Unusual calcination temperature dependent tetragonal–monoclinic transitions in rare earth-doped zirconia nanocrystals

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Abstract
The dependence of tetragonal-monoclinic (t → m) transitions in the hydrothermally prepared (ZrO2)0.98(RE2O3)0.02 (RE = Sc, Y) nanocrystals upon the calcination between 400–1400 °C were explored by means of X-ray diffraction (XRD), high temperature X-ray diffraction (HTXRD), near infrared Fourier transform (FT) Raman and ultraviolet (UV) Raman spectroscopies. XRD and FT Raman scattering analysis indicated that the monoclinic fraction varied discontinuously with increasing the temperature from 400 to 1400 °C, which is unusual and is mainly ascribed to the microstrain effect. HTXRD analyses (from room temperature to 1400 °C) revealed that the as-prepared samples underwent tetragonal → monoclinic (t → m) transformation at 500 °C for (ZrO2)0.98(Sc2O3)0.02 and at 200 °C for (ZrO2)0.98(Y2O3)0.02 during the cooling process. UV Raman spectra suggested that the t → m transformation initially occurred at the surface region of the calcined samples. Hence, the monoclinic phase was enriched on the surface of the zirconia grains. Finally, the size effect of the stabilizers (Y3+ and Sc3+) on the critical crystallite size and the metastability of the tetragonal phase were also discussed.