Thermodynamic Assessment of the La-Cr-O System

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Section I: Basic and Applied Research

Thermodynamic Assessment of the La-Cr-O System E. Povoden, M. Chen, A.N. Grundy, T. Ivas, and L.J. Gauckler

(Submitted May 6, 2008; in revised form September 23, 2008) The La-Cr and the La-Cr-O systems are assessed using the Calphad approach. The calculated La-Cr phase diagram as well as LaO1.5-CrO1.5 phase diagrams in pure oxygen, air, and under reducing conditions are presented. Phase equilibria of the La-Cr-O system are calculated at 1273 K as a function of oxygen partial pressure. In the La-Cr system reported solubility of lanthanum in bcc chromium is considered in the modeling. In the La-Cr-O system the Gibbs energy functions of La2CrO6, La2(CrO4)3, and perovskite-structured LaCrO3 are presented, and oxygen solubilities in bcc and fcc metals are modeled. Emphasis is placed on a detailed description of the perovskite phase: the orthorhombic to rhombohedral transformation and the contribution to the Gibbs energy due to a magnetic order-disorder transition are considered in the model. The following standard data of stoichiometric perovskite are calculated: Df;oxides  H 298K ðLaCrO3 Þ =  73:7 kJ mol1 , and  S298 K ðLaCrO3 Þ = 109:2 J mol1 K1 . The Gibbs energy of formation from the oxides, Df;oxides  GðLaCrO3 Þ =  72:403  0:0034T (kJ mol21) (1273-2673 K) is calculated. The decomposition of the perovskite phase by the reaction LaCrO3 ! 12 La2 O3 + Cr + 34 O2 ðgÞ " is calculated as a function of temperature and oxygen partial pressure: at 1273 K the oxygen partial pressure of the decomposition, pO2 ðdecompÞ = 1020:97 Pa. Cation nonstoichiometry of La1–xCrO3 perovskite is described using the compound energy formalism (CEF), and the model is submitted to a defect chemistry analysis. The liquid phase is modeled using the two-sublattice model for ionic liquids.

Keywords

defect chemistry, LaCrO3, lanthanum chromate, SOFC

1. Introduction In solid oxide fuel cells (SOFC), the thermodynamic stability of the cathode is particularly important for efficient long-term operation. Sr-doped lanthanum manganites (LSM) with the perovskite structure are used as cathode materials in SOFC. Diffusion of chromium from the metallic interconnect with high chromium content into the cathode leads to the formation of Mn(Cr,Mn)2O4 spinel and Cr2O3 along with a severe cell voltage decrease.[1-4] As the thermal expansions of LaCrO3-based interconnect and conventional perovskite cathode materials are similar, and Cr-diffusion into the cathode from LaCrO3-based interconnects is significantly lower than from Cr-containing metallic interconnects, recently Sr-, V-doped[5] and Zn-doped[6] La1–xCaxCrO3–d have been considered as promising alternative interconnect materials for SOFC. Furthermore alkaline earth containing LaCrO3 has been proposed as a cathode material in a recent study by Jiang et al.[7]

E. Povoden, T. Ivas, and L.J. Gauckler, Nonmetallic Inorganic Materials, ETH Zurich, Zurich, Switzerland; M. Chen, Fuel Cells and Solid State Chemistry Department, Risø National Laboratory, Technical Unive