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PAC Experiments for a Short Range Study of the Zr(10%Pr)O2 Solid Solution
Abstract
A Zr(10 mol % Pr)O2 powder obtained by high-energy ball milling has been investigated at nanoscopic scale using primarily the Perturbed Angular
Correlations technique. The aim has been to determine the nanoconfigurations around Zr4+ cations present in the solid solution and their thermal evolution with the intention of providing knowledge on the stability
of the system. Results indicate that the milled product is a substitutional cubic solid solution described by two hyperfine
interactions: a highly disordered interaction due to oxygen vacancies located very close to Zr4+ and an ordered interaction probably depicting a charge distribution including Pr3+ as nearest neighbor to Zr4+ probes. On cooling from high temperatures, monoclinic zirconia appears mostly at the expense of the oxygen defective cubic
form. A gradual cooling indicates that destabilization of the solid solution takes place around 500°C. Thermal cycling leads
to increasing amounts of the monoclinic phase.
... The mechanical treatment can also lead to disorder in the material, phase transformations and formation of solid solutions. The perturbed angular correlations technique (PAC) is a powerful tool to study these processes in oxide blends [4][5][6]. PAC gives information of the electric field gradients (EFG) at atomic sites in solids allowing the determination of the atomic configuration around them. The method requires a radioactive probe with special nuclear properties located at the site where the EFG is to be measured. ...
... The quadrupole parameters for the stabilized phases have been reported for different aliovalent impurities (e.g. yttrium, iron and praseodymium) [5,6,9] and the typical values are: ...
Ball milling of oxide blends can result in the formation of solid solutions depending on the characteristics of the oxides. In this paper the possibility of doping oxides with radioactive 181Hf through the formation of these solutions is analyzed. The 181Hf isotope decays to 181Ta, which is an adequate probe for perturbed angular correlations (PAC) studies. Through the measurement of the hyperfine interactions of 181Ta nuclei it is possible to determine the atomic distribution around the probes. We have thus studied the behavior of the La2O3–HfO2 ceramic system subjected to high-energy ball milling. An oxide blend, containing few atomic percent of hafnium oxide, was milled during several hours resulting in the formation of hafnium oxide defective phases. The sample was finally annealed at high temperatures in order to facilitate the formation of solid solutions. This thermal treatment produced a solid-state reaction given place to Hf2La2O7 pyrochlore and also the apparition of another phase or compound. The possibility of associating this last finding with a stabilized cubic phase of hafnium oxide resulting from lanthanum doping is analyzed.
... It is probably the cubic phase stabilized by a small amount of impurities [6]. The characteristic hyperfine parameters for this phase have been reported for different aliovalent impurities and typical values are in the following ranges: ω Q = 90-100 Mrad/s, Á 1 = 0.75-1 and ı 1 = 5-15% [7][8][9]. This phase contains many oxygen vacancies, which produce a nonzero EFG at the probe site. ...
Thin oxide films (100nm) grown on silicon (100) substrates were characterized by Perturbed Angular Correlation (PAC). Three samples were analyzed: (a) prepared by electron beam evaporation; (b) prepared by atomic-layer chemical vapor deposition method, with H2O and HfCl4 as precursors; (c) prepared by metal organic chemical vapor deposition with tetrakis-diethyl-aminohafnium and O2 as precursors. The radioactive 181Ta isotopes used as PAC probes appeared in the samples after the β-decay of 181Hf. These isotopes were introduced, in the cases of samples prepared by chemical vapor deposition, by ion implantation. In the case of samples prepared by electron beam evaporation, the PAC isotopes 181Hf were produced by neutron activation of the film by the reaction 180Hf(n,γ)181Hf. PAC measurements were carried out at room temperature after annealing at different temperatures from 773K up to 1273K in air. The PAC technique allows to determine the electric field gradient at the probe sites. In this way, the crystallization of the hafnium oxide films was monitored, characterizing at a nanoscopic level the atomic surrounding of the probes.
