INTERACTION OF YTTRIUM, LANTHANUM, AND SAMARIUM OXIDES AT 1600 °С

Abstract

Phase equilibria and structural transformations in the La₂O₃–Y₂O₃–Sm₂O₃ system at 1600 °C were studied by X-ray diffraction and petrography analyses over the entire composition range. Solid solutions based on hexagonal (A) modification of La₂O₃, cubic (C) modification of Y₂O₃, and monoclinic (B) modification of La₂O₃, (Sm₂O₃) are found to form in the system. The starting materials were La₂O₃, Sm₂O₃, and Y₂O₃ (99.99 %) powders. The samples were prepared from nitrate solutions with subsequent evaporation and decomposition at 800 °C for 2 h. The samples were subjected to heat treatment in three stages: at 1100 °C (for 2464 h), at 1500 °C (for 50 h), and then at 1600 °C (for 10 h) in furnaces with heating elements based on Fechral (H23U5T) and Superkanthal (MoSi₂), respectively. The isothermal section of the La₂O₃—Y₂O₃—Sm₂O₃ phase diagram at 1600 °C is characterized by three single-phase (A–La₂O₃, B–La₂O₃ (Sm₂O₃), C–Y₂O₃) and two-phase (A + B, B + C) regions. The ordered phase of perovskite-type was not found at 1600 °C in this system. An infinite series of solid solutions based on monoclinic modification of B–La₂O₃ (Sm₂O₃), which occupies the largest area of the isothermal section, forms in the system. Yttrium oxide stabilizes the total mutual solubility of lanthanum and samarium oxides. The lattice parameters of the B-phase decrease, the lattice volume increases with the addition of a heavier ion, and the lattice of solid solutions based on B–form of rare-earth metal oxides becomes more densely packed with increase in the content of yttrium oxide. The lattice parameters of the B-phase lattice vary from а = 1.3988 nm, b = 0.3774 nm, and с = 0.8427 nm in the single-phase sample containing 15 mole% Y₂O₃–42.5 mole% La₂O₃–42.5 mole% Sm₂O₃ to а = 1.3806 nm, b = 0.3709 nm, and с = 0.8312 nm in the two-phase sample containing 45 mole% Y₂O₃–27.5 mole% La₂O₃–27.5 mole% Sm₂O₃.