Ismael O. Fábregas

Synchrotron X‐ray diffraction study of the tetragonal–cubic phase boundary of nanocrystalline ZrO2–CeO2 synthesized by a gel‐combustion process

The crystal structures of a number of nanocrystalline ZrO2–CeO2 solid solutions, synthesized by a pH-controlled nitrate-glycine gel-combustion process, were studied. By using a synchrotron X-ray diffractometer, small peaks of the tetragonal phase, which correspond to forbidden reflections in the case of a perfect cubic fluorite structure, were clearly detected. By monitoring the most intense of these reflections, 112, as a function of the CeO2 content, the tetragonal–cubic phase boundary was found to be at 85 (5) mol% CeO2. For a CeO2 content up to 68 mol%, a tetragonal phase with c/a > 1 (known as the t′ form) was detected, whereas, between 68 and 85 mol% CeO2, the existence of a tetragonal phase with c/a = 1 and oxygen anions displaced from their ideal positions in the cubic phase (the t′′ form) was verified. Finally, solid solutions with higher CeO2 contents exhibit the cubic fluorite-type phase.

Local structure of the metal-oxygen bond in compositionally homogeneous, nanocrystalline zirconia-ceria solid solutions synthesized by a gel-combustion process

Compositionally homogeneous ZrO2–CeO2 nanopowders have been characterized by Raman and extended x-ray absorption fine structure (EXAFS) spectroscopies. These techniques revealed a tetragonal-to-cubic phase transition as a function of CeO2 content, as observed in a previous synchrotron x-ray diffraction study. The tetragonal–cubic phase boundary was found to be at (85 ± 5) mol% CeO2. The EXAFS study demonstrated that this transition is related to a tetragonal-to-cubic symmetry change of the Zr–O first neighbour coordination sphere, while the Ce–O coordination sphere preserves its cubic symmetry over the whole composition range.

Local atomic structure in tetragonal pure ZrO2 nanopowders

The local atomic structures around the Zr atom of pure (undoped) ZrO2 nanopowders with different average crystallite sizes, ranging from 7 to 40 nm, have been investigated. The nanopowders were synthesized by different wet-chemical routes, but all exhibit the high-temperature tetragonal phase stabilized at room temperature, as established by synchrotron radiation X-ray diffraction. The extended X-ray absorption fine structure (EXAFS) technique was applied to analyze the local structure around the Zr atoms. Several authors have studied this system using the EXAFS technique without obtaining a good agreement between crystallographic and EXAFS data. In this work, it is shown that the local structure of ZrO2 nanopowders can be described by a model consisting of two oxygen subshells (4 + 4 atoms) with different Zr—O distances, in agreement with those independently determined by X-ray diffraction. However, the EXAFS study shows that the second oxygen subshell exhibits a Debye–Waller (DW) parameter much higher than that of the first oxygen subshell, a result that cannot be explained by the crystallographic model accepted for the tetragonal phase of zirconia-based materials. However, as proposed by other authors, the difference in the DW parameters between the two oxygen subshells around the Zr atoms can be explained by the existence of oxygen displacements perpendicular to the z direction; these mainly affect the second oxygen subshell because of the directional character of the EXAFS DW parameter, in contradiction to the crystallographic value. It is also established that this model is similar to another model having three oxygen subshells, with a 4 + 2 + 2 distribution of atoms, with only one DW parameter for all oxygen subshells. Both models are in good agreement with the crystal structure determined by X-ray diffraction experiments.

Synchrotron X-ray powder diffraction and extended X-ray absorption fine structure spectroscopy studies on nanocrystalline ZrO2–CaO solid solutions

The crystal structure and the local atomic order of a series of nanocrystalline ZrO2–CaO solid solutions with varying CaO content were studied by synchrotron radiation X-ray powder diffraction and extended X-ray absorption fine structure (EXAFS) spectroscopy. These samples were synthesized by a pH-controlled nitrate–glycine gel-combustion process. For CaO contents up to 8 mol%, the t′ form of the tetragonal phase (c/a > 1) was identified, whereas for 10 and 12 mol% CaO, the t′′ form (c/a = 1; oxygen anions displaced from their ideal positions in the cubic phase) was detected. Finally, the cubic phase was observed for solid solutions with CaO content of 14 mol% CaO or higher. The t′/t′′ and t′′/cubic compositional boundaries were determined to be at 9 (1) and 13 (1) mol% CaO, respectively. The EXAFS study demonstrated that this transition is related to a tetragonal-to-cubic symmetry change of the first oxygen coordination shell around the Zr atoms.

