Interaction of Lanthanum, Lutetium, and Ytterbium Oxides at 1600°C

Phase equilibria in the La2O3-Lu2O3-Yb2O3 system were studied by X-ray diffraction and electron microscopy for the first time at 1600 degrees C over the entire composition range. Solid solutions based on various crystalline modifications of the starting components and ordered perovskite-type phases were found to form in the system. The starting materials were La2O3, Lu2O3, and Yb2O3 (99.99%). The samples were prepared with a concentration step of 1-5 mol.%. Weighed oxides were dissolved in HNO3 (1 : 1) and then the solutions were evaporated and decomposed at 800 degrees C for 2 h. The samples were heat treated in three stages: at 1100 degrees C (for 163 h), at 1500 degrees C (for 70 h), and at 1600 degrees C (for 10 h) in air. The samples were subjected to X-ray powder diffraction employing a DRON-3 diffractometer at room temperature (Cu-K-alpha radiation, Ni filter). The scan angle was 0.05-0.1 degrees in the range 2 theta = 15-90 degrees. The isothermal section of the La2O3-Lu2O3-Yb2O3 phase diagram at 1600 degrees C shows three single-phase (A-La2O3, R, C-Lu2O3 (Yb2O3)) and two two-phase (C + R, A + R) regions. The system forms continuous areas of solid solutions based on the cubic modification of C-Lu2O3 (Yb2O3) and the ordered perovskite-type phase (R phase). The solubility limits were determined and composition dependences for lattice parameters of the phases formed in the system were plotted. The boundary solubility of lutetium oxide in the R phase is 5 mol.% in the Lu2O3-(50 mol.% La2O3-50 mol.% Yb2O3) section. The homogeneity range of the C-Lu2O3 solid solutions extends from 93 to 100 mol.% Lu2O3 in the Lu2O3-(50 mol.% La2O3-50 mol.% Yb2O3) section. The solid solutions form through isovalent substitution, and the stability of ordered phases and solid solutions is determined by the shape factor of lanthanides.