EXAFS study of nanostructured ZrO2-CeO2 solid solutions

R.O. FUENTES; D.G. LAMAS; I.O. FÁBREGAS; J.R. CASANOVA; M.E. FERNÁNDEZ DE RAPP; G.E. LASCALEA; N.E. WALSÖE DE RECA; M.C.A. FANTINI; A.F. CRAIEVICH

Activity Report of the Brazilian Synchrotron Light Laboratory

Año: 2004 p. 157 - 158

    Zirconia-based ceramics have been widely investigated due to their excellent mechanical and electrical properties, which strongly depend on the crystal structures and phase changes. For example, the conditions required for the retention at room temperature of the high-temperature tetragonal phase (which is the most important for applications) is still under investigation, although it is well-known that this phase can only be retained in the case of nanocrystalline powders and fine-grained ceramics.    One of the most notable characteristics of some zirconia-based solid solutions is the existence of three tetragonal forms, all belonging to the P42 /nmc space group. The stable tetragonal form is called the t-form, which is restricted to the solubility limit predicted by the equilibrium phase diagram. There is also a t´-form with a wider solubility, but unstable in comparison with the mixture of the t-form and cubic phase. Finally, the t´´-form has an axial ratio c/a of unity, but with the oxygen atoms displaced along the c axis from their ideal sites of the cubic phase (8c sites of the Fm3m space group). The t´/t´´ and t´´/c boundaries have been investigated in several systems, including ZrO2-Y2O3 and ZrO2-CeO2. However, the influence of the synthesis method and the crystallite size on these boundaries has not been established yet.    Although several authors have studied nanostructured zirconia-ceria solid solutions, the crystal structure of these materials is still under discussion. Several synthesis methods have been explored, such as hydrothermal synthesis, high-energy mechanical milling, polymerised complex method, coprecipitation, sol-gel and gel-combustion. Some discrepancies and misunderstandings can be found in these works, since several authors have found problems to discriminate between the t´ and t´´ forms of the tetragonal phase or between the t´´-form and the cubic phase.    XAFS studies in ZrO2-CeO2 solid solution have been made for the purpose of the investigation on the local structure around Ce and Zr. The relationship between the oxygen storage/release capacity (OSC) and the local structure has been also studied in this system.    The aim of the present work was to study the local structure of compositionally homogeneous ZrO2-CeO2 nanopowders synthesised by a pH-controlled nitrate-glycine gel-combustion process.