High-temperature X-ray powder diffraction study of the tetragonal-cubic phase transition in nanocrystalline, compositionally homogeneous ZrO2-CeO2 solid solutions

, CO and hydrocarbons from automotiveexhausts , as anodes in fuel cell technologies, and others. Inparticular, the metastable forms of the tetragonal phase havebeen investigated widely since they are the most suitable forapplications Trovarelli, 2002; Di Monte and Kaspar, 2005 .The crystal structure of compositionally homogeneousZrO

Structure of nanoporous zirconia-based powders synthesized by different gel-combustion routes

Zirconia-based ceramics that retain their metastable tetragonal phase at room temperature are widely studied due to their excellent mechanical and electrical properties. When these materials are prepared from precursor nanopowders with high specific surface areas, this phase is retained in dense ceramic bodies. In this work, we present a morphological study of nanocrystalline ZrO2–2.8 mol% Y2O3 powders synthesized by the gel-combustion method, using different organic fuels – alanine, glycine, lysine and citric acid – and calcined at temperatures ranging from 873 to 1173 K. The nanopore structures were investigated by small-angle X-ray scattering. The experimental results indicate that nanopores in samples prepared with alanine, glycine and lysine have an essentially single-mode volume distribution for calcination temperatures up to 1073 K, while those calcined at 1173 K exhibit a more complex and wider volume distribution. The volume-weighted average of the nanopore radii monotonically increases with increasing calcination temperature. The samples prepared with citric acid exhibit a size distribution much wider than the others. The Brunauer–Emmett–Teller technique was used to determine specific surface area and X-ray diffraction, environmental scanning electron microscopy and transmission electron microscopy were also employed for a complete characterization of the samples.

Crystal structure and local order of nanocrystalline zirconia-based solid solutions

CITEFA CONICET, Inst Invest Cient & Tecn Fuerzas Armadas, Ctr Invest Solidos CINSO, Buenos Aires, DF, Argentina

Synchrotron XRD and XAS studies of nanocrystalline ZrO2-Y2O3 solid solutions

C494 We have determined the phase diagram of nanopowdered ZrO2 containing 10 to 14 mol% Sc2O3 with average crystallite sizes ranging from 30 nm up to 100 nm by X-ray powder diffraction, using the D10B-XPD synchrotron beam line of LNLS (Campinas, Brazil). Our study shows that the boundaries of c or t”, β and γ phases strongly depend on both, Sc2O3 content and crystallite size. The transition temperatures related to cooling and heating processes decrease for decreasing crystallite sizes. In addition, the experimental results corresponding to a whole heating/cooling cycle indicate the presence of a clear hysteresis effect. This work was supported by FAPESP (Brazil), LNLS (Brazil), CNPq (Brazil, PROSUL Program); ANPCyT (Argentina) and LatinAmerican Centre for Physics (CLAF).

Crystal structure, local atomic order and metastable phases of zirconia-based nanoceramics for solid-oxide fuel cells

C489 to compare these structures with a range of similar structural motifs present in the literature. Conformational differences between our investigated dyes and those complexes with a different counter ion were revealed. This analysis motivated a general enquiry into the influence of the position of attached ligands on the pyridine unit within all known Ruthenium or Iron based dyes containing three bipyridyl groups. The extent of delocalisation of electron density in these pyridine groups was calculated via a bond length variation analysis [2]. This revealed that our complexes were much less delocalised (~4%) than N719 (up to 37%). This delocalisation of the pyridine ring is attributed to the superior charge transfer properties of N719 and thus its better performance in DSC operation. This work has contributed to the basic foundation of molecular engineering wherein lies the ultimate goal of being able to tailor dyes to meet a specific component design of a DSC device.