Navegando por Autor "Krambrock, Klaus Wilhelm Heinrich"

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High throughput investigation of an emergent and naturally abundant 2D material : clinochlore.
(2022) Gonçalves, Raphaela de Oliveira; Guallichico, Luis Antonio Guallichico; Policarpo, Eduardo; Cadore, Alisson Ronieri; Freitas, Raul de Oliveira; Silva, Francisco Mateus Cirilo da; Teixeira, Veronica de Carvalho; Paniago, Roberto Magalhães; Chacham, Helio; Matos, Matheus Josué de Souza; Souza, Angelo Malachias de; Krambrock, Klaus Wilhelm Heinrich; Barcelos, Ingrid David
Phyllosilicate minerals, which form a class of naturally occurring layered materials (LMs), have been recently considered as a low-cost source of two-dimensional (2D) materials. Clinochlore [Mg5Al(AlSi3)O10(OH)8] is one of the most abundant phyllosilicate minerals in nature, exhibiting the capability to be mechanically exfoliated down to a few layers. An important characteristic of clinochlore is the natural occurrence of defects and impurities which can strongly affect their optoelectronic properties, possibly in technologically interesting ways. In the present work, we carry out a thorough investigation of the clinochlore structure on both bulk and 2D exfoliated forms, discussing its optical features and the influence of the insertion of impurities on its macroscopic prop- erties. Several experimental techniques are employed, followed by theoretical first-principles calculations considering several types of naturally-ocurring transition metal impurities in the mineral lattice and their effect on electronic and optical properties. We demonstrate the existence of requirements concerning surface quality and insulating properties of clinochlore that are mandatory for its suitable application in nanoelectronic devices. The results presented in this work provide important informations for clinochlore potential applications and establish a basis for further works that intend to optimize its properties to relevant 2D technological applications through defect engineering.
Influence of the matrix on the red emission in europium self-activated orthoceramics.
(2015) Siqueira, Kisla Prislen Félix; Lima, Patrícia Pereira de; André, Maria Rute de Amorim e Sá Ferreira; Carlos, Luís António Ferreira Martins Dias; Bittar, Eduardo Matzenbacher; Matinaga, Franklin Massami; Paniago, Roberto Magalhães; Krambrock, Klaus Wilhelm Heinrich; Moreira, Roberto Luiz; Dias, Anderson
Different oxide host matrices of ABO4 with A3+ (Eu) and B5+ (Nb, Ta, and Sb) were prepared to investigate the solid state luminescence behavior of Eu3+ as a self-activated emitter in orthoceramics. Crystal structures, phonon modes, metal valence states, optical excitation, and emission luminescence properties including emission decay curves, colorimetry, and nonstoichiometry defects were studied using X-ray diffraction (XRD), Raman spectroscopy, photoluminescence (PL), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR). Classical intraconfigurational (f−f transitions), interconfigurational (4fn−15d → 4fn), and charge transfer bands (X5+−O2−) were observed, besides split Stark levels indicating low local symmetry. The influence of host matrices on the optical behavior of orthocompounds was noticed by changes in the energy of the typical Eu3+ transitions. In this case, the larger blue-shifted peaks were observed for EuSbO4 and the larger red-shifted positions for EuNbO4, with respect to emission spectra of orthocompounds studied here. These results were correlated to the respective fifth ionization energies and covalent fraction as well as to the crystal structures exhibited by the different host matrices (I2/a: Nb, Ta; P21/c: Sb). The presence of more than one component for the 5D0 → 7F0 transition in each compound was observed at low temperature, and it could be justified by nonstoichiometric defects like the oxygen vacancy and Eu2+. These typical defects are observed in nonstoichiometric oxide materials, and they were analyzed in the orthoceramics by EPR and XPS spectroscopy. Finally, the lifetime of the 5D0 state and chromaticity diagrams confirmed our orthoceramics as good optical emitters in the red-end region.
Metavivianite, Fe2+Fe3+2(PO4)2(OH)2 6H2O : new data and formula revision.
