Structure, phase transformations, and defects ofHfO2andZrO2nanoparticle

Abstract

Structure, phase transformations, grain growth, and defects of bare and alumina-coated nanoparticles of HfO 2 and ZrO 2 synthesized in a microwave-plasma process have been investigated by x-ray diffraction ͑XRD͒, transmission electron microscopy ͑TEM͒, and perturbed angular correlation ͑PAC͒ spectroscopy. The PAC technique was used to measure the electric quadrupole interactions ͑QIs͒ of the nuclear probes Ta in the bulk oxides was determined in the same temperature range 300 K ഛ T ഛ 1550 K. The oxygen-metal ratio of the as-synthesized particles was determined by x-ray photoelectron spectroscopy to be in the range 1.4ഛ x ഛ 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-ZrO 2 , Al 2 O 3 -coated n-ZrO 2 , and Al 2 O 3 -coated n-HfO 2 are synthesized in the tetragonal or cubic modification with a particle size d Ͻ 5 nm, whereas bare n-HfO 2 is mainly monoclinic. The grain growth activation enthalpy of bare n-ZrO 2 is Q A =32͑5͒ kJ/ mol. Coating with Al 2 O 3 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 ͑m͒ phase up to T ഛ 1400 K. In coated n-ZrO 2 / Al 2 O 3 , the monoclinic and tetragonal ͑t͒ phases coexist over a large temperature range, whereas uncoated, initially tetragonal or cubic ͑t or c͒ n-ZrO 2 presents a sharp m ↔ t transition. A "defect" component involving 30%-40% of the probe nuclei appears in the 181Ta PAC spectra of all nanoparticles when these are cooled from high temperatures T ജ 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 Ͻ 100 nm suggests that mobile electrons are available only in the surface region of the oxide particles, either from oxygen vacancies ͑V O ͒ and/or V O -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 HfO 2 and ZrO 2 . The temperature dependence of the 181 Ta defect fraction is consistent with a Ta + impurity level at E d ϳ 0.9 and 0.6 eV below the hafnia and zirconia conduction band, respectively.

Coexistence of different charge states in Ta-doped monoclinicHfO2: Theoretical and experimental approaches

“…For these reasons, HfO 2 has been extensively studied by means of TDPAC. 3 The TDPAC technique provides a high-resolution determination of quadrupolar electric, dipolar magnetic hyperfine interactions, or a combination of both. 4 The electric quadrupolar interaction is essentially given by the product of the nuclear quadrupole moment, Q, characteristic of a given nuclear state of spin I, and the electric field gradient ͑EFG͒ at the nuclear site.…”

“…Ayala et al 9 reported a second interaction in coarse grained m-HfO 2 characterized by high V ZZ and values. Forker et al 3 investigated the structure, phase transformations, grain growth, and defects of coarse-grained and bare nanoparticles of m-HfO 2 synthesized in a microwave plasma. The authors found that, in addition to the well-known HFI1, a second component involving 20-30 % of the probe nuclei appeared in the 181 Ta PAC spectra.…”

“…The theoretical predictions were compared with a new analysis of the TDPAC experiments reported in Ref. 3.…”

Impurity Centers in Oxides Investigated by γ-γ Perturbed Angular Correlation Spectroscopy and <i>Ab Initio</i> Calculations

“…More recently, investigations of thin films [121,122] and nanoparticles [123] of HfO 2 have been reported. The last reference is particularly important for this chapter.…”

“…The last reference is particularly important for this chapter. Indeed, the formation of probe-defect complexes (and insights into the nature of these defects) in hafnia nanoparticles was studied in great detail as a function of the thermal history of the samples by Forker et al [123] by means of PAC experiments using 181 Ta as probe. Before the results obtained by Forker et al using hafnia nanoparticles are discussed, it is important to describe the results, obtained by the same group, in commercial coarse-grained (cg) HfO 2 .…”