Phase development in the metastable solid solutions of ZrO2-YO1.5 system

Metastable solid solutions of ZrO2-YO1.5 system (from 0 to 100 mol%) were prepared by crystallization of the amorphous precursors, coprecipitated from aqueous solutions of nitrate salts, using two synthesis routes: hydrothermal treatment at 150 °C for 24 h and annealing in the air atmosphere for 2 h at temperatures between 400 and 1200 °C. Thermal behavior of the amorphous precursors were followed by differential scanning calorimetry (DSC), structural analysis of the crystallization product was performed by X-ray powder diffraction (XRPD) and Raman spectroscopy, and the morphology of the crystalline grains was determined by the scanning electron microscope (FE-SEM). It was found that the crystallization temperature of ZrO2-type solid solutions increase with an increase in the YO1.5 content, from 487 °C (0 mol% of YO1.5) to 516 °C (20 mol% of YO1.5). Comparison of the obtained results with the results obtained for mixed oxides of zirconium with different trivalent dopants indicates that the crystallization temperature rises with the increase of dopand content and the difference between the ionic radius of the zirconium and the doped cation. The results of Rietveld refinements show that, regardless of the significant differences in the crystallite size and the morphology of the products obtained by two different synthesis routes, the phase composition and the unit cell parameters of the solid solutions obtained are very similar and depend mostly on the Zr/Y ratio. Crystallization products with YO1.5 content ≥5 mol% contain only one type of solid solution structurally closely related to tetragonal ZrO2 (from 5 to 15 mol% of YO1.5), cubic ZrO2 (from 15 to more than 60 mol% of YO1.5) and cubic Y2O3 (from 70 to 100 mol% of YO1.5). The unit-cell volume of the ZrO2- and Y2O3-type solid solutions increases linearly with the increase in the fraction of the yttrium. This linear dependence continued even after the transitions from ZrO2-type to Y2O3-type lattice.