ISOTHERMAL SECTION OF THE ZrO2–HfO2–Eu2O3 SYSTEM PHASE DIAGRAM AT 1100 °C

Phase equilibria and structural transformations in the ternary ZrO2–HfO2–Eu2O3 system at 1100 °C were studied by X-ray diffraction over the entire composition range. The samples of different compositions have been prepared from nitrate acid solutions by evaporation, drying, and calcinations at 1100 °C. Fields of solid solutions based on the cubic (F) modification with fluorite-type structure and tetragonal (T) and monoclinic (M) modifications of ZrO2 (HfO2), cubic (C) modification of Eu2O3, and an ordered intermediate phase with pyrochlore-type structure, Ln2Zr2O7 (Ln2Hf2O7)(Py), were established to exist in the system. The solubility of Eu2O3 in M-HfO2(ZrO2) is pretty low and approximately less than 1 mol%, which is confirmed by XRD. The boundaries of phase fields and lattice parameters of the phases were determined. The lattice parameters for F phase vary from а = 0.5305 nm in two-phase sample (C+F) containing 6.75 mol% ZrO2–8.25 mol% HfO2–85 mol% Eu2O3 to а = 0.5283 nm for sample containing 27 mol% ZrO2–33 mol% HfO2–40 mol% Eu2O3 and to а = 0.5275 nm in two-phase sample (Py+F) containing 29.25 mol% ZrO2–35.75 mol% HfO2–35 mol% Eu2O3, and from а = 0.5232 nm in two-phase sample (Py+F), containing 32.625 mol% ZrO2–39.875 mol% HfO2–27.5 mol% Eu2O3 to а = 0.5218 nm sample containing 33.75 mol% ZrO2–41.25 mol% HfO2–25 mol% Eu2O3 and to а = 0.5176 nm in two-phase sample (F+M) containing 40.5 mol% ZrO2–49.5 mol% HfO2–10 mol% Eu2O3 along the section Eu2O3–(55 mol% HfO2–45 mol% ZrO2). The fluorite-type structure (F) is in equilibrium with all phases that exist in the ternary ZrO2–HfO3–Eu2O3 system at 1100 °C and forms substitutional solid solutions with phases of the binary systems. In the ZrO2–HfO2–Eu2O3 system, an infinite series of solid solutions form from the Ln2Zr2O7 (Ln2Hf2O7) (Py) phase. The isothermal section of the ZrO2–HfO2–Eu2O3 phase diagram at 1100 °C contains one three-phase regions (T + M + F) and six two-phase regions (F + C, two-F + Py, F + M, F + T, T + M). No new phases were found in the ZrO2–HfO2–Eu2O3 system at 1100 °C and the nature of phase equilibria was determined by the constitution of the boundary binary systems.