Influence of crystallite size on the tetragonal-cubic phase transition in nanocrystalline Zr0.5Ce0.5O2

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

2007 Activity Report of the Brazilian Synchrotron Light Laboratory

Laboratorio Nacional de Luz Sincrotrón (LNLS)

Año: 2008 p. 1031 - 1031

1518-0204

Crystal structure of compositionally homogeneous zirconia-ceria (ZrO2-CeO2) solid solutions exhibit three forms of the tetragonal phase, all belonging to the P42/nmc space group. The stable form of the tetragonal phase is called the t-form. The t´-form has a wider solubility range, but it is unstable; the stable phase being in this compositional range a mixture of the t-form and the cubic phase. Finally, the t"-form has an axial ratio c/a equal to unity, but with the oxygen ions displaced from their ideal sites of the cubic structure (8c sites of the Fm3m space group) along the c axis. The t´/t" and t"/c boundaries reported by us in previous studies were of 68 and 85 mol% CeO2, respectively. The aim of this work is to study the effect of crystallite size on the tetragonal-cubic phase transition as function of temperature on nanocrystalline powders of ZrO2-50 mol% CeO2 by high-temperature X-ray diffraction (HT-XRD) employing synchrotron radiation. Powders were synthesized by the gelcombustion method and citrate complexation method with an average crystallite size about 30 nm and 8 nm, respectively. The evolution of the (112) peak with temperature was studied in all systems. The (112) peak is a characteristic reflection of t"-form, while it is a forbbiden reflection in a fluorite-like structure of the cubic phase. This reflection is related to the displacement of the oxygen 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 of the tetragonal unit cell, z(O), can be determined from the measured integrated intensities, I(112) and I(111).