Local atomic structure in tetragonal pure ZrO₂nanopowders

Abstract

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.

XANES and EXAFS study of the local order in nanocrystalline yttria-stabilized zirconia

“…Several XAS studies have been reported to date for solid ZrO 2 -Y 2 O 3 solutions or nanocrystalline YSZ materials for which the structural disorder plays an important role. 27,28 The results were controversial, mainly due to the interpretation of the coordination numbers and Debye-Waller factors extracted from EXAFS. 29 For this reason not only the EXAFS spectra but also the XANES region, more sensitive to the bonding geometry and stereochemical details, have been analyzed in order to reduce the possible arbitrariness of the EXAFS results arising from the large uncertainties in coordination numbers and Debye-Waller factors due to their strong correlation.…”

Water-gas shift: Characterization and testing of nanoscale YSZ supported Pt catalysts

“…The first peak corresponds to the contribution of the first oxygen shell around the Zr absorber, while the second peak contains information related to Zr Zr coordination. At the same temperature, different parameters can influence the intensity of these peaks such as: the degree of reduction, the crystallite size [59], and the local order in the system [60].…”

Caracterização estrutural e eletrônica da zircônia pura e com defeitos e impurezas

“…Contudo, apesar de muitos estudos experimentais, pouco tem sido estudado do ponto de vista teórico sobre propriedades estruturais e impurezas em zircônia. Materiais baseados em zircônia têm sido muito investigados devidoàs suas grandes aplicações tecnológicas como calalizadores [1], células solares e sensores de oxigênio [2,3]. A zircônia apresenta 3 fases estruturais, dependendo da temperatura.…”

“…A zircônia apresenta 3 fases estruturais, dependendo da temperatura. Até 1170 • C sua fase mais estávelé a monoclínica, de 1170 • C até 2370 • C a fase tetragonalé a mais estável e acima de 2370 • C até seu ponto de fusão (2780 • C) a estruturaé cúbica [2]. As fases mais importantes para aplicações práticas são a tetragonal e a cúbica.…”

“…Alguns modelos foram propostos para tentar explicar a estrutura local do ZrO 2 , baseados em experimentos de EXAFS e raios-X [2]. Porém, ainda não se sabe qual desses modelos melhor representa a estrutura da zircônia.…”