Oxygen Nonstoichiometry and Defect Equilibria of Yttrium Manganite Perovskites with Strontium A‑Site and Aluminum B‑Site Doping

• Published in: Journal of physical chemistry. C 2020-02, Vol.124 (8), p.4448-4458
• Main Authors: Carrillo, Richard J, Hill, Caroline M, Warren, Kent J, Scheffe, Jonathan R
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.9b11308

In this work, we investigate YMnO3 perovskites with Sr2+ and Al3+ doping, Y1–x Sr x Mn1–y Al y O3, for use in thermochemical H2O or CO2 splitting for the first time. Oxygen nonstoichiometries (δ) with x = 0.1 or 0.2 and y = 0.4 or 0.6 were measured over a wide range of temperatures and oxygen partial pressures (pO2). Experiments were performed from 973 to 1173 K and pO2 from 1.24 × 10–20 to 2.26 × 10–13 atm in a high-temperature tubular reactor coupled with residual gas analysis by delivering controlled mixtures of H2 and H2O. Higher temperature and pO2 experiments, from 1173 to 1473 K and 1.61 × 10–4 to 3.23 × 10–2 atm, respectively, were performed via thermogravimetric analysis by delivering controlled mixtures of O2 and Ar. In the low-temperature and pO2 region, each material showed low sensitivity to changes in pO2; thus, the slopes of the equilibrium δ isotherms were small. These experiments, under precisely controlled reaction conditions, help to explain some of the discrepancies in the literature regarding Y0.5Sr0.5MnO3 that, in some cases, highlight its superior behavior compared to ceria and other perovskite compositions. Defect models considering oxygen vacancy formation accompanied by either reduction of Mn4+ to Mn3+ or Mn4+ to Mn2+ along with the formation of extended defects were formulated and tested to describe the chemical equilibria of the synthesized perovskites. Y0.8Sr0.2Mn0.4Al0.6O3 could be well described by Mn4+ to Mn3+, but other compositions showed mixed results because of phase transformations, as confirmed by powder X-ray diffraction.