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15. Temperature- and Pressure-Dependent Photoluminescence Emission of Bulk GaSe0.5Te0.5Alloy

Author

Leite, F. F., N. Silva, F. W., Oliveira, Victor V., Guerra, Yuset, Paraguassu, W., Souza Filho, Antonio G., Viana, Bartolomeu C., and Alencar, Rafael S.

Abstract

In this study, we conduct a comprehensive investigation into the temperature and pressure dependencies of photoluminescence (PL) in a bulk GaSe0.5Te0.5alloy. By using density functional theory (DFT) calculations and experimental measurements, we identify and distinguish the contributions of free excitons and indirect transitions to the PL spectrum. Our analysis reveals a nonlinear redshift for these transitions over the temperature range of 90–667 K, evolving in accord with the modified Varshni equation. We observe a pronounced influence of electron–phonon coupling in the GaSe0.5Te0.5alloy compared to that of GaTe and GaSe crystal structures. Below 180 K, we detect the emergence of new broad bands associated with bound excitons and radiative recombination of trap states. Furthermore, by employing the Arrhenius plots, we determine activation energies for nonradiative recombination of the indirect and free exciton transitions. Concerning the pressure dependence of the PL spectra, the free exciton and indirect transitions undergo a linear redshift within the specific pressure range of 0.3 to 4.3 GPa, accompanied by a continuous reduction in PL intensity, leading to complete quenching at 4.8 GPa. This phenomenon is attributed to a direct-to-indirect band gap crossover. Pressure-dependent band structure calculation via DFT supports this assumption and shows a further metallization of the GaSe0.5Te0.5alloy at ∼8.0 GPa. This study sheds new light on understanding the optical properties of the GaSe0.5Te0.5alloy under extreme pressure and temperature conditions, thereby opening avenues for tailored applications and guiding future research efforts in this field.

Published
2024
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33. Structural and optical properties of rare earth-doped (Ba0.77Ca0.23)1-x(Sm, Nd, Pr, Yb)xTiO3.

Author

Moraes, A. P. A., Filho, A. G. Souza, Freire, P. T. C., Filho, J. Mendes, M'Peko, J. C., Hernandes, A. C., Antonelli, E., Blair, Michael W., Muenchausen, Ross E., Jacobsohn, Luiz G., and Paraguassu, W.

Subjects
CRYSTAL grain boundaries, CERAMICS, RARE earth metals, DIELECTRICS research, SOLID state physics
Abstract

The structural, dielectric, and vibrational properties of pure and rare earth (RE)-doped Ba0.77Ca0.23TiO3 (BCT23; RE = Nd, Sm, Pr, Yb) ceramics obtained via solid-state reaction were investigated. The pure and RE-doped BCT23 ceramics sintered at 1450 °C in air for 4 h showed a dense microstructure in all ceramics. The use of RE ions as dopants introduced lattice-parameter changes that manifested in the reduction of the volume of the unit cell. RE-doped BCT23 samples exhibit a more homogenous microstructure due to the absence of a Ti-rich phase in the grain boundaries as demonstrated by scanning electron microscopy imaging. The incorporation of REs led to perturbations of the local symmetry of TiO6 octahedra and the creation of a new Raman mode. The results of Raman scattering measurements indicated that the Curie temperature of the ferroelectric phase transition depends on the RE ion and ion content, with the Curie temperature shifting toward lower values as the RE content increases, with the exception of Yb3+ doping, which did not affect the ferroelectric phase transition temperature. The phase transition behavior is explained using the standard soft mode model. Electronic paramagnetic resonance measurements showed the existence of Ti vacancies in the structure of RE-doped BCT23. Defects are created via charge compensation mechanisms due to the incorporation of elements with a different valence state relative to the ions of the pure BCT23 host. It is concluded that the Ti vacancies are responsible for the activation of the Raman mode at 840 cm-1, which is in agreement with lattice dynamics calculations. [ABSTRACT FROM AUTHOR]

Published
2011
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37. Pressure-induced structural transformations in the molybdate Sc-2(MoO4)(3)

Author

Paraguassu, W., Maczka, M., Filho, A. G. Sonza, Freire, P. T. L., Filho, J. M., Melo, F. E. A., Macalik, L., Gerward, Leif, Olsen, J. Staun, Waskowska, A., and Hanuza, J.

Subjects
NEGATIVE THERMAL-EXPANSION, CRYSTALS, ZRW2O8, INDUCED AMORPHIZATION, PHASE-TRANSITIONS, TUNGSTATE, CONDUCTIVITY, BEHAVIOR, SC-2(WO4)(3)
Abstract

High pressure Raman scattering and x-ray diffraction studies of the molybdate Sc-2(MoO4)(3) are presented. A sequence of changing symmetry effects is observed through two structural phase transitions ending up with an amorphous state. The observed two structural phase transformations are reversible. The crystal to amorphous transition is irreversible. Our results point out that the amorphization process in Sc-2(MoO4)(3) may be due to a kinetic hindrance of a phase transition rather than due to chemical decomposition effects as have been proposed to occur for some molybdates and tungstates.

Published
2004
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40. Pressure-induced structural transformations in In2-xYx(MoO4)3 systems.

Author

Mendonça, R., Paraguassu, W., Mendes Filho, J., Marinkovic, B. A., Souza Filho, A. G., Maczka, M., and Freire, P. T. C.

Subjects
*PRESSURE-induced transformations, *MOLYBDENUM oxides, *CRYSTAL structure, *PHASE transitions, *AMORPHIZATION, *OCTAHEDRAL molecules
Abstract

In this work, we report results of high-pressure Raman experiments (P < 8 GPa) on In2-xYxMo3O12 for x = 0.0 and 0.5. A crystalline to crystalline structural phase transition and pressure-induced amorphization (PIA) have been identified. The structural phase transition takes place at 1.5 and 1.0 GPa for In2(MoO4)3 and In1.5Y0.5(MoO4)3, respectively, resulting in the change of structure from monoclinic P21/a to a more denser structure. The PIA started at 5 and 3.4 GPa for In2Mo3O12 and In1.5Y0.5Mo3O12, respectively. The amorphization process takes place in two stages in the case of In1.5Y0.5Mo3O12 phase, while for In2Mo3O12, it is not complete until the pressure is as high as 7 GPa. Our results also suggest that with increase of ionic size of the A3+ ions, the octahedral distortion increases and consequently larger local structural disorder is introduced in the A2(MoO4)3 system, where A is a trivalent ion (In, Y3+, Sc3+, Fe3+, etc.). Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Published
2016
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41. Pressure-induced structural transformations in the molybdate Sc2 (MoO4)3

Author

Paraguassu, W., Maczka, M., Souza Filho, A.G., Freire, P.T.C., Mendes Filho, J., Melo, F.E.A., Macalik, L., Gerward, L., Olsen, Janus Staun, Waskowska, A., Hanuza, J., Paraguassu, W., Maczka, M., Souza Filho, A.G., Freire, P.T.C., Mendes Filho, J., Melo, F.E.A., Macalik, L., Gerward, L., Olsen, Janus Staun, Waskowska, A., and Hanuza, J.

Abstract

Udgivelsesdato: 16 March

Published
2004

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