(2012) Chukanov, Nikita V.; Cipriano, Ricardo Augusto Scholz; Aksenov, Sergey M.; Rastsvetaeva, Ramiza K.; Pekov, Igor V.; Belakovskiy, Dmitriy I.; Krambrock, Klaus Wilhelm Heinrich; Paniago, Roberto Magalhães; Righi, Ariete; Martins, R. F.; Belotti, Ricardo Augusto; Bermanec, Vladimir
The composition, structure, X-ray powder diffraction pattern, optical properties, density, infrared, Raman and Mo¨ssbauer spectra, and thermal properties of a homogeneous sample of metavivianite from the Boa Vista pegmatite, near Galile´ia, Minas Gerais, Brazil are reported for the first time. Metavivianite is biaxial (+) with a = 1.600(3), b = 1.640(3), g = 1.685(3) and 2Vmeas = 85(5)º. The measured and calculated densities are Dmeas = 2.56(2) and Dcalc = 2.579 g cm 3. The chemical composition, based on electronmicroprobe analyses, Mo¨ssbauer spectroscopy (to determine the Fe2+:Fe3+ ratio) and gas chromatography (to determine H2O) is MgO 0.70, MnO 0.92, FeO 17.98, Fe2O3 26.60, P2O5 28.62, H2O 26.5; total 101.32 wt.%. The empirical formula is (Fe3+ 1.64Fe2+ 1.23Mg0.085Mn0.06)S3.015(PO4)1.98(OH)1.72·6.36H2O. Metavivianite is triclinic, P1¯ , a = 7.989(1), b = 9.321(2), c = 4.629(1) A ˚ , a = 97.34(1), b = 95.96(1), g = 108.59(2)º, V = 320.18(11) A ˚ 3 and Z = 1. The crystal structure was solved using a single-crystal techniques to an agreement index R = 6.0%. The dominant cations in the independent sites are Fe2+ and Fe3+, with multiplicities of 1 and 2, respectively. The simplified crystal-chemical formula for metavivianite is Fe2+(Fe3+,Fe2+)2(PO4)2(OH,H2O)2·6H2O; the endmember formula is Fe2+Fe3+ 2 (PO4)2(OH)2·6H2O, which is dimorphous with ferrostrunzite.
A new appraisal of sri lankan bb zircon as a reference material for LA-ICP-MS U-Pb geochronology and Lu-Hf isotope tracing.
(2017) Santos, Maristella Moreira; Lana, Cristiano de Carvalho; Buick, Ian S.; Schmitz, Mark D.; Kamo, Sandra L.; Gerdes, Axel; Corfu, Fernando; Tapster, Simon; Lancaster, Penelope; Storey, Craig Darryl; Basei, Miguel Ângelo Stipp; Tohver, Eric; Alkmim, Ana Ramalho; Nalini Júnior, Hermínio Arias; Krambrock, Klaus Wilhelm Heinrich; Leite, Cristiano Fantini; Wiedenbeck, Michael
A potential zircon reference material (BB zircon) for laser abla tion-in ductively coupled plasma-mass spectrometry (LA-ICP-M S)U-Pb geochronology and Hf isotope geochemistry is described. A batch of twenty zircon megacrysts (0.5–1.5 cm3) from SriLanka was studied. Within-grain rare earth element (REE) compositions are largely homogeneous, albeit with somevariation seen between fractured and homogeneous domains. Excluding fractured cathodoluminescence bright domains,the variation in U content for all analysed crystals ranged from 227 to 368 lgg-1and the average Th/U ratios werebetween 0.20 and 0.47. The Hf isotope composition (0.56–0.84 g/100 g Hf) is homogeneous within and between thegrains – mean176Hf/177Hf of 0.281674 ± 0.000018 (2s). The calculated alpha dose of 0.59 3 1018g-1for a numberof BB grains falls within the trend of previously studied, untreated zircon samples from Sri Lanka. Aliquots of the samecrystal (analysed by ID-TIMS in four different laboratories) gave consistent U-Pb ages with excellent measurementreproducibility (0.1–0.4% RSD). Interlaboratory assessment (by LA-ICP-MS) from individual crystals returned results that arewithin uncertainty equivalent to the TIMS ages. Finally, we report on within- and between-grain homogeneity of theoxygen isotope systematic of four BB crystals (13.16‰ VSMOW).
