DSpace Coleção:

Crescimento e caracterização de monocristais das famílias RCuGa3 (R = Ce, Pr, Nd, Gd) e RNiIn4 (R = La, Ce, Pr, Nd) por meio do método de fluxo metálico: estudos estruturais, magnéticos e do campo elétrico cristalino

2026-09-04T18:39:29Z

Título: Crescimento e caracterização de monocristais das famílias RCuGa3 (R = Ce, Pr, Nd, Gd) e RNiIn4 (R = La, Ce, Pr, Nd) por meio do método de fluxo metálico: estudos estruturais, magnéticos e do campo elétrico cristalino Autor(es): Possidonio, Daykson Neves Abstract: This work presents a systematic experimental study on the synthesis, structural characterization, and investigation of the magnetic properties of single crystals belonging to two families of rare-earth-based intermetallic compounds, RCuGa3 (R = Ce, Pr, Nd, Gd) and RNiIn4 (R = La, Ce, Pr, Nd). The single crystals were grown using the high-flux method, employing gallium (Ga) and indium (In) as solvents for the RCuGa3 and RNiIn4 series, respectively, enabling the growth of samples with high crystalline quality. Structural characterization was performed by X-ray diffraction (XRD), with crystallographic parameters refined using the Rietveld method implemented in the DBWS software, revealing that the RCuGa3 compounds crystallize in a tetragonal structure with space group I4/mmm, whereas the RNiIn4 compounds adopt an orthorhombic YNiAl4-type structure belonging to the Cmcm space group. Complementary scanning electron microscopy (SEM) analyses, particularly for PrNiIn4, confirmed the chemical homogeneity and high structural quality of the crystals, in good agreement with the XRD results. The magnetic properties were investigated through temperature-dependent magnetic susceptibility measurements for both families, and through specific heat (Cp) measurements for the RNiIn4 series, revealing antiferromagnetic behavior at low temperatures, with magnetic transitions characterized by N´eel temperatures (TN ) ranging from 2.2 K to 9.3 K, namely TN ≈ 3 K (Ce), 4.4 K (Pr), 2.2 K (Nd), and 9.3 K (Gd) for the RCuGa3 series, and 2.8 K < TN < 4.2 K for the RNiIn4 series. The analysis of the inverse magnetic susceptibility in the paramagnetic regime, based on the Curie-Weiss model, allowed the determination of the Curie-Weiss temperature (θCW), the experimental effective magnetic moment (µexp), and the magnetic frustration factor (f), revealing a pronounced magnetic anisotropy in the RNiIn4 series, expressed by ratios χeasy/χhard > 1, which is attributed to the significant influence of crystalline electric field (CEF) effects. Similar CEF-related effects were also identified in the RCuGa3 series, particularly for the Ce- and Pr-based compounds, influencing both the values of TN and the deviations observed from the scaling behavior predicted by the De Gennes rule. The specific heat measurements corroborated the magnetically determined transition temperatures and further reinforced the central role of CEF effects in defining the magnetic ground state of these systems, providing, overall, a comprehensive comparative analysis of the correlations between crystal structure, electronic interactions, and magnetic properties in rare-earth intermetallic compounds.

Modelo de regressão simbólica informado por física para a temperatura de transição vítrea em vidros alcalino borato

2026-09-04T17:55:35Z

Título: Modelo de regressão simbólica informado por física para a temperatura de transição vítrea em vidros alcalino borato Autor(es): Vitoria, Leonardo dos Santos Abstract: In this thesis, a new analytical model for the glass transition temperature of borate glasses of the form xM2O–(100 − x)B2O3, with M = Li, Na, and K, is proposed. The model was induced through a physics-informed symbolic regression framework. The physical descriptors employed in the induction process were based on the elastic deformation model originally proposed by Makishima and Mackenzie, in which the Young’s modulus of multicomponent glasses is expressed in terms of the average bond dissociation energy of the glass network and the manner in which atoms are structurally arranged, represented by the packing density factor. In addition, a contemporary revision of the original model was considered, introducing a new formulation of the packing factor based on rigid unit packing fraction called RUPF. The validation of the induced model was carried out through three complementary stages designed to ensure both physical and statistical rigor: (i) evaluation of the extrapolation capability to systems chemically distinct from those used in training, specifically glasses containing Rb and Cs; (ii) estimation of bond dissociation energies via error minimization with respect to experimental glass transition temperature data, followed by comparison with reference values from the literature in order to assess physical plausibility; and (iii) knowledge transfer to the prediction of the Young’s modulus of Rband Cs-containing glasses using the model proposed by Shi et al. The results from these three validation stages indicate that the induced model is physically consistent, as the estimated bond dissociation energies fall within physically reasonable ranges, while also exhibiting good statistical accuracy, with mean deviations on the order of 17 K relative to experimental Tg values between 300 and 800K. The limitations of the model were further investigated along two fronts: (i) uncertainty analysis through Monte Carlo uncertainty propagation and (ii) extrapolation to systems containing divalent modifiers, specifically Sr and Ba. A region of increased uncertainty was observed in the compositional range between 20% and 40% alkali addition, which was attributed to the larger uncertainties associated with the dissociation energies of BO4 structural units characteristic of this range. Extrapolation to alkaline-earth modifiers revealed a clear limitation of the model, which systematically underestimated the glass transition temperature by more than 100 K for both systems. Overall, this thesis demonstrates a methodological framework capable of inducing data-driven, physics-informed analytical models that exhibit physical consistency by extrapolating beyond the physical-chemical domain used in training and by transferring knowledge to simillar property models. This approach establishes a promising pathway for the development of interpretable models capable of generating new physical insight from experimental data.

