(2) (atm) <=similar to 1) depending on temperature. Isothermal experiments show that the larger the Ln3+ cation the lower pO(2) for phase conversion. At some temperatures and pO(2) conditions, the LnBaMn(2)O(5+delta) compounds are unstable with respect to decomposition to BaMnO3-delta and LnMnO(3). This instability is more apparent in Coulometric titration experiments than in thermogravimetric analysis. The Coulometric titration experiments are necessarily slow in order to achieve equilibrium oxygen compositions. (C) 2016 Elsevier Inc. All rights reserved.