On the yellow color of gamma‐irradiated brazilianite from Minas Gerais (Brazil).
(2021) Pinheiro, Mauricio Veloso Brant; Cipriano, Ricardo Augusto Scholz; Karfunkel, Joachim; Chaves, Mario Luiz de Sá Carneiro; Krambrock, Klaus Wilhelm Heinrich
The origin of gamma irradiation-induced strong yellow color in brazilianite from Brazil is investigated. The irradiation- induced optical absorption band responsible for the color shows an onset in the blue spectral region at about 2.5 eV and maximum centered in the UV at ~ 4.24 eV. From the ratio between the squared value of the line width (W2 ) and the peak energy (M), a value of about 0.09 eV is estimated, which is consistent with an absorption band caused by a Schirmer ́s-type O− bound small polaron. By electron paramagnetic resonance (EPR), we are able to confrm its microscopic structure. This O− hole center is in fact the Al3+–O−X2+–P5+ hole center already identifed by EPR earlier in the literature (where X2+ stands for a nearby divalent cation with negligible abundance of magnetic isotopes). The EPR spectrum of the Al3+–O−X2+–P5+ hole center, along with the O− bound small polaron absorption band responsible for the yellow color, appears simultane- ously at high concentrations after gamma irradiation, and vanishes together for thermal annealing above 300 °C, returning after re-irradiation in a reversible way. Their appearance is concomitant with the H0 centers and Ti3+ electron centers, and possible charge-compensating centers.
Optical phonon features of triclinic montebrasite : dispersion analysis and non-polar Raman modes.
(2015) Almeida, Rafael Mendonça; Höfer, Sonja; Mayerhöfer, Thomas G.; Popp, Jürgen; Krambrock, Klaus Wilhelm Heinrich; Lobo, Ricardo P. S. M.; Dias, Anderson; Moreira, Roberto Luiz
Polarized infrared and Raman spectra of triclinic LiAl(PO4)(OH) [montebrasite] single crystal were recorded for appropriate optical configurations. Dispersion analysis was applied on the infrared reflectivity spectra taken at low incidence angle (11 ) to determine the oscillator parameters and the dipole directions of the polar phonons. In particular, all the 27 polar phonons, predicted by group theory for triclinic P1 structure,were determined. The obtained dielectric tensor parameters have been checked by comparison between predicted and measured infrared spectra at higher incidence angle (34 ). The azimuth and co-elevation angles obtained from the dispersion analysis showed that the response of several polar phonons is close to that of an orthorhombic system. Polarized Raman spectra obtained in several scattering geometries allowed us to obtain well-defined 24 non-polar modes, also in perfect agreement with group theory. The selection rule between Raman and infrared phonons was respected, confirming the centrosymmetric structure and ruling out any relevant influence of defects. The relatively narrow phonon bands are compatible with a highly ordered structure with fully occupied atomic sites.
Origin of the color in cobalt-doped quartz.
(2011) Pinto, Luiz Carlos Barbosa de Miranda; Righi, Ariete; Lameiras, Fernando Soares; Araújo, Fernando Gabriel da Silva; Krambrock, Klaus Wilhelm Heinrich
Synthetic Co-doped quartz was grown hydrothermally in steel autoclaves at the Technological Center of Minas Gerais (CETEC), Brazil. The quartz samples, originally yellow in the as-grown state acquired blue coloration after prolonged heat treatment times at 500°C near the alpha–beta transition temperature. UV–VIS–NIR absorption spectroscopy shows the characteristic spectra of Co3+ before heat treatment. After heat treatment, the optical absorption spectrum is dominated by two split-triplet bands the Wrst in the near infrared region centered at about 6,700 cm¡1 (1,490 nm) and the second in the visible spectral range at about 16,900 cm¡1 (590 nm). Both split-triplet bands are typical for Co2+ ions in tetrahedral coordination environments. From the absence of electron paramagnetic resonance (EPR) spectra, we conclude that the Co2+ found in the optical absorption spectra of the blue quartz is not due to an isolated structural site in the quartz lattice. Instead, the blue color is associated with electronic transitions of Co2+ in small inclusions in which the Co site has tetrahedral symmetry. The non-observation of polarizationdepend optical absorption spectra is also in agreement with this model. The results for Co2+ in quartz are diVerent from Co-bearing spinel and staurolite and other silicates like orthopyroxene, olivine, and beryls. The formation process of the color center is discussed.
