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Bomio M.R.D.,Federal University of Rio Grande do Norte | Cavalcante L.S.,UESPI CCN | Cavalcante L.S.,Sao Paulo State University | Almeida M.A.P.,Federal University of Sao Carlos | And 6 more authors.
Polyhedron | Year: 2013

Lead molybdate (PbMoO4) crystals were synthesized by the co-precipitation method at room temperature and then processed in a conventional hydrothermal (CH) system at low temperature (70 °C for different times). These crystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, micro-Raman (MR) and Fourier transformed infrared (FT-IR) spectroscopies. Field emission scanning electron microscopy images were employed to observe the shape and monitor the crystal growth process. The optical properties were investigated by ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) measurements. XRD patterns and MR spectra indicate that these crystals have a scheelite-type tetragonal structure. Rietveld refinement data possibilities the evaluation of distortions in the tetrahedral [MoO 4] clusters. MR and FT-IR spectra exhibited a high mode ν1(Ag) ascribed to symmetric stretching vibrations as well as a large absorption band with two modes ν3(Eu and Au) related to anti-symmetric stretching vibrations in [MoO 4] clusters. Growth mechanisms were proposed to explain the stages involved for the formation of octahedron-like PbMoO4 crystals. UV-Vis absorption spectra indicate a reduction in optical band gap with an increase in the CH processing time. PL properties of PbMoO4 crystals have been elucidated using a model based on distortions of tetrahedral [MoO4] clusters due to medium-range intrinsic defects and intermediary energy levels (deep and shallow holes) within the band gap. © 2012 Elsevier Ltd. All rights reserved. Source


Cavalcante L.S.,Sao Paulo State University | Moraes E.,Federal University of Sao Carlos | Almeida M.A.P.,Federal University of Sao Carlos | Dalmaschio C.J.,Federal University of Sao Carlos | And 6 more authors.
Polyhedron | Year: 2013

In this paper, a combined theoretical and experimental study on the electronic structure and photoluminescence (PL) properties of beta zinc molybdate (β-ZnMoO4) microcrystals synthesized by the hydrothermal method has been employed. These crystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, Fourier transform Raman (FT-Raman) and Fourier transform infrared (FT-IR) spectroscopies. Their optical properties were investigated by ultraviolet-visible (UV-Vis) absorption spectroscopy and PL measurements. First-principles quantum mechanical calculations based on the density functional theory at the B3LYP level have been carried out. XRD patterns, Rietveld refinement, FT-Raman and FT-IR spectra showed that these crystals have a wolframite-type monoclinic structure. The Raman and IR frequencies experimental results are in reasonable agreement with theoretically calculated results. UV-Vis absorption measurements shows an optical band gap value of 3.17 eV, while the calculated band structure has a value of 3.22 eV. The density of states indicate that the main orbitals involved in the electronic structure of β-ZnMoO4 crystals are (O 2p-valence band and Mo 4d-conduction band). Finally, PL properties of β-ZnMoO4 crystals are explained by means of distortions effects in octahedral [ZnO6] and [MoO6] clusters and inhomogeneous electronic distribution into the lattice with the electron density map. © 2013 Elsevier Ltd. All rights reserved. Source


Cavalcante L.S.,Sao Paulo State University | Almeida M.A.P.,Federal University of Sao Carlos | Avansi W.,Sao Paulo State University | Tranquilin R.L.,Federal University of Sao Carlos | And 5 more authors.
Inorganic Chemistry | Year: 2012

In this paper, we report our initial research to obtain hexagonal rod-like elongated silver tungstate (α-Ag 2WO 4) microcrystals by different methods [sonochemistry (SC), coprecipitation (CP), and conventional hydrothermal (CH)] and to study their cluster coordination and optical properties. These microcrystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinements, Fourier transform infrared (FT-IR), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The shape and average size of these α-Ag 2WO 4 microcrystals were observed by field-emission scanning electron microscopy (FE-SEM). The optical properties of these microcrystals were investigated by ultraviolet-visible (UV-vis) spectroscopy and photoluminescence (PL) measurements. XRD patterns and Rietveld refinement data confirmed that α-Ag 2WO 4 microcrystals have an orthorhombic structure. FT-IR spectra exhibited four IR-active modes in a range from 250 to 1000 cm -1. XANES spectra at the W L 3-edge showed distorted octahedral [WO 6] clusters in the lattice, while EXAFS analyses confirmed that W atoms are coordinated by six O atoms. FE-SEM images suggest that the α-Ag 2WO 4 microcrystals grow by aggregation and the Ostwald ripening process. PL properties of α-Ag 2WO 4 microcrystals decrease with an increase in the optical band-gap values (3.19-3.23 eV). Finally, we observed that large hexagonal rod-like α-Ag 2WO 4 microcrystals prepared by the SC method exhibited a major PL emission intensity relative to α-Ag 2WO 4 microcrystals prepared by the CP and CH methods. © 2012 American Chemical Society. Source


Cavalcante L.S.,UESPI CCN | Cavalcante L.S.,Sao Paulo State University | Batista N.C.,UESPI CCN | Badapanda T.,National Institute of Technology Rourkela | And 7 more authors.
Materials Science in Semiconductor Processing | Year: 2013

Ba(Zr0.75Ti0.25)O3 (BZT-75/25) powders were synthesized by the polymeric precursor method. Samples were structurally characterized by X-ray diffraction (XRD), Rietveld refinement, X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. Their electronic structures were evaluated by first-principle quantum mechanical calculations based on density functional theory at the B3LYP level. Their optical properties were investigated by ultraviolet-visible (UV-Vis) spectroscopy and photoluminescence (PL) measurements at room temperature. XRD patterns and Rietveld refinement data indicate that the samples have a cubic structure. XANES spectra confirm the presence of pyramidal [TiO5] clusters and octahedral [TiO6] clusters in the disordered BZT-75/25 powders. EXAFS spectra indicate distortion of Ti-O and Ti-O-Ti bonds the first and second coordination shells, respectively. UV-Vis absorption spectra confirm the presence of different optical bandgap values and the band structure indicates an indirect bandgap for this material. The density of states demonstrates that intermediate energy levels occur between the valence band (VB) and the conduction band (CB). These electronic levels are due to the predominance of 4d orbitals of Zr atoms in relation to 3d orbitals of Ti atoms in the CB, while the VB is dominated by 2p orbitals related to O atoms. There was good correlation between the experimental and theoretical optical bandgap values. When excited at 482 nm at room temperature, BZT-75/25 powder treated at 500 C for 2 h exhibited broad and intense PL emission with a maximum at 578 nm in the yellow region. © 2013 Elsevier Ltd. All rights reserved. Source

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