ANASTASIA BURIMOVA
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Resumo IPEN-doc 27305 Effect of the magnetic impurity on the charge diffusion in highly dilute Ce doped LaMnO32019 - CABRERA-PASCA, G.; BOSCH-SANTOS, B.; BURIMOVA, A.; CORREA, E.; CARBONARI, A.W.LaMnO3+δ is a complex oxide, which, depending on the oxygen excess concentration δ, presents different crystalline structure and interesting magnetic and electric properties such as colossal magnetoresistance, polaron dynamics, multiferroic behavior, and charge-orbital ordering. This complexity requires different characterization techniques to draw a picture as complete as possible allowing a good understanding of these phenomena. Here, we have used the perturbed angular correlation (PAC) technique to measure hyperfine interactions at La and Mn sites of LaMnO3+δ (δ ~0.15) using 140Ce (at La sites) and 111Cd (at both La and Mn sites) as probe nuclei in order to investigate within an atomic scale the magnetic and electric interactions in this compound. Results show that 111Cd nuclei occupy quite high symmetric local sites in agreement with a rhombohedral structure. The magnetic hyperfine field (Bhf) measured with 111Cd at La sites is very small (Bhf = 0.40 T) due to the supertransferred magnetic field from Mn neighbors through oxygen orbitals. On the other hand, 140Ce nuclei at La sites present a saturation field of around 3.7 T much higher than that expected for La sites (due to the weak transfer field by superexchange mechanism). In addition, for temperature range above the magnetic ordering (200-300 K) a dynamic hyperfine interaction was observed characterized by the attenuation parameter λ(T) whose temperature dependence allowed to determine the activation energy (Ea) associated to the polaron diffusion, as displayed in Fig. 1.Resumo IPEN-doc 27096 Mechanisms of reduction in V2O5 and MoO3 studied by PAC spectroscopy2019 - BURIMOVA, A.; CARBONARI, A.W.; RICHARD, D.; PEREIRA, L.F.D.; GENEZINI, F.A.Structural evolution of vanadium (V) and molybdenum (VI) oxides with temperature was studied by Time Differential Perturbed Angular Correlation (TDPAC) spectroscopy in different atmospheres with 111Cd probe incorporated by wet impregnation. In case of V2O5 sealed in vacuum, the critical temperature for probe diffusion was found to be above 500oC, whereas for MoO3 this barrier was 600oC. The diffusion was suppressed in low pressure oxygen atmosphere. The behavior of hyperfine parameters on heating to melting point of V and Mo oxides and subsequent cooling to room temperature was analyzed. TDPAC results were complemented by X-ray diffraction data. We discuss comproportionation, topotactic, competitive and consecutive mechanisms as the candidates to describe V2O5 → VO2 and MoO3 → MoO2 reduction.Artigo IPEN-doc 25614 Low temperature synthesis of pure and Fe-doped HfSiO4: Determination of Si and Fe fractions by neutron activation analysis2019 - SALES, T.N.S.; BOSCH-SANTOS, B.; SAIKI, M.; BURIMOVA, A.; PEREIRA, L.F.D.; SAXENA, R.N.; CARBONARI, A.W.A new method of synthesis of hafnium silicate HfSiO4 (also known as hafnon) is reported. We observed a selfcontrolled incorporation of SiO2 from the quartz tube in which a sample of hafnium oxide nanoparticles was heated. This approach was then adapted to Fe-doped hafnon production. Sample structure, morphology and composition were characterized by X-ray diffraction, electron microscopy and neutron activation analysis. Diffraction data has shown that lattice parameters of doped HfSiO4 thus obtained are very close to those previously known for bare hafnon. The hafnon-like phase stabilized at T=900 °C which is about 500 °C lower than the corresponding transition of bare bulk hafnium silicate. The fractions of Si and Fe in the composite matrices were determined with neutron activation analysis. These results completed by X-ray diffraction data allowed to assume that (i) Fe initially substituted Hf in the HfO2 lattice; (ii) there was no migration of iron atoms from Hf to Si sites at the formation of hafnon-like phase; (iii) doped and undoped hafnium oxide has taken as much Si from the quartz as was needed for the arrangement of Fe1-xHfxSiO4 tetragonal system, 0≤x<0.2. Our results are consistent with those obtained for similar materials, such as metal (Fe,V) doped zircon, where the dopant also demonstrated catalytic effect on phase stabilization.Resumo IPEN-doc 25561 Tuning the dielectric response of HfO 2 nanoparticles by Si-doping2018 - SALES, T.S.N.; BURIMOVA, A.; CARBONARI, A.W.; BOSCH-SANTOS, B.; PEREIRA, L.F.D.The properties of bulk materials and corresponding nanostructures may differ significantly which leads to distinct scopes of their applications. As the miniaturization continues, nanostructures show great potential in microelectronics. Among