PHASE EQUILIBRIA IN THE ZrO₂–La₂O₃–Sm₂O₃ SYSTEM AT 1100 ºС

Abstract

Phase equilibria and structural transformations in the ternary ZrO₂–La₂O₃–Sm₂O₃ system at 1100ºС were studied by X-ray diffraction over the entire composition range. Fields of solid solutions based on the hexagonal (A) modification of La₂O₃, cubic (F) modification with fluorite-type structure and tetragonal (Т) and monoclinic (M) modifications of ZrО₂, monoclinic (B) modification of Sm₂O₃, and an ordered intermediate phase with pyrochlore-type structure, Ln₂Zr₂O₇ (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 ZrO₂ are formed. The solubility of La₂O₃ in T-ZrO₂ 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 ZrО₂ cannot be quenched from high temperatures in the cooling conditions. The X-ray diffraction patterns recorded at room temperature show peaks of the M-ZrO₂ monoclinic phase. The ordered pyrochlore-type phase, Ln₂Zr₂O₇ (Ру), is in equilibrium with all phases that exist in the ternary ZrО₂–La₂O₃–Sm₂O₃ 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 Ln₂Zr₂O₇ (Py) phase. The isothermal section of the ZrО₂–La₂O₃–Sm₂O₃ phase diagram at 1100 °C contains three three-phase regions (T + M + Py, T + F + Py, A + B + Py) and eight two-phase regions (А + В, А + Py, B + Py, F + Py, F + T, T + M, T + Py, Py + M). New phases were not found to form at 1100 °C.