High-temperature XPD study of tetragonal-to-cubic phase transition in nanocrystalline, compositionally homogeneus ZrO2-CeO2 solid solutions

L.M. ACUÑA; R.O. FUENTES; D. LAMAS; I. FÁBREGAS; M.G. ZIMICZ; M.E. FERNANDEZ DE RAAP; N. WALSÖE DE RECA; A.F. CRAIEVICH

Campinas, Brasil.

Workshop; Latin American Workshop on Applications of Powder Diffraction; 2007

ZrO2-CeO2 ceramics are widely investigated due to their excellent mechanical and catalytical properties. Compositionally homogeneous ZrO2-CeO2 solid solutions exhibit 3 tetragonal forms (P42/nmc space group). The stable tetragonal form is called the t-form and is restricted to the solubility limit predicted by the equilibrium phase diagram. The t´-form has an expanded solubility but is unstable against the mixture of 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). If CeO2 content is further increased, the cubic phase is retained. In previous works carried out at LNLS, we investigated the crystal structure and the local order at room temperature of compositionally homogenous ZrO2-CeO2 nanopowders synthesized by gel-combustion routes. By means of XPD, we determined the crystal structure of the solid solutions as a function of CeO2 content and found that the tetragonal/cubic boundary is located at 85 mol% CeO2 (D.G. Lamas et al., J. Appl. Cryst. 38 (2005) 867). In addition, EXAFS study showed that this tetragonal-cubic transition is associated to a symmetry change in the Zr-O coordination sphere, while the Ce-O one has a cubic symmetry in the whole compositional range (I.O. Fábregas et al., J. Phys.: Condens. Matter 18 (2006) 7863). In this work, we investigated the crystal structure of compositionally homogeneous, nanocrystalline ZrO2-CeO2 solutions by XPD as a function of temperature for compositions between 50 and 65 mol% CeO2. ZrO2 -50 and 60 mol% CeO2 solid solutions, that exhibit the t´-form of the tetragonal phase, transform into the cubic phase in two steps, t´-to-t" followed by t"-to-cubic. Differently, the ZrO2-65 mol% CeO2, which exhibits the t"-form, directly transforms to the cubic phase. The results suggest that t´-to-t" transition is of first order, while the t"-to-cubic one seems to be of second order.