Phase equilibria in the ZrO2–HfO2–Sm2O3 system at 1500 °С

The phase equilibria in the ternary system ZrO₂–HfO₂–Sm₂O₃ were studied by X-ray diffraction and microstructural analyses. The formation of new phases in the ZrО₂–HfO₂–Sm₂O₃ system at 1500 °С was not observed. It is established that in the studied system at 1500°C the fields of solid solutions based on tetragonal (T) modification of ZrО₂, monoclinic (M) modifications of HfO₂, and monoclinic (B) modifications of Sm₂O₃, and ordered phase with the structure of pyrochlore-type (Py) Ln₂Zr₂O₇ (Ln₂Hf₂O₇) are present. The boundaries of the phase fields and the parameters of the unit cells of the formed phases are determined. The studied isothermal cross section of the ZrО₂–HfO₂–Sm₂O₃ system is characterized by the formation of continuous series of cubic solid solutions based on a phase with a structure of the pyrochlore-type Sm₂Zr₂O₇ (Sm₂Hf₂O₇) and a structure of the fluorite type F-ZrO₂ (HfO₂). It is established that two regions of homogeneity of cubic solid solutions are formed in the determined system. The existence of these regions of homogeneity is due to the rupture of the solubility of the F-ZrO₂ (HfO₂) phase in the region of the existence of an ordered phase with a pyrochlore-type structure Sm₂Zr₂O₇ (Sm₂Hf₂O₇). It is established that the solid solution based on a cubic modification with fluorite-type structure exists in equilibrium with all phases observed in the system. Isothermal section of the phase diagram of the system ZrO₂–HfO₂–Sm₂O₃ at 1500°C has been characterized by the presence of one three-phase (F + T + M), as well as five two-phase (B + F, two - F + Py, M + F, T + F) regions.