Interaction of Cerium, Lanthanum, and Samarium Oxides at 1250°C

Materials produced from cerium oxides stabilized by rare-earth metal (REM) oxides are promising for medical and engineering applications for their unique properties. The CeO2–La2O3–Ln2O3 phase diagrams serve as a physicochemical basis for developing solid electrolytes for fuel cells, oxygen sensors, catalyst carriers, protective coatings on alloys, etc. Phase equilibria and structural transformations in the CeO2–La2O3–Sm2O3 system at 1250°C were studied by X-ray diffraction over the entire composition range. The studies revealed fields of solid solutions based on the cubic (F) fluorite-type modification of CeO2, monoclinic (B) and cubic (C) modifications of Sm2O3, and hexagonal (A) modification of La2O3 in the ternary CeO2–La2O3–Sm2O3 system at 1250°C had predominantly cubic phases in equilibrium: CeO2-based F-phase with Fm3m space group and Sm2O3-based C-phase with Ia3 space group. The lattice parameters of the fluorite-type (F-CeO2) cubic solid solutions changed from a = 0.5409 nm for pure CeO2 to a = 0.5515 nm for the two-phase (F + C) 55 CeO2–22.5 La2O3–22.5 Sm2O3 sample (mol.%) along section La2O3 : Sm2O3 = = 1 : 1, to a = 0.5548 nm for the two-phase 55 CeO2–33.75 La2O3–11.25 Sm2O3 sample along section La2O3: Sm2O3 = 3 : 1, and to a = 0.5478 nm for the two-phase (F + C) 60 CeO2–10 La2O3–30 Sm2O3 sample along section La2O3 : Sm2O3 = 1 : 3. At 1250°C, the cubic F-CeO2 fluorite-type solid solutions are in equilibrium with all phases formed in the system. The isothermal section of the CeO2–La2O3–Sm2O3 phase diagram at 1250°C contains two three-phase (A + F + B, F +B + C) and five two-phase (A + F, A + B, F + B, B + C, F + C) regions.