Structure, phase transformations, grain growth, and defects of bare and alumina-coated nanoparticles of HfO2 and ZrO2 synthesized in a microwave-plasma process were investigated by X-ray diffraction, transmission electron microscopy, and perturbed angular correlation spectroscopy. The perturbed angular correlation technique was used to measure the electric quadrupole interactions of the nuclear probes 181Ta and 111Cd in nanocrystalline HfO2 and ZrO2 as a function of temperature. For comparison, the quadrupole interactions of 181Ta in the bulk oxides was determined at 300 to 1550K. The O-metal ratio of the as-synthesized particles was determined by X-ray photo-electron spectroscopy to be 1.4 to 1.8. A hydrate surface layer with a H content of 5–10wt%, consisting of chemisorbed hydroxyl groups and organic precursor fragments, was detected by 1H magic-angle spinning nuclear magnetic resonance. X-ray diffraction and transmission electron microscopy show that bare n-ZrO2, Al2O3-coated n-ZrO2, and Al2O3-coated n-HfO2 were synthesized in the tetragonal or cubic modification with a particle size d<5nm, whereas bare n-HfO2 was mainly monoclinic. The grain growth activation enthalpy of bare n-ZrO2 was QA=32kJ/mol. Coating with Al2O3 stabilizes the tetragonal over the monoclinic phase, both in hafnia and zirconia nanoparticles. While transmission electron micrographs of the native nanoparticles reveal a well-ordered cation sub-lattice, the observation of a broad quadrupole interaction distribution in the perturbed angular correlation spectra suggests a high degree of disorder of the O sub-lattice. The gradual transformation of the disordered state and the phase evolution were studied by high-temperature quadrupole interaction measurements. Hafnia nanoparticles persist in the monoclinic (m) phase up to 1400K. 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 presented a sharp m ↔ t transition. A “defect” component involving 30 to 40% of the probe nuclei appears in the 181Ta perturbed angular correlation spectra of all nanoparticles when these were cooled from temperatures above 1200K. The temperature dependence of this component could 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<100nm suggests that mobile electrons were available only in the surface region of the oxide particles, either from O vacancies (VO) and/or VO-hydrogen donors at the interface of the nanoparticles and their hydrate layers. This conclusion was supported by the absence of a size effect for 111Cd probes in HfO2 and ZrO2. The temperature dependence of the 181Ta defect fraction was consistent with a Ta+ impurity level at Ed~0.9 and 0.6eV below the hafnia and zirconia conduction band, respectively.

Structure, Phase Transformations, and Defects of HfO2 and ZrO2 Nanoparticles Studied by 181Ta and 111Cd Perturbed Angular Correlations, 1H Magic-Angle Spinning NMR, XPS, and X-Ray and Electron Diffraction. M.Forker, P.de la Presa, W.Hoffbauer, S.Schlabach, M.Bruns, D.V.Szabó: Physical Review B, 2008, 77[5], 054108 (18pp)