The electrical properties of nanostructured, heavily yttria‐ or samaria‐doped ceria ceramics are studied as a function of grain size using electrochemical impedance spectroscopy (EIS). A remarkable enhancement in the total ionic conductivity of about one order of magnitude is found in nanostructured samples, compared with the intrinsic bulk conductivity of conventional microcrystalline ceramics. This effect is attributed to the predominance of grain‐boundary conduction in the nanostructured materials, coupled with an increase in the grain‐boundary ionic diffusivity with decreasing grain size.

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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.

Ionic transport in nanostructured solid electrolytes is investigated using an atomistic model that clarifies the enhanced conductivity (see figure) of these materials. It is based on the fast diffusion of free oxygen vacancies through the grain boundaries. A transition observed in the ionic conductivity of the nanoceramics as a function of temperature, caused by a change in the transport mechanism, is also explained.

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