INTERACTION OF LANTHANUM, YTTRIUM, AND GADOLINIUM OXIDES AT 1600 ºC

Abstract

Phase equilibria and structural transformations in the La₂O₃–Y₂O₃–Gd₂O₃ system at 1600 °C were studied by X-ray diffraction, electron microscopy, and petrography in the entire composition range. Fields of solid solutions based on hexagonal (A) modification of La₂O₃, cubic (C) modification of Y₂O₃, and monoclinic (B) modification of La₂O₃, (Gd₂O₃) were established to exist in the system. The starting materials were La₂O₃, Gd₂O₃, and Y₂O₃ (99.99%) powders. The samples were prepared with a concentration step of 1–5 mol.%. Weighed portions of the oxides were dissolved in HNO₃ (1 : 1) followed by evaporation of the solutions and decomposition of the nitrates at 800 ºC for 2 h. The samples were heat-treated in three stages: at 1100 °C (168 h), 1500 °C (70 h), and 1600 °C (10 h) in air in furnaces with FeCrAl (H23U5T) and molybdenum disilicide (MoSi₂) heating elements. X-ray diffraction of the samples was performed using the powder method with a DRON-3 diffractometer at room temperature (Cu-Kα radiation). The scanning step was 0.05–0.1° at angles 2θ = 15–90°. The isothermal section of the phase diagrams for the La₂O₃–Y₂O₃–Gd₂O₃ system at 1600 °C was characterized by three single-phase (A-La₂O₃, B-La₂O₃ (Gd₂O₃), C-Y₂O₃) and two-phase (A + B, B + C) regions. The solubility limits were determined, and the composition dependences of the lattice parameters for the phases formed in the system were plotted. The ordered perovskite-type phase in this system was not found at 1600 °C. A continuous series of solid solutions based on the monoclinic modification of B-La₂O₃(Gd₂O₃) formed in the system, occupying the largest area of the isothermal section. Yttrium oxide stabilizes the total mutual solubility of lanthanum and samarium oxides. With the addition of heavier ions, the lattice parameters of the B-phase cell reduce, the lattice volume increases, and the density increases as well. The lattice of solid solutions based on the B-form of rare-earth metal oxides becomes more densely packed with a higher concentration of yttrium oxide.