Structural Disorder in the Anion Lattice of Nanocrystalline Zirconia and Hafnia Particles
- PDF / 200,975 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 66 Downloads / 161 Views
Structural Disorder in the Anion Lattice of Nanocrystalline Zirconia and Hafnia Particles Dieter Vollath1), Manfred Forker2), Michael Hagelstein3), D. Vinga Szabó1) 1) Forschungszentrum Karlsruhe, Institut für Materialforschung III, D-76021 Karlsruhe, Germany, P.O.Box 3640, Germany, E-mail: [email protected] 2) Universität Bonn, Institut für Strahlen- und Kernphysik, Nußallee , D-53115Bonn, Germany 3) ANKA GmbH, Hermann-von-Helmholtz-Platz, D-76344 Eggenstein-Leopoldshafen, Germany ABSTRACT Nanoparticles of ZrO2 and HfO2 with sizes around 5 nm are – depending on the production route - found in the cubic or tetragonal or monoclinc phase. To shed some light into this difference, an analysis of crystal structure, distances, and symmetry around the cations was performed on alumina coated nanoparticles prepared by the microwave plasma process. Within the range of accuracy, electron-diffraction and -microscopy revealed a possibly cubic cation lattice for both materials. In the case of ZrO2, the evaluation of the amplitude of the modified radial distribution function derived from extended x-ray absorption fine structure (EXAFS) data lead to a structure with a narrow distribution of oxygen ions in the first shell of neighbors. This is expected for the cubic structure. Comparing the imaginary part with model calculations, a distinct similarity with the cubic structure is obvious. In the second shell of neighbors, consisting of metal ions, one radial distribution peak is expected and found experimentally. Perturbed angular correlation measurements indicate a pronounced disorder of the positions of the first neighbors of the metal ions. Together with the EXAFS observation of a narrow radial distribution of the first neighbor oxygen ions, these measurements provide evidence for a random distribution of Zr-O bond angles in the vicinity of their crystallographic positions. Above 500°C a partially reversible transformation to almost perfect tetragonal symmetry is observed. Therefore, one has to conclude that the cubic structure with disordered anion lattice is more stable than the tetragonal one at room temperature. Structural relationships like this are unknown in coarse-grained material. INTRODUCTION Undoped zirconia, ZrO2 and hafnia, HfO2, undergo a series of phase transformations. The first solid phase at high temperatures is the cubic fluorite structure with the highest density, lowering the temperature the tetragonal phase occurs, and finally, at room temperature, the stable structure is the monoclinic one exhibiting the lowest density [1]. In general, compressive stresses promote the stability of the phases with higher density. Therefore, decreasing grain size lowers the temperature of the phase transformations. This behavior is well proven for zirconia and expected for hafnia, too [1]. The materials used in this study, undoped nanocrystalline zirconia and hafnia with particle size around 5 nm with and without alumina, Al2O3, coating were synthesized using the microwave plasma process [2,3]. As in this process the p
Data Loading...