This paper deals with the phase stability of an aqueous suspension of tetragonal Zr(0.9)Pr(0.1)O(2) (20 wt%/vol%) at room temperature as a function of the aging time. The suspension is investigated in situ using the highly localized Perturbed Angular Correlations technique. The results indicate that an almost fully reversible degradation process toward monoclinic zirconia takes place through a first-order reaction of rate constant k = 0.7 day(-1). Two successive diffusion mechanisms are observed that are interpreted as OH(-) ions' migration in the grain surface and then, as proton defects' diffusion into the bulk.
Based on X-ray diffraction (XRD) patterns and high resolution transmission electron microscopy (HRTEM) the mechanically alloyed ZrO2-Y2O3 (CoO, Fe2O3, or α -Al2O3 ) end-products have been investigated. Starting from the mixture of 2.5 g of monoclinic ZrO2 and 0.5 g Y2O3 (CoO, Fe2O3, or α -Al2O3) powders, ZrO2 based tetragonal/or cubic solid solutions were produced. Also a refinement of crystalline sizes down to nanometer-sizes was observed during milling.
The intermetallic compound AlFe in its ordered B2 phase has been ground under Ar in a ball-milling device. The system evolution under mechanical work has been examined as a function of time of milling by x-ray diffraction, transmission Mössbauer spectroscopy, and differential scanning calorimetry. Short and long range disorder was observed. The evolution of the long range order parameter with milling time is analyzed up to the completion of the solid state reaction when a steady state value is reached. The short range order evolution is consistent with a nonstatistical order distribution throughout the sample but mainly distributed in two regions with different degrees of order each. Kinetic analysis of the reordering and recrystallization has been performed under continuous heating and isothermal calorimetric regimes. Modeling of these processes is presented as well as the estimated values of both the enthalpy and the activation energy of the reordering process. Results are compared with preceding information on related systems.
Following a brief review of its properties, discussions are presented of the application of synchrotron radiation to: (a) the determination of atomic arrangements using extended x-ray absorption fine structure (EXAFS); (b) the determination of atomic arrangements using x-ray anomalous scattering; (c) dynamic small angle x-ray scattering; (d) x-ray lithography and microscopy; (e) the determination of surface electronic states using vacuum ultraviolet radiation; (f) time resolved fluorescence from protein molecules.
The effects on the molar compositions (ZrO2)0· 92 (Y2O3)0· 04 and (ZrO2)0· 82 (Y2O3)0· 04 (Pr2O3)0· 05 of preparation method, including the traditional ceramic method and several sol–gel methods, and of doping with praseodymium have been studied. Powders fired at 500°C were sieved at 60 µm, uniaxially pressed, and fired at 1600°C for 2 h. The resulting compacts were characterised by XRD, Raman spectroscopy, SEM–EDX, density measurements, and Vickers microhardness testing. The results indicate that praseodymium addition stabilises cubic zirconia rather than the tetragonal polymorph and that it also improves microhardness and densification of the material.
Powders and coatings of zirconia doped with 2.5 mole% yttria have been produced via the sol-gel route. The phase structure and subsequent thermal evolution in heating and cooling cycles have been investigated using mainly perturbed angular correlations spectroscopy. Thermal analyses and XRD as a function of temperature have also been performed to obtain complementary information. Upon heating, the amorphous gels crystallized into the tetragonal structure and showed the same hyperfine pattern and thermal behavior as observed in tetragonal zirconia obtained by the ceramic route: the two configurations of vacancies around zirconium ions denoted as t1 and t2 forms and their mutual t1 → t2 transformation. While the powder sample exhibited an incipient thermal instability above 1000 °C and underwent completely the t2 form to m–ZrO2 transition during subsequent, gradual cooling below 500 °C, the coating retained the tetragonal phase within the whole temperature range investigated. Hyperfine results suggest that the tetragonal phase stabilization is favored by the highly defective nature of the t1 form and consequently hardened by the availability of oxygen. The PAC derived activation energy for the fast diffusion of the oxygen vacancies inherent to the t2 form was determined as 0.54 ± 0.14 eV.
