The crystal structures of a number of nanocrystalline ZrO 2 -CeO 2 solid solutions, synthesized by a pH-controlled nitrate-glycine gel-combustion process, were studied. By using a synchrotron X-ray diffractometer, small peaks of the tetragonal phase, which correspond to forbidden reflections in the case of a perfect cubic fluorite structure, were clearly detected. By monitoring the most intense of these reflections, 112, as a function of the CeO 2 content, the tetragonal-cubic phase boundary was found to be at 85 (5) mol% CeO 2 . For a CeO 2 content up to 68 mol%, a tetragonal phase with c/a > 1 (known as the t 0 form) was detected, whereas, between 68 and 85 mol% CeO 2 , the existence of a tetragonal phase with c/a = 1 and oxygen anions displaced from their ideal positions in the cubic phase (the t 00 form) was verified. Finally, solid solutions with higher CeO 2 contents exhibit the cubic fluorite-type phase.

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Compositionally homogeneous ZrO2–CeO2 nanopowders have been characterized by Raman and extended x-ray absorption fine structure (EXAFS) spectroscopies. These techniques revealed a tetragonal-to-cubic phase transition as a function of CeO2 content, as observed in a previous synchrotron x-ray diffraction study. The tetragonal–cubic phase boundary was found to be at (85 ± 5) mol% CeO2. The EXAFS study demonstrated that this transition is related to a tetragonal-to-cubic symmetry change of the Zr–O first neighbour coordination sphere, while the Ce–O coordination sphere preserves its cubic symmetry over the whole composition range.

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The transition between tetragonal and cubic phases in nanostructured ZrO2-Sc2O3 solid solutions by high-temperature X-ray powder diffraction using synchrotron radiation is presented. ZrO2-8 and 11 mol% Sc2O3 nanopowders that exhibit the t′- and t″-forms of the tetragonal phase, respectively, were synthesized by a stoichiometric nitrate-lysine gel-combustion route. The average crystallite size treated at 900°C was about 25 nm for both compositions. Our results showed that t′-t″ and t″-cubic transitions take place for the 8 and 11 mol% Sc2O3 samples, respectively.