Phase relations in the La2O3-ZrO2-HfO2 system at 1250 °C and 1500 °C

In the present study, the phase equilibria in the ternary system La₂O₃-ZrO₂HfO₂ have been studied in the whole concentration range. In the course of the study, isothermal sections of the phase diagram of the system were constructed at temperatures of 1500 and 1250 °C. The obtained results indicate the no new phase formation in the studied system. It was found that in the La₂O₃-ZrO₂HfO₂ ternary system at temperatures of 1500 and 1250 °C, solid solutions are formed on the basis of monoclinic (M, space group P21/C) HfO₂ and tetragonal (T, space group P42/nmc) modifications of ZrO₂, solid solutions based on a hexagonal modification (A, space group P3m1) of La₂O₃, and an ordered phase with a pyrochlore-type structure of La₂Zr₂О₇ (La₂Hf₂О₇) (Py, space group Fd-3 m). The system studied is characterized by the formation of a continuous series of solid solutions with a pyrоchlore-type structure. A decrease in temperature from 1500 °C to 1250 °C leads to the shift of the three-phase region, containing solid solutions based on a pyrochlore-type structure of Ln₂Zr₂О₇ (Ln₂Hf₂О₇), monoclinic M-HfO₂, and tetragonal T-ZrO₂, towards the concentration area with higher ZrO₂ content and also to an increase in the extent of the solid solution based on the monoclinic modification M-HfO₂.