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Your search keyword '"Pinheiro, M. V. B."' showing total 28 results
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28 results on '"Pinheiro, M. V. B."'

21. Spectroscopic characterization of transition metal impurities in natural montebrasite/amblygonite.

Author

DIAS, L. N., PINHEIRO, M. V. B., MOREIRA, R. L., KRAMBROCK, K., GUEDES, K. J., FILHO, L. A. D. MENEZES, KARFUNKEL, J., SCHNELLRATH, J., and SCHOLZ, R.

Subjects
*CRYSTALS, *TRANSITION metals, *RAMAN spectroscopy, *ELECTRON paramagnetic resonance, *VANADIUM, *NIOBIUM
Abstract

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 Al 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. [ABSTRACT FROM AUTHOR]

Published
2011
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25. Correlation of irradiation-induced yellow color with the O - hole center in tourmaline

Author

Krambrock, K., Pinheiro, M. V. B., Guedes, K.J., Medeiros, S.M., Schweizer, S., and Spaeth, J.-M.

Abstract

Tourmaline with the general formula XY 3 Z 6(BO 3) 3Si 6O 18(OH,O) 3(OH,F) and the trigonal space group R3 m ( C 3 v 5) is known as a gemstone with great variety of colors. Some color centers are related to transition metal ions, and others to electron or hole traps. In this paper we report on a combined study using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and the optical detection of EPR (ODEPR) on a yellow color center produced by ?-irradiation in colorless Li-bearing elbaite tourmaline from Brazil. The color center is an O - hole trap center, which is stabilized within the plane spanned by three Y sites, and is located in the structural channels formed by Si 6O 18. We suggest that two of the Y sites are substituted by 27Al and the other by 6,7Li. During the irradiation process atomic hydrogen H 0 is also produced, which shows the same thermal stability as the hole center (250 °C). Therefore, we assign H 0 to be the local charge compensator for the hole trap. From the ODEPR measurements we conclude that the yellow color is caused by the O - hole center. The large negative isotropic Al superhyperfine interaction of the O - hole trap center is consistent with a calculation of the transferred hyperfine interactions by exchange polarization supporting the proposed defect model of an O - at the O 1 sites, whereby the O - is relaxed into the plane formed by three Y ions.

Published
2004
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27. New material for low-dose brachytherapy seeds: Xe-doped amorphous carbon films with post-growth neutron activated 125I.

Author

Gonçalves RG, Pinheiro MV, Lacerda RG, Ferlauto AS, Ladeira LO, Krambrock K, Leal AS, Viana GA, and Marques FC

Subjects
Humans, Iodine Radioisotopes pharmacology, Spectrometry, Gamma, Brachytherapy methods, Carbon chemistry, Iodine Radioisotopes chemistry, Xenon Isotopes chemistry
Abstract

We report a novel material for use in (125)I brachytherapy that consists of amorphous carbon films grown by ion-beam-assisted deposition and doped with Xe (5 at%) by implantation. Samples of these films grown on Si substrates were irradiated with neutrons in a TRIGA-I nuclear reactor for the production (125)Xe, and latter characterized by gamma spectroscopy. The results indicate that the (124)Xe was efficiently converted into (125)Xe, the precursor of (125)I, and support the activity calculations for a model brachytherapy seed., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Published
2011
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28. Identification of lamivudine conformers by Raman scattering measurements and quantum chemical calculations.

Author

Pereira BG, Vianna-Soares CD, Righi A, Pinheiro MV, Flores MZ, Bezerra EM, Freire VN, Lemos V, Caetano EW, and Cavada BS

Subjects
HIV Infections drug therapy, HIV Infections enzymology, HIV Infections virology, HIV Reverse Transcriptase chemistry, HIV Reverse Transcriptase therapeutic use, HIV-1 drug effects, HIV-1 enzymology, HIV-1 genetics, Humans, Lamivudine pharmacology, Models, Molecular, Powders, Reverse Transcriptase Inhibitors pharmacology, Lamivudine chemistry, Quantum Theory, Reverse Transcriptase Inhibitors chemistry, Spectrum Analysis, Raman methods
Abstract

Characterization of nucleoside and non-nucleoside human immunodeficiency virus (HIV) reverse transcriptase inhibitors conformers, NRTIs and NNRTIs, respectively, is fundamental for an improved treatment of infected individuals. Three conformers in lamivudine I powder are quickly identified in this work by assignment of some Raman peaks to their vibrational frequencies, as obtained by first principles quantum chemical calculations. The method is proposed as a practical procedure for non-destructive identification, analysis, and process monitoring of NRTIs and NNRTIs conformers.

Published
2007
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