ISOTHERMAL SECTION OF THE La2O3–ZrO2–HfO2 TERNARY DIAGRAM AT 1100 °C

The phase equilibria in the system based on zirconium, hafnium, and lanthanum oxides were studied by XRD. Based on the obtained data, an isothermal section of the La2O3–ZrO2–HfO2 ternary diagram at 1100 °C was constructed. The samples of different compositions were prepared from nitrate solutions by evaporation, drying, and calcination at 1100 °C. It was found that a decrease in the number of phase fields is observed in the studied system, along with a reduction in temperature from 1500 to 1100 °C. This is probably due to the tetragonal-monoclinic transformation M↔T. At this temperature, three series of continuous solid solutions of different lengths are formed in the ZrO2–HfO2–La2O3 system based on: monoclinic (M, P21/C space group) modification of ZrO2(HfO2), hexagonal (A, P-3m1 space group) modification of La2O3, and an ordered pyrochlore-type structure (Py, Fd-3m space group) of La2Zr2O7(La2Hf2O7). The solubility of La2O3 in the monoclinic modification is less than 2 mol%. The formation of new phases in the studied system was not found. Cubic solid solutions based on an ordered structure, such as pyrochlore, occupy the largest area of the isothermal section studied, considering the heterophase regions. It was found that the region of homogeneity based on the ordered phase with a pyrochlore structure is located in the concentration range of 35–38 mol% La2O3 along the La2O3–(50 mol% ZrO2–50 mol% HfO2) constant ratio line. Along the same line, the unit cell parameters of the ordered pyrochlore type structure vary from a = 1.0761 nm for the two-phase sample (M+Py) 47.5 mol% ZrO2 – 47.5 mol% HfO2 – 5 mol% La2O3 to a = 1.0772 nm for the boundary solid solution and to a = 1.0781 nm for the heterogeneous composition (A+Py) 10 mol% ZrO2 – 10 mol% HfO2 – 80 mol% La2O3. It was found that the amount of cubic phase (Py) in heterogeneous samples (M+Py) is 35%, 72%, 86%, 94%, and 98% for the corresponding compositions of 5 mol%, 10 mol%, 15 mol%, 20 mol%, and 30 mol% La2O3. The determined volume of unit cells of the monoclinic structure varies from V = 0.144 nm3 for the sample of 50 mol% ZrO2 – 50 mol% HfO2 – 0 mol% La2O3 to V = 0.131 nm3 for the heterogeneous sample (A+Py) of 40 mol% ZrO2 – 40 mol% HfO2 – 20 mol% La2O3. From the dependence shown it follows that the unit cell parameter of an ordered cubic phase with a pyrochlore (Py) structure increases with increasing lanthanum oxide content.