Phase Equilibria in the ZrO2–La2O3–Sm2O3 System at 1100°C

Phase equilibria and structural transformations in the ternary ZrO 2 –La 2 O 3 –Sm 2 O 3 system at 1100°C were studied by X-ray diffraction over the entire composition range. Fields of solid solutions based on the hexagonal (A) modification of La 2 O 3 , cubic (F) modification with fluorite-type structure and tetragonal (T) and monoclinic (M) modifications of ZrO 2 , monoclinic (B) modification of Sm 2 O 3 , and an ordered intermediate phase with pyrochlore-type structure, Ln 2 Zr 2 O 7 (Py), were established to exist in the system. The boundaries of phase fields and lattice parameters of the phases were determined. In the zirconia-rich corner, solid solutions based on the tetragonal modification of ZrO 2 are formed. The solubility of La 2 O 3 in T-ZrO 2 is low and amounts to ~0.5 mol.%, which is evidenced by X-ray diffraction and microstructural analyses. The solid solutions based on the tetragonal modification of ZrO 2 cannot be quenched from high temperatures in the cooling conditions. The X-ray diffraction patterns recorded at room temperature show peaks of the M-ZrO 2 monoclinic phase. The ordered pyrochlore-type phase, Ln 2 Zr 2 O 7 (Py), is in equilibrium with all phases that exist in the ternary ZrO 2 –La 2 O 3 –Sm 2 O 3 system at 1100°C and forms substitutional solid solutions with phases of the binary systems. In the system, an infinite series of solid solutions form from the Ln 2 Zr 2 O 7 (Py) phase. The isothermal section of the ZrO 2 –La 2 O 3 –Sm 2 O 3 phase diagram at 1100°C contains three three-phase regions (T + M + Py, T + F + Py, A + B + Py) and eight two-phase regions (A + B, A + Py, B + Py, F + Py, F + T, T + M, T + Py, Py + M). New phases were not found to form at 1100°C.