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Your search keyword '"de Sousa, F.F."' showing total 30 results
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30 results on '"de Sousa, F.F."'

27. Low-temperature phase transformation studies in the stearic acid: C form.

Author

de Sousa, F.F., Freire, P.T.C., de Menezes, A.S., Pinheiro, G.S., Cardoso, L.P., Jr.Alcantara, P., Moreira, S.G.C., Melo, F.E.A., Mendes Filho, J., and Saraiva, G.D.

Subjects
*LOW temperatures, *PHASE transitions, *STEARIC acid, *X-ray diffraction, *DIFFERENTIAL scanning calorimetry, *RAMAN spectra
Abstract

This paper reports the temperature-dependent measurements in the C form of stearic acid. Raman scattering, X-ray diffraction, and differential scanning calorimetry measurements were performed at low temperatures. The polarized Raman spectra were measured for temperatures ranging from 8 to 300 K over the spectral range of 30–3000 cm −1 . The spectral changes observed in both the lattice vibrational modes and the internal vibrational modes regions of the Raman spectrum, allowed to identify a phase transition undergone by the stearic acid crystal occurring between 210 and 170 K and a change in the structure continues to be observed down to 8 K. The anharmonicity of some vibrational modes and the possible space groups presented by the crystal at low temperatures were also discussed. Low-temperature X-ray diffraction measurements were performed from 290 to 80 K and the results showed slight changes in the lattice parameters at ∼200 K. Furthermore, the evidence of the phase transformation was provided by the differential scanning calorimetry measurements, which identified an enthalpic anomaly at about 160 K. [ABSTRACT FROM AUTHOR]

Published
2015
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29. Temperature-dependent vibrational spectroscopic study and DFT calculations of the sorbic acid.

Author

Saraiva, G.D., Nogueira, C.E.S., Freire, P.T.C., de Sousa, F.F., da Silva, J.H., Teixeira, A.M.R., and Mendes Filho, J.

Subjects
*SORBIC acid, *TEMPERATURE measurements, *POLYCRYSTALS, *VIBRATIONAL spectra, *DENSITY functional theory
Abstract

This work reports a temperature-dependent vibrational spectroscopic study of the sorbic acid (C 6 H 8 O 2 ), as well as the mode assignment at ambient conditions, based on the density functional theory. Temperature-dependent vibrational properties have been performed in polycrystalline sorbic acid through both Raman and infrared spectroscopy in the 20–300 K and 80–300 K temperature ranges, respectively. These studies present the occurrence of some modifications in the Raman spectra that could be interpreted as a low temperature phase transition undergone by sorbic acid from the monoclinic phase to an unknown phase with conformational change of the molecules in the unit cell. [ABSTRACT FROM AUTHOR]

Published
2015
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30. Temperature dependence Raman spectroscopy and DFT calculations of Bi2(MoO4)3.

Author

Saraiva, G.D., da Silva Filho, J.G., Saraiva-Souza, A., de Castro, A.J. Ramiro, Teixeira, A.M.R., Luz-Lima, C., Oliveira, F.G.S., Neto, V.O. Sousa, Freire, P.T.C., and de Sousa, F.F.

Subjects
*RAMAN spectroscopy, *SEMICONDUCTOR materials, *CHEMICAL properties, *CONDUCTION bands, *ENERGY bands
Abstract

This work reports a theoretical and experimental study on the electronic and vibrational properties of Bi 2 (MoO 4) 3. First-principle calculations were applied to increase the understanding on the properties of the chemical composition through the energy bands. The conduction band minimum (CBM) is found at the high symmetric Γ-point, while the valence-band maximum (VBM) is located between the Z and the Γ-points. Therefore, these facts confirm that the Bi 2 (MoO 4) 3 crystal is a semiconductor compound with an indirect band-gap of about 2.1 eV. Moreover, lattice dynamic properties were calculated using density functional perturbation theory (DFPT) in order to assign the experimental Raman bands. In addition, we performed temperature-dependent Raman spectroscopic studies in the Bi 2 (MoO 4) 3 crystals to obtain information on structural changes induced by effects of the temperature change. From the changes observed in the Raman spectra phase transitions at ∼ 668 and 833 K were inferred, with the last one possibly related to the disorder due to the heating process. Unlabelled Image • Electronic properties of the Bi 2 (MoO 4) 3 semiconductor material • Lattice dynamic calculations based on the density-functional theory (DFT). • Effect of the temperature on the vibrational properties of the Bi 2 (MoO 4) 3. • Temperature-induced phase transformation in the Bi 2 (MoO 4) 3. [ABSTRACT FROM AUTHOR]

Published
2020
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