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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.

Comportamento dos fônons e magnetismo gerado por vacâncias em nanocristais de CeO2− dopado com Mn

2026-09-04T17:33:16Z

Título: Comportamento dos fônons e magnetismo gerado por vacâncias em nanocristais de CeO2− dopado com Mn Autor(es): Assis, Maria Suêd Macêdo Abstract: We investigated the influence of doping CeO2 nanocrystals with Mn through structural, spectroscopic, and magnetic analyses. The materials were synthesized by the sol-gel method and characterized by X-ray diffraction, X-ray absorption spectroscopy (XANES), Raman spectroscopy, and magnetic measurements. The results of Rietveld refinement together with the Raman spectrum confirm the formation of a single phase of CeO2 with a fluorite-type cubic structure. These results show that Mn insertion modifies the lattice parameters, crystallite size, and microdeformation in a non-monotonic manner, revealing a balance between local disorder and lattice relaxation induced by phonons and vacancy formation. The XANES results reveal mixed valence of Mn ions as Mn3+ /Mn2+ , resulting in charge compensation through partial reduction of Ce4+ to Ce3+ . The ZFC/FC curves suggest an anisotropic distribution originating from Mn–vacancy and show blocked superparamagnetic behavior at low temperatures. The study identified an antiferromagnetic regime derived from Dzyaloshinskii-Moriya (DM) influenced by oxygen vacancies. These results present a framework that quantifies the defect chemistry and magnetic behavior in (Ce1−xMnxO2−, 0 ≤ x ≤ 0.09) nanocrystals, highlighting the role of oxygen vacancies and doping-induced structural disorder.

Avaliação de risco radiológico e radioepidemiológico em centros urbanos: simulação de um incidente nuclear hipotético em uma cidade litorânea

2026-09-03T20:03:05Z

Título: Avaliação de risco radiológico e radioepidemiológico em centros urbanos: simulação de um incidente nuclear hipotético em uma cidade litorânea Autor(es): Silva, Karla Pereira Carneiro da Abstract: uclear accidents release large quantities of radioactive materials into the atmosphere, exposing the population to radiation doses and contamination. Given the threat of accidental and/or intentional radiological events, it is essential to employ tools that aid in decisionmaking for risk analysis and the protection of the occupationally exposed population. This study presents an in-depth assessment of the effects of a hypothetical nuclear incident involving an LWR reactor in the city of Aracaju, accounting for the population and affected area. The analysis of the event is based on the Pasquill-Gifford classification of atmospheric stability (PG Classes) at the time, the prevailing wind direction, and census data on the individuals. Simulations were performed using the HotSpot Health Physics Code v3.1.2 software to determine the Total Equivalent Effective Dose (TEDE) and the Excess Relative Risk (ERR) associated with radiation-induced cancers, such as leukemia. The estimates considered low-dose radioepidemiological models, Biological Effects of Ionizing Radiation (BEIR) V and VII, and vulnerability criteria such as age and sex, analyzing groups of males, females, children, and the elderly in the affected areas. To collect population data, Geographic Information Systems (GIS) were used to delineate impacted zones and groups, and information from the National Institute of Meteorology (INMET) was used to assess wind patterns. The results indicated variations between exposure and risk, attributed to local atmospheric conditions and wind direction, predominantly from the East-Southeast (ESE) and Southeast (SE). The expansion of isodoses is explained by the capital’s flat and rugged terrain, combined with PG classes and wind direction. The study highlights the importance of geospatial models for demographic analysis, census data, and informed decisions about the exposed population.

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.