Crystal structure, local atomic order and metastable phases of zirconia-based nanoceramics for Solid-Oxide Fuel Cells

D. G. LAMAS; ABDALA, P.M; ACUÑA, L.M.; FABREGAS ISMAEL OSCAR; FUENTES R.O.; N. E. WALSÖE DE RECA; FANTINI, M.C.A; PRADO, R.J.; A. F. CRAIEVICH

Madrid

Congreso; XXII congress and general assembly IUCR; 2011

ZrO2-based ceramics have many technological uses because of their high ionic conductivity at high temperatures and their excellent mechanical properties. They are usually employed as solid electrolytes in solid-oxide fuel cells (SOFCs). In addition, due to their excellent oxygen-storage capability, ZrO2-CeO2 solid solutions are used as SOFC anodes and in three-way catalysis.Pure ZrO2 exhibits three polymorphs of monoclinic, tetragonal and cubic symmetries. The monoclinic phase is stable at room temperature and on heating transforms to the tetragonal phase at 1170°C which by further heating transforms to the cubic phase at 2370°C. This phase exhibits a fluorite-type crystal structure and can be fully stabilized at room temperature by doping with other oxides (Y2O3, CaO, etc.). The tetragonal phase has a crystal structure similar to the cubic phase, but with its unit cell c parameter slightly longer than the other two axes.The tetragonal phase can be retained, under metastable condition, in nanopowders or fine-grained ceramics. Since the monoclinic phase has poor electrical properties, only the high-temperature phases are useful for technological applications. These materials can exhibit 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, in this phase, the oxygen atoms are displaced along the c axis from their ideal sites of the fluorite-type structure of the cubic phase (8c sites of the Fm3m space group).During the last decade, we have investigated the crystallographic features of a number of ZrO2-based systems (ZrO2-CeO2, ZrO2-CaO, ZrO2-Y2O3 and ZrO2-Sc2O3). We have mainly focused on the retention of metastable tetragonal forms in nanocrystalline and compositionally homogeneous zirconia-based solid solutions. In this communication, we review the main results of our investigations in a number of nanocrystalline ZrO2-based solid solutions, most of them obtained by means of X-ray powder diffraction (XPD) and EXAFS techniques at the  Brazilian Synchrotron Light Laboratory (LNLS, Campinas, Brazil). We demonstrate how the use  of a high intensity synchrotron source in XPD experiments allows to detect and precisely determine the intensity of very weak Bragg peaks, which are related to small displacements of oxygen atoms in the tetragonal phase, even those observed for the t"-form with a cubic unit cell. By analyzing high temperature XPD data, we determined the influence of the average crystallite size on the features of different phase diagrams of the studied materials. We also discuss our results for a  number of ZrO2-based solid solutions related to (i) the local atomic structure, which yields a new insight on the structural disorder of the oxygen sublattice, and (ii) the mechanisms responsible for the retention of the observed metastable phases.