https://tedebc.ufma.br/jspui/handle/tede/1267 2026-07-21T08:10:15Z https://tedebc.ufma.br/jspui/handle/tede/7124 Título: PROPRIEDADES MAGNÉTICAS E EFEITO MAGNETOCALÓRICO DOS SISTEMAS RMAl3 (R = La, Ce, Pr E Nd; M = Ag E Cu) E R2AgCuAl6 (R = Pr E Nd) Autor: LOPES, Maurício Silva Primeiro orientador: SILVA, Luzeli Moreira da Abstract: ABSTRACT: This study investigated the structural, magnetic, and magnetocaloric properties of intermetallic compounds in the RMAl3 (R = La, Ce, Pr, and Nd; M = Ag and Cu) and R2CuAgAl6 (R = Pr and Nd) systems. The samples were synthesized in an arc furnace under an argon atmosphere, subjected to heat treatment for 7 days at 700 °C, and structurally characterized via X-ray diffraction (XRD) and Rietveld refinement. Specific heat (Cp) measurements as a function of temperature and magnetic field revealed that CeAgAl3, PrAgAl3, and NdCuAl3 order magnetically at 4.8 K, 5.3 K, and 2.21 K, respectively, whereas NdAgAl3, PrCuAl3, and Nd2AgCuAl6 showed no detectable magnetic ordering above 1.8 K. Temperature-normalized Cp data indicate a Schottky anomaly in all analyzed samples, with the associated peak being most intense in the NdAgAl3, PrCuAl3, NdCuAl3, and Nd2AgCuAl6 compounds. The 4f-electron contribution to Cp in the CeAgAl3, PrAgAl3, PrCuAl3, NdAgAl3, and NdCuAl3 compounds was determined by subtracting the contributions of the non-magnetic analogs LaAgAl3 or LaCuAl3. This enabled the derivation of magnetic entropy (S4f) curves. These curves allowed for the estimation of the number of microstates contributing to the system's S4f within the 1.8–100 K temperature range. Parameters quantifying the Magnetocaloric Effect (MCE)—namely magnetic entropy change (-ΔS), adiabatic temperature change (ΔTad), and Relative Cooling Power (RCP)—were determined from the entropy (S) curves. For a magnetic field change (ΔH) of 5 T, the CeAgAl3 compound exhibited maximum values ​​of -ΔS (-ΔSmax) and ΔTad (ΔTad,max), as well as an RCP of approximately 10.32 J/kg·K, 3.15 K, and 56.87 J/kg, respectively. PrAgAl3 exhibits lower values ​​of -ΔSmax (~4.68 J/kg·K) and ΔTad,max (~2.61 K) but maintains a relatively high RCP (~62.49 J/kg). In this case, a significant portion of the contribution to the MCE is associated with crystal field effects, as evidenced by the results obtained for the non-magnetic compound PrCuAl3 within the measured range (-ΔSmax = 2.71 J/kg·K, ΔTad,max = 1.14 K, and RCP = 41.69 J/kg). On the other hand, NdCuAl3 demonstrates the best magnetocaloric performance (-ΔSmax = 17.08 J/kg·K, ΔTad,max = 3.80 K, and RCP = 159.66 J/kg). The results obtained for these materials are comparable to—and in the case of CeAgAl3, even superior to—intermetallics reported in the literature, indicating their potential for application in MCE-based magnetic refrigeration (MR) systems at cryogenic temperatures. Instituição: Universidade Federal do Maranhão Tipo do documento: Dissertação 2026-06-02T00:00:00Z https://tedebc.ufma.br/jspui/handle/tede/7080 Título: ESTUDO DAS PROPRIEDADES MAGNÉTICAS E MAGNETOCALÓRICAS DOS COMPÓSITOS METÁLICOS Dy-Pt-In e Gd-Pt-In Autor: FREITAS, Aline Alves de Primeiro orientador: SILVA, Luzeli Moreira da Abstract: This study presents the synthesis of metallic alloy composites based on Dy-Pt-In and Gd-Pt-In, followed by characterization through X-ray diffraction (XRD), Scanning Electron microscopy (SEM) magnetization, and specific heat measurements. XRD and SEM analysis revealed that the Dy-Pt-In composite consists of DyPtIn, DyPt and DyPt2In phases. Similarly, the Gd-Pt-In composite comprises GdPtIn and GdPt phases. Magnetic properties analysis indicates that both composites exhibit the magnetic transitions corresponding to the crystalline phases present in each material. The Dy composite displays two consecutive ferromagnetic transitions at 32.5 K and 23 K, and an antiferromagnetic transition at 7.5 K, which undergoes a metamagnetic phase transition to the ferromagnetic state upon applying a magnetic field, as demonstrated by specific heat data. Magnetocaloric properties analysis revealed that the three consecutive magnetic transitions extend the magnetocaloric effect (MCE) over a wide temperature range between 2.5 and 57 K for the Dy-Pt-In composite. Although the two ferromagnetic transitions in the Gd-Pt-In composite are relatively close, they are sufficiently broad to promote a significant MCE between 6.0 and 89 K. The maximum values obtained for