Synchrotron X-ray diffraction study of the tetragonal-cubic phase boundary of nanocrystalline ZrO₂-CeO₂ synthesized by a gel-combustion process

The crystal structures of a number of nanocrystalline ZrO₂-CeO₂ 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 CeO₂ content, the tetragonal-cubic phase boundary was found to be at 85 (5) mol% CeO₂. For a CeO₂ content up to 68 mol%, a tetragonal phase with c/a > 1 (known as the t' form) was detected, whereas, between 68 and 85 mol% CeO₂, the existence of a tetragonal phase with c/a = 1 and oxygen anions displaced from their ideal positions in the cubic phase (the t'' form) was verified. Finally, solid solutions with higher CeO₂ contents exhibit the cubic fluorite-type phase.