Fully stabilized cubic zirconia doped with iron oxide has been synthesized by high-energy ball milling from powder mixtures of monoclinic zirconia and hematite. It is found that the iron ions dissolved in cubic ZrO2 are in substitutional positions with a maximum solubility of approximately 18.5 mol% α–Fe2O3. The unit-cell volume of the cubic ZrO2 phase decreases with increasing iron content. During heating the cubic-to-tetragonal transition occurs at approximately 827 °C and the tetragonal-to-monoclinic transition seems to be absent at temperatures below 950 °C. During cooling the tetragonal-to-monoclinic transition occurs at 900–1100 °C.
This work focuses on the microwave-assisted hydrothermal synthesis of praseodymium-doped zirconia and on the subsequent evaluation of the effect of synthesis conditions on powder properties. Pure and 10 mol% Pr-doped zirconia samples have been analyzed by X-ray diffraction (XRD) and the perturbed angular correlations hyperfine technique, which probes the nearest environments of zirconium ions. At atomic scale, as determined from perturbed angular correlation data, the XRD amorphous fraction of the as-obtained powders exhibits a tetragonal-like structure. The pure powder becomes partially stabilized and the doped powder is a substitutional solid solution of praseodymium in tetragonal zirconia.
The Mössbauer effect and perturbed angular correlation techniques have been used to investigate the nature and thermal stability of Zr and Fe surroundings in a solid solution of zirconia and 15 mol% hematite obtained by mechanical mixing. Hematite incorporates completely into the zirconia lattice, resulting in a substitutional cubic structure that remains stable over a wide thermal range. Upon annealing at 900°C, a 20% decrease in the iron solubility occurs, accompanied by the cubic to tetragonal zirconia transformation. Both the cubic and the tetragonal crystalline solid solutions exhibit two local coordinations for each one of the probes. The presence of the charge-balancing oxygen vacancy, which appeared after Fe3+ substitution for Zr4+ and located as a nearest neighbor to host and dopant cations, is reflected by the most disordered interaction, which shows similar characteristics for the cubic and the tetragonal polymorphs. The other local coordination, described at Zr sites by an electric field of phase-dependent intensity, has been thought to depict a direct interaction between very near Fe and Zr cations.
The PrxZr1-xO2 ceramic system obtained by coprecipitation has been investigated at zirconium sites as a function of Pr content (1, 10, and 15 mol %) and of annealing temperature by means of the perturbed angular correlations (PAC) technique. Praseodimium enters zirconia forming a tetragonal solid solution whose extent increases with Pr content. In 10 and 15 mol % Pr-doped samples, metastable tetragonal zirconia is present in its two known t- and t'-forms, the latter form being the one which involves a higher dopant concentration. While Rietveld refinement and thermal analyses indicate the uncompleted crystallization of the as-obtained powders, PAC allows the phase identification of the amorphous residues. Minor changes observed in the hyperfine interaction at high temperatures reveal the incipient ceramic degradation. Experiments indicate that, up to 15 mol % of dopant content, Pr behaves as an efficient zirconia stabilizer.
Characteristic extended x-ray-absorption fine structure and x-ray-absorption near-edge structure spectra of zirconia polymorphs (monoclinic, orthorhombic, tetragonal, and cubic) have been identified at 10 K. For the host cations, these spectra are phase dependent but dopant independent because the dopant perturbation is short ranged. In the tetragonal structure, the first Zr-O shell is bifurcated into two tetrahedra, with outer O ions loosely bound and severely distorted. Although the tetragonal Zr cation network is nearly fcc and higher-order Zr-Zr coordinated shells are resolvable at distances up to 9 Å, other evidence points to incoherent-distortion reflecting phase instability. In monoclinic, orthorhombic, and cubic structures, a similar Zr-O polyhedron with sevenfold coordination and a comparable bond length is observed, along with a large distortion or splitting of the next-nearest-neighbor Zr-Zr shell. Progressive departure from the ideal fluorite structure by reducing the Zr-O coordination and dilating or distorting the local Zr-Zr network is seen to be correlated to increased phase stability but not to long-range crystal symmetry.
Powder Diffraction File, Card No. 27-997
- R Jenkins
- R Anderson
- G J Mccarthy