About La₂Zr₂O₇

About the thermodynamic data of La₂Zr₂O₇, recent investigations have been examined carefully. As a result of reconsiderations, a decision was made not to change the data of La₂Zr₂O₇. These considerations were reported in the proceedings of the European Fuel Cells Forum 2014, which was held in Luzern, Switzerland:

Harumi Yokokawa
Thermodynamics of LSM-YSZ Interfaces- A Revisit with Confirmed La₂Zr₂O₇ Thermodynamic Data
Proc. 11th European SOFC-SOE Forum 2014, Chap. 10-B0624, 1344-1353, 2014.

Abstract:

From late 1980's to the early 90's, Yokokawa made a series of thermodynamic analyses on the perovskite electrode and the YSZ electrolyte. For the LSM-YSZ interface, evaluation of the thermodynamic data of La₂Zr₂O₇ was made to obtain consistency among LSM-YSZ diffusion couple experiment by Lau and Singhal, thermogravimetric results on nonstoichiometric La_1-xMnO₃ by Shimomura et al, and calorimetric measurements by *V. R. Korneev. After this compilation, a new calorimetric value determined by M. Bolech et al. was more negative than the complied value. The calphad-type optimization was made by Gaukler's group by relying on the new value, so that they criticized Yokokawa's evaluated data of La₂Zr₂O₇ and his analytical results of phase relations related to LSM-YSZ interface. Even so, Yokokawa's analyses relied on Lau and Singhal data, so the issue to be confirmed is which diffusion couple data by Lau and Singhal or the calorimetric data by Bolech is incorrect. Upon Yokokawa's request, Navrotsky's group made calorimetric measurements again on La₂Zr₂O₇ and their new data indicate less negative. It becomes more consistent with diffusion couple data by Lau and Singhal. These chronological events eventually confirmed that Yokokawa's initial treatment is essentially valid. Since Yokokawa's first work on the LSM-YSZ interface, many interesting and important experimental results have been reported not only for SOFC mode but also for SOEC mode. These new findings will be discussed in terms of equilibrium data as well as some kinetic considerations. For the SOFC mode, the direct confirmation of Yokokawa's results was made by Siemens. Quite recently, Matsui et al reported the interesting features of slightly A-site deficient LSM cathodes having different La content, indicating the formation of a dense LSM layer at the LSM-YSZ interface more significantly for the La-rich composition, where the A-site deficiency becomes more significant with growing possibility of La₂Zr₂O₇ formation at a given A-site deficient value. Analogous consideration to the sintering behavior of the A-site deficient cathode on heat treatment suggests that the oxide ion vacancies formed during the electrochemical polarization together with the cation vacancies enhance the sintering of LSM cathode without precipitation of Mn oxides.

Behavior under the SOEC mode should depend largely on the magnitude of polarization, since the LSM-YSZ interface chemistry depends on the oxygen potential mainly because the A-site deficient width or the La₂Zr₂O₇ formation region depends on the valence of Manganese ions. With increasing oxygen potential, the La₂Zr₂O₇ is partially formed at the interface, and then finally La₂Zr₂O₇ as well as MnO₂ is formed as a result of oxidative decomposition.