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. Th e products were characterized by X-ray diffraction (XRD), scanning electron ic microscope (SEM), X-ray photoelectron spectroscopy (XPS), and electron p aramagnetic resonance (EPR). Different from the solid-state reaction system s, the solution limit of Bi2O3 in ceria by hydrothermal conditions was as h igh as ca. 50%. XRD data showed that all solid solutions crystallized in si ngle-phase cubic fluorite-type structure. The average grain size of all sol id solutions was within nanometer scale, XPS data gave evidence of the pres ence of Bi(III) and Ce(IV) on the surface of the doped ceria. EPR measureme nts 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 th e as-made doped ceria up to 800 degrees C did not lead to any structural tr ansformation. 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 measure d by impedance spectroscopy showed that the solid solutions with dopant con tent 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 s ilver 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)(B iO1.5)(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.