Solid solubility and transport properties of nanocrystalline(CeO2)1-x(BiO1.5)x by hydrothermal conditions

A series of nanocrystalline solid solutions (CeO2)(1-x)(BiO1.5)(x) (x = 0.0-0.5) were synthesized by mild hydrothermal conditions at 240 degrees C. The products were characterized by X-ray diffraction (XRD), scanning electronic microscope (SEM), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). Different from the solid-state reaction systems, the solution limit of Bi2O3 in ceria by hydrothermal conditions was as high as ca. 50%. XRD data showed that all solid solutions crystallized in single-phase cubic fluorite-type structure. The average grain size of all solid solutions was within nanometer scale, XPS data gave evidence of the presence of Bi(III) and Ce(IV) on the surface of the doped ceria. EPR measurements confirmed Ce(III) ions in the bulk of the sintered solutions. When the content of dopant Bi2O3 in ceria was lower than the limit, air firing of the as-made doped ceria up to 800 degrees C did not lead to any structural transformation. For the solution (CeO2)(0.5)(BiO1.5)(0.5), however, sintering it in air at 800 degrees C would destabilize the cubic fluorite structure and result in segregation of an unknown phase. The ionic conduction measured by impedance spectroscopy showed that the solid solutions with dopant content lower than the limit exhibited primarily the bulk conduction, whereas for the sintered (CeO2)(0.5)(BiO1.5)(0.5), both the bulk and grain boundary resistance decreased dramatically with increasing temperature when using silver electrode. The solution (CeO2)(0.6)(BiO1.5)(0.4) was determined to be the best conducting phase. For the nanocrystalline solutions (CeO2)(1-x)(BiO1.5)(x), the bulk conduction was due to oxide ions. The variations of the activation energy and conductivity with dopant content were interpreted in terms of the relative content of the dopant-defect complexes, Ce-Ce'Vo/Bi-Ce'Vo.