Thermodynamic Assessment of the La-Cr-O System

Abstract

The La-Cr and the La-Cr-O systems are assessed using the Calphad approach. The calculated La-Cr phase diagram as well as LaO 1.5 -CrO 1.5 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 La 2 CrO 6 , La 2 (CrO 4 ) 3 , and perovskite-structured LaCrO 3 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;oxides H 298K ðLaCrO 3 Þ = À 73:7 kJ mol À1 , and S 298 K ðLaCrO 3 Þ = 109:2 J mol À1 K À1 . The Gibbs energy of formation from the oxides, D f;oxides GðLaCrO 3 Þ = À 72:403 À 0:0034T (kJ mol 21 ) (1273-2673 K) is calculated. The decomposition of the perovskite phase by the reaction LaCrO 3 ! 1 2 La 2 O 3 + Cr + 3 4 O 2 ðgÞ " is calculated as a function of temperature and oxygen partial pressure: at 1273 K the oxygen partial pressure of the decomposition, p O 2 ðdecompÞ = 10 À20:97 Pa. Cation nonstoichiometry of La 1-x CrO 3 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.

Thermodynamic modeling of La2O3–SrO–Mn2O3–Cr2O3for solid oxide fuel cell applications

“…Thus, we believe that the experimental increase of d from 1173 to 1073 K might be too small, possibly caused by equilibration difficulties due to slow diffusion at these lower temperatures. 69,70 in the case of LaCrO 3 , 20 we did not obtain satisfying results in higher-order perovskites, most likely due to more complex interactions that cannot be sufficiently described by the model. Since the magnetic transitions are low temperature features, their modeling was omitted without consequences for the applicability of the database for SOFC.…”

“…No quaternary phases or solid solutions were found in the Sr-Mn-Cr-O oxide system. 16 Previous assessments of the La-O, Cr-O, La-Cr-O, and Cr-Mn-O oxide databases are adopted from Povoden et al, [17][18][19][20][21] with some modifications denoted in the supplement file to this paper, LSCM_v1.0.tcm (ThermoCalc macrofile), and the La-Sr-Mn-O oxide database is taken from Grundy et al, [22][23][24][25][26][27] also with the following slight modification: Grundy et al 23,24 allowed Mn 31 on the A-site of LSM to reproduce experimental oxygen nonstoichiometries under low oxygen partial pressures. Due to large differences between the ionic radii of La 31 and Mn 31 and possible coordination numbers (1.5 Å for 12-fold coordinated La 31 , 0.785 Å for at maximum 6-fold coordinated Mn 31 ), 28 we omit Mn 31 on the A-site.…”

Advanced Cell Development and Degradation Studies

“…Regarding the thermodynamics of the La-Cr-O system, a recent study of thermodynamic calculations using CALPHAD [31] showed that LaCrO 3 is stable up to 1000°C in atmosphere ranging from pure O 2 to PO 2 = 10 -16.1 Pa. On the other hand, La 2 CrO 6 was shown to form within a wide temperature range above 7000°C at PO 2 at and above 10 5 Pa. While the SOEC stacks are operated at atmospheric pressure, the local PO 2 in anode and contact layer of the cells can be greater than 10 5 Pa because of the generation and evolution of O 2 .…”

Thermodynamic assessment of the CoOx–CrO1.5 system

“…The thermodynamic descriptions for the 3 binary sub-systems (Co-O, Cr-O, Co-Cr) are from Chen et al [19], Povoden et al [29], and Kusoffsky and Jansson [30]. The oxygen solubilities in pure Co and pure Cr were modeled by Chen et al [19] and Povoden et al [29], respectively. No information on the oxygen solubility in the Co-Cr alloy is available in the literature.…”

“…There are nine end-members according to the current model of the cubic spinel phase. The four Gibbs energies of the end-members belonging to the [Co 2+ , Co 3+ ] 1 (Co 2+ , Co 3+ ) 2 O 4 system have been taken from Chen et al [19], and the Gibbs energy of the [Cr 2+ ] 1 (Cr 3+ ) 2 O 4 end-member has been taken from Povoden et al [29]. The Gibbs energies of the four end-members [ …”