Structure and phase transformations in scandia, yttria, ytterbia and ceria-doped zirconia-based solid solutions during directional melt crystallization

Abstract

(ZrO2)0.999-x(Sc2O3)х(Eu2O3)0.001 (х = 0.08–0.11) and (ZrO2)0.889(Sc2O3)0.1(R2O3)0.01 (Eu2O3)0.001 (R = Y, Yb, or Ce) crystals were grown by directional melt crystallization with direct high-frequency heating. The microstructure, phase composition and local structure of the crystals were studied using optical microscopy, X-ray diffraction, Raman scattering and optical spectroscopy. Scandia, yttria and ytterbia were shown to be distributed quite homogeneously in the crystals, and their concentrations corresponded to their contents in the charge. Ceria co-doped crystals exhibited ceria displacement in the longitudinal direction, leading to a decrease in the scandia concentration. We analysed the effect of the crystallization conditions on the formation of microstructure, crystallographic phases and local crystalline structure as a function of crystal length.Two-phase (ZrO2)0.999-x(Sc2O3)х(Eu2O3)0.001 (х = 0.08–0.11) crystals were synthesized using this method. At a Sc2O3 concentration of ≤9 mol.%, tetragonal crystals containing small local regions with cubic structures were obtained. At concentrations above Sc2O3 ≥ 10 mol.%, the crystals contained the pseudocubic (t’’ phase) and rhombohedral zirconia modifications and had an inhomogeneous phase distribution in the longitudinal direction.A study of the (ZrO2)0.889(Sc2O3)0.1(R2O3)0.01(Eu2O3)0.001 (R = Y, Yb, or Ce) crystals showed that only the yttria-doped crystals were homogeneous and had a single-phase composition that comprised only the t” phase. The microstructure of the ytterbia- and ceria-doped crystals varied in the longitudinal direction. These crystals were mixtures of rhombohedral and pseudocubic phases (t’’ phase), and the rhombohedral phase was predominant in the top and bottom portions of the crystals.We show that the experimentally observed variations in the microstructure, local structure and phase composition of the crystals were largely determined by the difference in the thermal pretreatment history of different crystal regions, which is typical of this growth method.