Crystal structure and local order of nanocrystalline zirconia-based solid solitions (original) (raw)
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Journal of Applied Crystallography, 2008
The crystal structure and the local atomic order of a series of nanocrystalline ZrO 2 -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 pHcontrolled nitrate-glycine gel-combustion process. For CaO contents up to 8 mol%, the t 0 form of the tetragonal phase (c/a > 1) was identified, whereas for 10 and 12 mol% CaO, the t 00 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 0 /t 00 and t 00 /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.
Structure of nanocrystalline zirvonia and yttria
Nanostructured Materials, 1995
A combination of spectroscopic and imaging methods was chosen to investigate extended (XRD, TEM) and local (EXAFS, HR-TEM) structure of nanosized ZrO2 and Y2O3. Phase content of the powders was determined by Rietveld analysis. We investigated phase stability of metastable polymorphs in n-oxides by in-situ high temperature and high pressure XRD. EXAFS oscillations in nano oxide powders (5 to 30 nm) are distinctly weaker compared to coarse grained samples. In a compacted and sintered sample of n-Zr02 (80 nm) the EXAFS oscillation is nearly identical to the monoclinic coarse grained powder. High-Resolution-TEM micrographs show crystalline grains with grain sizes between 5 and 30 nm. Image contrast simulations of n-ZrO, powder micrographs indicate at least for some particles a shell like structure with different ph.ases.
Journal of Materials Chemistry, 2003
The crystal structure of compositionally homogeneous, nanocrystalline ZrO 2 -10, 30, 50, 70 and 90 mol% CeO 2 powders synthesised by a pH-controlled nitrate-glycine gel-combustion process has been studied by X-ray diffraction and Raman spectroscopy. All the powders with a CeO 2 content up to 70 mol% exhibited the tetragonal phase (P4 2 /nmc space group), whereas the ZrO 2 -90 mol% CeO 2 solid solution presented the cubic phase (Fm3m space group). The axial ratio c/a decreased with increasing CeO 2 content and became unity for ZrO 2 -70 mol% CeO 2 powders. By analysing X-ray diffraction data using the Rietveld method, it was found that this material exhibited the t@-form of the tetragonal phase (the oxygen atoms were displaced from their ideal sites of the cubic phase along the c axis). Raman spectroscopy study confirmed the results found by X-ray diffraction. The morphology of the nanopowders was also evaluated.
XANES and EXAFS study of the local order in nanocrystalline yttria-stabilized zirconia
The local order around Zr and Y atoms of nanocrystalline yttria-stabilized zirconia (YSZ) powders with different grain sizes has been investigated by x-ray absorption spectroscopies. The samples were prepared by means of mechanical alloying with or without subsequent sintering treatment and also by milling commercial YSZ. Our study is motivated by the interest in the electrical properties of grain boundaries and the controversy about the level of disorder in the intergrain regions in nanocrystalline YSZ. The x-ray absorption near edge structure (XANES) analysis indicates that the local order of all the sintered samples is independent of the grain size. This is confirmed by the analysis of the extended x-ray absorption fine structure, which points out also that, in contrast to that found in sintered samples, the local order around the cation in the samples milled without further sintering treatment extends only to the first coordination shell. Finally, the results of ab initio Zr K-edge XANES calculations lead us to conclude that the observed changes of the shape of the white line are not related to a phase transformation but reflects the short-range order present in the as-milled samples.
Structure of nanoporous zirconia-based powders synthesized by different gel-combustion routes
Journal of Applied Crystallography, 2006
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 ZrO 2-2.8 mol% Y 2 O 3 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.
Journal of Solid State Chemistry, 1999
Nanostructured yttrium-stabilized zirconia powders, with yttria concentrations between 0.0 and 10.0 mol%, were prepared via the hydrolysis of an aqueous solution of zirconyl and yttrium chloride, and ammonium hydroxide. Powder phases were characterized by using X-ray powder diffraction; their crystalline structures were refined with the Rietveld technique. When samples were annealed below 200°C, their diffraction patterns corresponded to an amorphous atom distribution and were independent of yttria concentration. The doped amorphous phases crystallized, at 400°C, into tetragonal or cubic nanocrystalline zirconia, which were stabilized by yttrium. These results suggest that yttrium atoms served as a substitute for zirconium atoms not only in the crystalline phases but also in the amorphous phases, which are determined by the fast condensation of zirconyl clusters. Nondoped samples contained a mixture of monoclinic and tetragonal nanocrystalline zirconia; those with 2.5 to 5.0 mol% yttria contained only the tetragonal zirconia nanophase, and those with 7.5 to 10.0 mol% had only the nanocrystalline cubic phase. The average crystallite size of the nanophases diminished when Y 2 O 3 concentration was increased.
Journal of Applied Physics, 2008
Local environment surrounding Zr atoms in the thin films of nanocrystalline zirconia ͑ZrO 2 ͒ has been investigated by using the extended x-ray absorption fine structure ͑EXAFS͒ technique. These films prepared by the ion beam assisted deposition exhibit long-range structural order of cubic phase and high hardness at room temperature without chemical stabilizers. The local structure around Zr probed by EXAFS indicates a cubic Zr sublattice with O atoms located on the nearest tetragonal sites with respect to the Zr central atoms, as well as highly disordered locations. Similar Zr local structure was also found in a ZrO 2 nanocrystal sample prepared by a sol-gel method. Variations in local structures due to thermal annealing were observed and analyzed. Most importantly, our x-ray results provide direct experimental evidence for the existence of oxygen vacancies arising from local disorder and distortion of the oxygen sublattice in nanocrystalline ZrO 2 . These oxygen vacancies are regarded as the essential stabilizing factor for the nanostructurally stabilized cubic zirconia.
Journal of Applied Crystallography, 2005
The crystal structures of a number of nanocrystalline ZrO 2 -CeO 2 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 CeO 2 content, the tetragonal-cubic phase boundary was found to be at 85 (5) mol% CeO 2 . For a CeO 2 content up to 68 mol%, a tetragonal phase with c/a > 1 (known as the t 0 form) was detected, whereas, between 68 and 85 mol% CeO 2 , the existence of a tetragonal phase with c/a = 1 and oxygen anions displaced from their ideal positions in the cubic phase (the t 00 form) was verified. Finally, solid solutions with higher CeO 2 contents exhibit the cubic fluorite-type phase.
Journal of Alloys and Compounds, 2011
By means of synchrotron X-ray powder diffraction (SXPD) and Raman spectroscopy, we have detected, in a series of nanocrystalline and compositionally homogeneous ZrO 2 -Y 2 O 3 solid solutions, the presence at room temperature of three different phases depending on Y 2 O 3 content, namely two tetragonal forms and the cubic phase. The studied materials, with average crystallite sizes within the range 7-10 nm, were synthesized by a nitrate-citrate gel-combustion process. The crystal structure of these phases was also investigated by SXPD. The results presented here indicate that the studied nanocrystalline ZrO 2 -Y 2 O 3 solid solutions exhibit the same phases reported in the literature for compositionally homogeneous materials containing larger (micro)crystals. The compositional boundaries between both tetragonal forms and between tetragonal and cubic phases were also determined.