RT Journal Article
T1 Structure, phase transformations, and defects of HfO₂ and ZrO₂ nanoparticles studied by ^(181)Ta and ^(111)Cd perturbed angular correlations, ^(1) H magic-angle spinning NMR, XPS, and x-ray and electron diffraction
A1 Forker, M.
A1 Presa Muñoz del Toro, Patricia de la
A1 Hoffbauer, W.
A1 Schlabach, S.
A1 Bruns, M.
A1 Szabó, D. V.
AB Structure, phase transformations, grain growth, and defects of bare and alumina-coated nanoparticles of HfO₂ and ZrO₂ synthesized in a microwave-plasma process have been investigated by x-ray diffraction (XRD), transmission electron microscopy (TEM), and perturbed angular correlation (PAC) spectroscopy. The PAC technique was used to measure the electric quadrupole interactions (QIs) of the nuclear probes ^(181)Ta and ^(111)Cd in nanocrystalline HfO₂ and ZrO₂ as a function of temperature. For comparison, the QI of ^(181)Ta in the bulk oxides was determined in the same temperature range 300 K ≤ T ≤ 1550 K. The oxygen-metal ratio of the as- ynthesized particles was determined by x-ray photoelectron spectroscopy to be in the range 1.4 ≤ x ≤ 1.8. A hydrate surface layer with a hydrogen content of 5–10 wt %, consisting of chemisorbed hydroxyl groups and organic precursor fragments, was detected by ^(1) H magic-angle spinning NMR. XRD and TEM show that bare n-ZrO₂, Al₂O₃-coated n-ZrO₂, and Al₂O₃-coated n-HfO₂ are synthesized in the tetragonal or cubic modification with a particle size d < 5 nm, whereas bare n-HfO₂ is mainly monoclinic. The grain growth activation enthalpy of bare n-ZrO₂ is Q_(A)=32(5)kJ/mol. Coating with Al₂O₃ stabilizes the tetragonal over the monoclinic phase, both in hafnia and zirconia nanoparticles. While TEM micrographs of the native nanoparticles reveal a well-ordered cation sublattice, the observation of a broad QI distribution in the PAC spectra suggests a high degree of disorder of the oxygen sublattice. The gradual transformation of the disordered state and the phase evolution were studied by high-temperature QI measurements. Hafnia nanoparticles persist in the monoclinic (m) phase up to  T ≤ 1400 K. In coated n-ZrO₂ /Al₂O₃, the monoclinic and tetragonal (t) phases coexist over a large temperature range, whereas uncoated, initially tetragonal or cubic (t or c) n-ZrO₂ presents a sharp m↔t transition. A “defect” component involving 30%–40% of the probe nuclei appears in the ^(181)Ta PAC spectra of all nanoparticles when these are cooled from high temperatures T ≥ 1200 K. The temperature dependence of this component can be reproduced by assuming that Ta impurities in hafnia and zirconia may trap electrons at low temperatures. The observation that the defect component appears only in nanoparticles with diameter d < 100 nm suggests that mobile electrons are available only in the surface region of the oxide particles, either from oxygen vacancies  (Vo) and/or Vo- hydrogen donors at the interface of the nanoparticles and their hydrate layers. This conclusion is supported by the absence of a size effect for ^(111)Cd probes in HfO₂ and ZrO₂. The temperature dependence of the ^(181)Ta defect fraction is consistent with a Ta_(+) impurity level at E_d ~ 0.9 and 0.6 eV below the hafnia and zirconia conduction band, respectively.
PB American Physical Society
SN 1098-0121
YR 2008
FD 2008-02
LK https://hdl.handle.net/20.500.14352/52147
UL https://hdl.handle.net/20.500.14352/52147
LA eng
NO © The American Physical SocietyThe authors gratefully acknowledge the financial support by Deutsche Forschungsgemeinschaft under Grants Nos. FO148/3-1,2 and VO861/1-1,2. They are also grateful to C. Odemer and J. Marquardt, Forschungszentrum Karlsruhe, for the support in the thermogravimetric and combustion measurements, respectively. The assistance of J. Penner and T. Geruschke, University of Bonn, in some of the PAC experiments is very much appreciated. The critical reading of the manuscript by A. F. Pasquevich, University of La Plata, has been very helpful. The neutron  irradiations have been carried out at GKKS, Geesthacht.
NO Deutsche Forschungsgemeinschaft
DS Docta Complutense
RD 1 may 2024