Thermodynamic Assessment of the La-Cr-O System

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, La-2(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: D-f,D-oxides degrees H-298K(LaCrO3) = -73.7 kJ mol(-1), and degrees S-298K(LaCrO3) = 109.2 J mol(-1) K-1. The Gibbs energy of formation from the oxides, Delta(f,oxides) degrees G(LaCrO3) = -72.403 - 0.0034T (kJ mol(-1)) (1273-2673 K) is calculated.

The decomposition of the perovskite phase by the reaction LaCrO3 -> 1/2 La2O3 + Cr + 3/4 O-2(g) up arrow is calculated as a function of temperature and oxygen partial pressure: at 1273 K the oxygen partial pressure of the decomposition, pO(2(decomp)) = 10(-20.97) 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.