Doping and grain size effects in nanocrystalline ZrO2–Sc2O3 system with complex phase transitions: XRD and Raman studies

Abstract

Weakly-agglomerated nanocrystalline (ZrO₂)_1−x(Sc₂O₃)_x (x = 0.02–0.16) powders with high surface area (106–186 m² g⁻¹) and uniform particle size (5.7–9.5 nm) were synthesized by a two-step hydrothermal process in the presence of urea, and their complex phase evolution upon calcinations over 400–1400 °C were studied by a combined utilization of X-ray diffraction, in situ high temperature X-ray diffraction, near infrared Fourier transform Raman and ultraviolet Raman spectroscopies. It was discovered that the growth of the grains comprised two stages, and the apparent activation energies, E_a1 (<800 °C) and E_a2 (>800 °C), are in the range of 5.5–12 kJ mol⁻¹ and 80–140 kJ mol⁻¹, respectively. The crystallite size, microstrain, surface area and growth activation energy of the samples showed a strong dependence on the Sc³⁺ doping concentration. The nanocrystalline (ZrO₂)_1−x(Sc₂O₃)_x samples underwent complex phase transitions upon calcination from 800 to 1000 °C. Tetragonal and cubic phases have been preserved when the samples calcined below 800 °C as a result of crystallite size and doping effects. At elevated calcination temperature of 1000 °C, the compositions at x = 0.04, 0.10 and 0.14 seemed to be in the metastable phase boundaries of m + t, t + β (Sc₂Zr₇O₁₇) and β + γ (Sc₂Zr₅O₁₃), respectively. As calcined at 1400 °C, the grain size was in the micrometre regime, and t → m, c → t″, c → β and c → γ phase transitions occurred on the cooling stage for x ≤ 0.04, x = 0.08, 0.10 ≤ x ≤ 0.12 and x ≥ 0.14, respectively, resulting in the existence of m, t, t″, β and γ phases in turn at room temperature. Moreover, it was found that the tetragonal phase has been enriched at the surface region for the as-calcined (ZrO₂)_0.94(Sc₂O₃)_0.06 and (ZrO₂)_0.92(Sc₂O₃)_0.08 samples.