others, nanostructured hafnium (IV) oxide (hafnia, HfO 2 ) has attracted considerable attention due to its chemical stability and physical characteristics 1 . It has high dielectric constant ( ∼ 25) and is relatively thermally stable (melting point at 3085 K) 2 . These qualities have made hafnia an alternative to SiO 2 for gate oxide layers in field-effect transistors, since the leakage current issue and consequent thickness limitations of silica were hindering miniaturization. Clearly, the features of HfO 2 can be further tuned by doping 3 . Understanding the relation between the macro-properties of such complex systems and their architecture at atomic scale should help select doping parameters adequate for particular applications. In this work 0at%, 5at% and 10at% Si-doped HfO 2 nanoparticles (NPs) were synthesized via sol-gel method. Basic characterization included scanning/transmission electron microscopy, electron backscattering and X-ray diffraction. NPs were demonstrated to have spherical shape, sizes in the range of 10─40nm and monoclinic structure (m-HfO 2 ). Local structure and electronic environment of the samples was probed with perturbed angular correlation (PAC) spectroscopy. Sample irradiation resulted in the formation of 181 Hf isotopes (PAC probes 4 ) at several sites whose decay to 181 Ta is preferentially through an intermediate (metastable) state with lifetime depending strongly on local extra-nuclear fields. Thus, time distribution of decay probability provided information on the ambient of the probe. For undoped sample PAC data has revealed two distinct Hf sites, one of those possessing well-defined quadrupole frequency ν Q ≈ 750 MHz and electric field distribution asymmetry η ≈ 0.36. Site occupation raised with temperature from 16% at T = 473K to 75% at T = 1073K, which agrees with the results found in the literature for the quadrupole interactions of 181 Ta in m-HfO 2 4 . For the doped samples (both Si 5at% and 10at%) additional site was observed. Symmetrical electric field distribution ( η = 0) and a greater site fraction at T = 673─873K allowed to attribute it to a tetragonal phase of hafnia. First principles analysis has shown that doping m-HfO 2 with certain cations can induce the formation of tetragonal phase 5 . We emphasize that this phase is desirable for transistor applications since it is associated with the highest dielectric constant.Artigo IPEN-doc 24375 Magnetic interactions at Ce impurities in REMn2Ge2 (RE = La, Ce, Pr, Nd) compounds2018 - BOSCH-SANTOS, B.; CABRERA-PASCA, G.A.; SAXENA, R.N.; BURIMOVA, A.N.; CARBONARI, A.W.In the work reported in this paper, the temperature dependence of the magnetic hyperfine field (Bhf) at 140Ce nuclei replacing Pr atoms in PrMn2Ge2 compound was measured by the perturbed angular correlation technique to complete the sequence of measurements in REMn2Ge2 (RE = La, Ce, Pr, Nd). Results show an anomalous behavior different from the expected Brillouin curve. A model was used to fit the data showing that the Ce impurity contribution (Bimp hf ) to Bhf is negative for NdMn2Ge2 below 210 K. The impurity contribution (Bimp hf ) at 0 K for all compounds is much smaller than that for the free Ce3+, showing that the 4f band of Ce is more likely highly hybridized with 5d band of the host. Results show that direction of the localized magnetic moment at Mn atoms strongly affects the exchange interaction at Ce impurities.Resumo IPEN-doc 23266 Determination of Si content in Fe-doped HfSiO4 nanoparticles by neutron activation analysis2017 - SALES, T.S.N.; BOSCH-SANTOS, B.; SAIKI, M.; PEREIRA, L.F.; CARBONARI, A.W.; SAXENA, R.N.; BURIMOVA, A.New synthesis method of Fe-doped hafnium silicate nanoparticles (NPs) with tetragonal structure is reported. The procedure implies auto-controlled incorporation of Si from the quartz to the iron-doped hafnium oxide NPs. X-ray diffraction data has shown that lattice parameters of Fe-HfSiO4 thus obtained are very close to those previously known for hafnon. It was established that the hafnon-like phase has stabilized at T=1173K which is ~500K lower than the corresponding transition of bare bulk hafnium silicate. The fractions of Si and Fe in the composite matrix were evaluated via neutron activation analysis (NAA). Delayed gamma NAA results allowed to assume that (i) Fe initially substituted Hf in the HfO2 lattice; (ii) there was no migration of iron atoms from Hf to Si sites throughout the formation of hafnon-like phase; (iii) Fe-doped hafnium oxide has taken as much Si from the quartz as was needed for the arrangement of Fe1-x-Hf x SiO4 tetragonal system. Our results are consistent with those observed for similar materials, such as metal (Fe,V) doped zircon, where metal has also demonstrated catalytic effect on phase stabilization.