Phase equilibria in the ZrO₂-CeO₂-Ln₂O₃ systems are important for the development of alternative ceramics for thermal barrier coatings (TBCs) other than state of the art yttria-stabilized zirconia (YSZ), aiming to lower thermal conductivity and to improve high temperature performance and durability. Ceria doped with aleovalent cations, such as rare earth oxides, has been considered as one of the most promising candidate materials for intermediate temperature solid oxide fuel cells (SOFCs) because of its much higher ionic conductivity at lower temperatures in comparison with that of stabilized zirconia. In addition, ceria finds its application in optical polishing, petroleum cracking catalyst, nuclear reactors components etc.

Present work is about phase equilibria in the ternary ZrO₂-CeO₂-Ln₂O₃ systems and in the boundary binary systems CeO₂-Ln₂O₃, ZrO₂-Ln₂O₃ (where Ln= La, Sm, Gd) at 1100-1500 °C in air in the whole concentration range. X-ray diffraction, petrography and electron microprobe X-ray diffraction were used to determine phase contents. The microstructures of the sintered ceramic samples were examined by using the scanning electron microscopy (SEM).

The isothermal sections at 1100, 1500 °C for the ternary systems ZrO₂-CeO₂-La₂O₃, ZrO₂-CeO₂-Sm₂O₃ and ZrO₂-CeO₂-Gd₂O₃ were developed. The main regularities of the phase diagram state (topology) have been revealed in both binary and ternary systems as a function of dopant ion radius. It was shown that the character of the phase equilibrium in the ternary system is determined by the binary phase diagram.

In the subsolidus area of the phase diagrams ZrO₂-CeO₂-Ln₂O₃ solid solutions based on tetragonal (T) ZrO₂, cubic with fluorite-type structure (F) CeO₂ and ZrO₂, hexagonal (A) La₂O₃, cubic form of rare-earth oxides (C-type) and monoclinic (B) forms Sm₂O₃ and Gd₂O₃ as well as intermediate phases with pyrochlore-type structure Ln₂Zr₂O₇ were found.

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