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 1273K 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: $$ \Updelta_{\text{f,oxides}} {^{\circ }H}_{{298{\rm K}}} ( {\text{LaCrO}}_{ 3} )= - 7 3. 7 {\text{ kJ}}\,{\text{mol}}^{-1} $$ , and $$ {}^{\circ } S_{{298\,{\text{K}}}} ( {\text{LaCrO}}_{ 3} )= 109.2{\text{ J}}\,{\text{mol}}^{ -1}\, {\text{K}}^{ -1} $$ . The Gibbs energy of formation from the oxides, $$ \Updelta_{\text{f,oxides}} {^{\circ } G} ( {\text{LaCrO}}_{ 3} )= -72.403 - 0.0034$$ T (kJmol−1) (1273-2673K) is calculated. The decomposition of the perovskite phase by the reaction $$ {\text{LaCrO}}_{3} \to \frac{1}{2}{\text{La}}_{2} {\text{O}}_{3} + {\text{Cr}} + \frac{3}{4}{\text{O}}_{2} ({\text{g}}) \uparrow $$ is calculated as a function of temperature and oxygen partial pressure: at 1273K the oxygen partial pressure of the decomposition, $$ p_{{{\text{O}}_{{\text{2}}} {\text{(decomp)}}}} = 10^{-20.97}\,{\text{Pa}}$$ . Cation nonstoichiometry of La1-x CrO3 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 liquid