High-temperature XRD study of the tetragonal-cubic phase transition in nanostructured ZrO2-CeO2 solid solutions synthesized by citrate complexation and gel-combustion methods

L.M. ACUÑA; M.G. ZIMICZ; S. LARRONDO; A.F. CRAIEVICH; D. LAMAS; R.O. FUENTES

Laboratorio Nacional de Luz Sincrotrón, Campinas, Brasil

Congreso; 18° Reunión anual de usuarios (RAU); 2008

LNLS

Since ZrO2-CeO2 materials have been proposed as anodes in intermediate-temperature solid oxide fuel cells (IT-SOFC), it is really important to know what is happening with the structural properties of the material at SOFC working temperatures (500-800°C). 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. 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 aim of this work is to study the tetragonal-cubic phase transition as function of temperature on nanocrystalline powders of ZrO2-50 and 65%mol CeO2 by  high-temperature X-ray diffion (HT-XRD) employing synchrotron radiation XPD 10B beamline of LNLS). Both compositions were synthesized by diffent chemical routes: citrate complexation and gel-combustion (glycine as fuel). ZrO2- 50%mol CeO2 solid solution exhibits the metastable t-form while the t-form is observed in ZrO2-65%mol CeO2. The evolution of the (112) peak with temperature was studied in all systems. The (112) peak is a characteristic reflection of t"-form (pseudo-fluorite), while it is a forbbiden reflection in a fluorite-like structure of the cubic phase. This reflection is related to the displacement of the oxigen anions. By monitoring of this peak it was possible to discriminate between tetragonal and cubic phases. The fractional z-coordinate of the oxygen atom in the asymmetric unit, z (O), can be determined from the measured integrated intensities, I(112) and I(111). The data were collected at temperatures ranging from room temperature to 900°C.