Polarized Raman scattering and infrared spectroscopy of a naturalmanganocolumbite single crystal.
(2010) Moreira, Roberto Luiz; Rubinger, Carla Patricia Lacerda; Krambrock, Klaus Wilhelm Heinrich; Dias, Anderson
A well-ordered natural manganocolumbite single crystal of high quality was used as a prototype for the first determination of the polarized optical phonon modes of materials with the columbite structure. Electron microprobe and X-ray diffraction characterizations determined the chemical formula asMn0.60Fe0.40(Nb0.80Ta0.20)2O6, a cationic ordering of 81%, and the crystal structure as belonging to the Pbcn group. Polarized Raman and infrared-reflectivity spectroscopies on oriented samples allowed us to discern 50 of the 54 predicted gerade (Raman) modes and 31 of the 38 predicted ungerade (infrared) modes for the columbite structure. The selection rules were verified, and polarization leaks only due to slight sample misorientation, confirming the high purity, ordering and quality of the material. From the polar phonon spectra, intrinsic dielectric merit factors <εr> = 29.2 and of 64 THz were determined, showing adequate values for designing applications in microwave circuitry.
Radiation-induced defects in montebrasite : an electron paramagnetic resonance study of O – hole and Ti3+ electron centers.
(2020) Toledo, José Roberto de; Gonçalves, Raphaela de Oliveira; Dias, Lorena Nunes; Chaves, Mario Luiz de Sá Carneiro; Karfunkel, Joachim; Cipriano, Ricardo Augusto Scholz; Pinheiro, Mauricio Veloso Brant; Krambrock, Klaus Wilhelm Heinrich
Montebrasite is a lithium aluminum phosphate mineral with the chemical formula LiAlPO4(Fx,OH1–x) and considered a rare gemstone material when exhibiting good crystallinity. In general, montebrasite is colorless, sometimes pale yellow or pale blue. Many minerals that do not have colors contain hydroxyl ions in their crystal structures and can develop color centers after ionization or particle irradiation, examples of which are topaz, quartz, and tourmaline. The color centers in these minerals are often related to O– hole centers, where the color is produced by bound small polarons inducing absorption bands in the near UV to the visible spectral range. In this work, colorless montebrasite specimens from Minas Gerais state, Brazil, were investigated by electron paramagnetic resonance (EPR) for radiation-induced defects and color centers. Although γ irradiation (up to a total dose of 1 MGy) did not visibly modify color, a 10 MeV electron irradiation (80 MGy) induced a pale greenish-blue color. Using EPR, O– hole centers were identified in both γ- or electron-irradiated montebrasite samples showing superhyperfine interactions with two nearly equivalent 27Al nuclei. In addition, two different Ti3+ electron centers were also observed. From the γ irradiation dose dependency and thermal stability experiments, it is concluded that production of O– hole centers is limited by simultaneous creation of Ti3+ electron centers located between two equivalent hydroxyl groups. In contrast, the concentration of O– hole centers can be strongly increased by high-dose electron irradiation independent of the type of Ti3+ electron centers. From detailed analysis of the EPR angular rotation patterns, microscopic models for the O– hole and Ti3+ electron centers are presented, as well as their role in the formation of color centers discussed and compared to other minerals.
Spectroscopic characterization of transition metal impurities in natural montebrasite/amblygonite.