Polimorfismo cristalino e estrutura eletrônica de ortofosfatos sob alta pressão e alta temperatura: um estudo de primeiros princípios

2026-07-09T17:50:32Z

Título: Polimorfismo cristalino e estrutura eletrônica de ortofosfatos sob alta pressão e alta temperatura: um estudo de primeiros princípios Autor(es): Pirôpo, Gilvan de Jesus Abstract: This thesis addresses two research topics involving a class of materials known as orthophosphates, which are of significant scientific and technological interest: (1) the investigation of the structural and electronic properties of APO4 compounds (A = Sc, Lu, Y) under hydrostatic pressure up to 100 GPa, and (2) the study of the structural, electronic, and optical properties of LiBaPO4 across its four experimentally observed crystalline phases up to a temperature of 1373 K. All investigations were carried out using ab initio calculations based on density functional theory. In the first topic, the results confirm the sequence of pressure-induced phase transitions observed experimentally: zircon > scheelite for ScPO4 (with a scheelite > monazite transition predicted at 94 GPa), zircon > monazite > scheelite for YPO4, and zircon > scheelite for LuPO4. The calculated critical pressures, pressure-induced volume collapses, and bulk moduli for each crystalline phase are in good agreement with available experimental data. It was found that, in the zircon phase, two distinct sets of A–O bonds within the AO8 dodecahedron exhibit significantly different compressibilities. This behavior was explained and correlated with an unusual increase in the AO8 polyhedral volume at the critical pressures triggering the zircon > scheelite phase transition. In the scheelite phase, both sets of A–O bonds display approximately equal compressibilities. Despite undergoing pressure-induced structural phase transitions, all compounds exhibit remarkable electronic stability, with consistent band characteristics dominated by O 2p states in the valence band and A d states in the conduction band. The band gap increases with pressure within each crystalline phase but decreases abruptly at the critical pressures due to the expansion of the more compressible set of A–O bonds. These findings highlight the exceptional mechanical resistance to compression and electronic stability of orthophosphates, supporting their potential for applications under extreme conditions. In the second topic, four crystalline phases of LiBaPO4 proposed experimentally were investigated: Cc, P31c, P63, and Pmcn. An improved description of the electronic structure of the low-temperature Cc and P31c phases revealed wide and nearly identical band gaps (≈ 7.6 eV) for both phases, in contrast to previous reports. This result is consistent with the similar Ba–O bond lengths, which play a key role in determining the band-gap width. For the high-temperature phases, the atomic positions of the P63 and Pmcn structures were determined. However, due to convergence issues and structural inconsistencies associated with the Pmcn phase, it is proposed that the correct space group is Pnma, which yields better agreement with experimental lattice parameters. Analysis of bond lengths, coordination numbers, and band-gap values supports a sequence of displacive phase transitions among the Cc, P31c, and P63 phases, as these structures share similar local atomic environments and electronic structures. In contrast, the transition from P63 to Pnma is characterized as reconstructive, evidenced by a change in Li coordination, significant modifications in Ba–O bond lengths, and a slight reduction in the band gap. In addition, optical absorption and reflectivity spectra were calculated for all four phases, providing theoretical guidance for future experimental investigations.

Caracterização estelar e abundâncias químicas de anãs M hospedeiras de exoplanetas a partir de espectros APOGEE

2026-07-09T17:50:00Z

Título: Caracterização estelar e abundâncias químicas de anãs M hospedeiras de exoplanetas a partir de espectros APOGEE Autor(es): Melo, Édypo Ribeiro de Abstract: The characterization of exoplanetary atmospheres, one of the objectives of the JWST, depends on precise knowledge of their host stars, especially M dwarfs, whose chemical compositions are difficult to determine. This thesis addresses this need through a detailed, homogeneous spectroscopic analysis of four planet-hosting M dwarfs that are high-priority targets for JWST: TOI-1685, GJ 436, TOI-2445, and GJ 3470. From high-resolution near-infrared (H-band) spectra from the APOGEE survey, atmospheric parameters and chemical abundances for twelve elements were derived using spectral synthesis with MARCS models and the Turbospectrum code. The analysis focused on rock-forming elements and the C/O ratio, which is crucial for planetary formation. The new stellar parameters enabled refinement of the associated exoplanet parameters. As a significant contribution, the results of this work were used to enable the prediction of the mass (3.276+0.448 −0.419 M⊕) and internal structure of TOI-2445 b using stellar chemistry as a reference. The results indicate a dense, rocky world with a core mass fraction similar to Earth’s. The derived stellar C/O ratios (ranging from 0.526 to 0.638) provide the primordial chemical context for the interpretation of JWST data. Placing these results within a broader population context enabled the investigation of potential chemical trends in M dwarfs. A comparison between stars with and without planets revealed no statistically significant differences in the C/O ratio, indicating that any effects associated with planetary formation remain inconclusive within this sample. Conversely, a robust difference was identified in the slope of the C/O versus [Fe/H] relationship between M and FGK dwarfs with exoplanets, suggesting that the local population of M dwarfs has a distinct chemical history, possibly associated with diverse Galactic origins. Finally, this Thesis presents an uncertainty Atlas for determining chemical abundances in M dwarfs with effective temperatures between 3200 and 4000 K, constructed from synthetic spectra in the 1.51–1.69 µm range. The Atlas systematically quantifies the impact of variations in atmospheric parameters, signal-to-noise ratio, and pseudo-continuum definition, providing a valuable tool for realistic uncertainty estimation in future H-band spectroscopic analyses.