(2010) Dias, Lorena Nunes; Pinheiro, Mauricio Veloso Brant; Moreira, Roberto Luiz; Krambrock, Klaus Wilhelm Heinrich; Guedes, Kassílio José; Menezes Filho, Luiz Alberto Dias; Karfunkel, Joachim; Schnellrath, Jurgen; Cipriano, Ricardo Augusto Scholz
Natural single-crystal specimens of the montebrasite/amblygonite series from Brazil, with general formula LiAlPO4(F,OH), were investigated by electron microprobe, Raman spectroscopy, X-ray diffraction, and infrared absorption. Since little is known about impurities and their local symmetries, electron paramagnetic resonance (EPR) was applied. Six different paramagnetic impurities and radiation defects were detected by EPR. Three of them, all substituting for Al3+ ions, namely, iron (Fe3+), vanadium (V4+), and niobium (Nb4+) impurities were characterized in this work. The Fe3+ (3d5)-related EPR spectra and angular dependencies show occupation of low-symmetry sites that are revealed in the high asymmetry parameter of the electronic fine structure, E/D = 0.27. Vanadium and niobium impurities are identified through their typical strong hyperfine interactions. Both form interesting examples for which the properties of 3d1 ion (V4+) and 4d1 ion (Nb4+) in the same host matrix can be compared. It is shown that both ions form complex defects of type VO2+ (vanadyl) and NbO2+ (niobyl), showing superhyperfine interaction with two equivalent hydrogen ions and not to fluorine. The EPR rotation patterns are analyzed in detail for three mutually perpendicular crystal planes. Spin Hamiltonian parameters are calculated and discussed.
Synthesis of a highly active Nb2O5 for 1,2-cyclohexanediol production.
(2022) Bruziquesi, Carlos Giovani Oliveira; Gabriel Filho, José Balena; Victoria, Henrique Fernandes Vieira; Krambrock, Klaus Wilhelm Heinrich; Mansur, Herman Sander; Mansur, Alexandra Ancelmo Piscitelli; Chagas, Poliane; Silva, Adilson Cândido da; Oliveira, Luiz Carlos Alves de
A commercial niobium oxide was used to form a gelled material, which was lyophilized producing a novel polyoxoniobate catalyst (L-AmNbO) containing peroxo groups. The activity of this new catalyst was assessed through oxidation reactions of the cyclohexene. In the presence of L-AmNbO catalyst, the oxidation of cyclohexene with H2O2 reached 90% of conversion after 12 h at 75 ◦C, with selectivity for 1,2-cyclohexanediol (~ 50%). The catalytic results and the surface properties of this new material indicated that it can be a promising catalyst for niobium compounds applications in the selective oxidation of cyclohexene.
Understanding photocatalytic activity and mechanism of nickel-modified niobium mesoporous nanomaterials.
(2020) Silva, Iza F. B.; Martins, Amanda R.; Krambrock, Klaus Wilhelm Heinrich; Rosmaninho, Marcelo Gonçalves; Binatti, Ildefonso; Moura, Flávia Cristina Camilo
To better understand the photocatalysis oxidation mechanism of different types of coupled semiconductors, mesoporous niobium(V) oxides (n-type) with relatively high surface areas (140 m2 g−1 ) were synthesized by Evaporation Induced Self-Assemble method and modified with different amounts of NiO (p-type). The materials are polycrystalline nanostructures, with mesoporous of approximately 12 nm and bandgap values in the UVA range of the spectrum (344–356 nm). The materials showed remarkable photocatalytic activity and the addition of nickel oxide increased the reducibility of the Nb5+ species and improved the charge transfer in the materials. Studies with indigo carmine dye have shown that the niobium-based materials are able to decolorize a 40 ppm solution in 20 and 60 min (1.0% NiO material) in photo-Fenton like reaction, and photocatalysis reactions, respectively, without the strong adsorption of the molecule to the material surface. In addition, in the presence of a radical scavenger the photocatalytic reactions half-life time showed an increase of up to 11 times, which indicates that the indirect oxidation mechanism, with the formation of radicals, is predominant. The formation of hydroxyl radicals during irradiation was confirmed by EPR analysis with the spin trapping method.