magnetic entropy change are similar for both composites (6.3 and 6 J/kg. K at ΔH = 50 kOe). However, the relative cooling power is 47% higher in the Gd composite (~327 J/kg) compared to the Dy composite (~221 J/kg). The synthesis and characterization of the metallic composites in this study present promising results for applications in magnetic systems, especially in the context of the magnetocaloric effect. These results highlight the importance of developing composites with magnetic systems, since a material with optimized properties for technological application can be obtained by combining the individual magnetic properties of two or more metallic compounds. Instituição: Universidade Federal do Maranhão Tipo do documento: Dissertação 2024-06-27T00:00:00Z https://tedebc.ufma.br/jspui/handle/tede/7034 Título: Investigação teórica e experimental das propriedades estruturais, eletrônicas e térmicas do cristal de malonato de L-fenilalanina L-fenilalaninium Autor: COSTA, Alexandre Saraiva Primeiro orientador: FAÇANHA FILHO, Pedro de Freitas Abstract: In this work, the structural, electronic and vibrational properties of the crystalline environment of the L-phenylalanine L-phenylalaninium malonate salt were investigated. The synthesis of the compound occurred by the method of slow evaporation of the solvent with a 2: 1 ratio; the pH value measured was 2.61 under ambient conditions of temperature and pressure. The structural characterization by powder X-ray diffraction, combined with the Rietveld refinement method, confirmed that the material crystallized in a monoclinic system with space group P21, containing two molecules per unit cell Z = 2 and with the following network parameters : a = 14.08 Å, b = 5.53 Å, c = 14.65 Å and angle β = 107.39 º. The volume of the unit cell obtained after refinement was 1089.5 Å3 . The structural parameters, lengths and bond angles, analyzed via DFT were described satisfactorily with the LDA exchange and correlation functional. The vibrational properties were analyzed using the density functional theory (DFT) and FTIR and Raman spectroscopy techniques. In addition, to assign the vibrational bands we use the computational approach of calculations with the quantum_ESPRESSO package. Characterizations by thermal analysis Thermogravimetry (TGA), Derivative thermogravimetry (DTG), Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) indicate decomposition of the molecules from 120 ° C and a large endothermic peak around 185 ° C regarding the decomposition of the material, and suggest that the material has five stages of decomposition. The main objective of this work is to investigate the structural and vibrational properties via DFT with those based on the functional B3LYP, B3LYP / PCM and LDA using the quantum-ESPRESSO package. The vibrational study at high temperatures, indicated that the material does not present a phase transition until the melting point, showing only small modifications for smaller wave numbers and a decrease in the relative intensity, showing the decomposition of the NH3 and CO2 groups. Instituição: Universidade Federal do Maranhão Tipo do documento: Dissertação 2020-09-16T00:00:00Z https://tedebc.ufma.br/jspui/handle/tede/6788 Título: Propriedades estruturais, espectroscópicas e térmicas do sal de Tutton Rb2Co(SO4)2(H2O)6: uma abordagem experimental e teórica visando aplicações em dispositivos ópticos Autor: GOMES, Letícia Fonseca Primeiro orientador: OLIVEIRA NETO, João Gomes de Abstract: This work investigates the structural, spectroscopic, thermal, and optical properties of the Rb2Co(SO4)2(H2O)6 crystal, a Tutton salt with potential application in selective optical devices. The material was synthesized by slow solvent evaporation and characterized by powder X-ray diffraction (PXRD) with Rietveld refinement, Fourier- transform infrared (FT-IR) and Raman spectroscopies, thermogravimetric and differential scanning calorimetry (TG-DSC) analyses, and ultraviolet–visible–near infrared (UV-Vis-NIR) spectroscopy. The monoclinic crystal structure, belonging to the P21/a space group, was confirmed, with lattice parameters consistent with literature data. The analysis of intermolecular interactions, via Hirshfeld surfaces and calculations of crystal voids, revealed a dense lattice packing, predominantly governed by O···H/H···O hydrogen bonds, O···Co/Co···O coordination interactions, and Rb···O/O···Rb electrostatic interactions. Periodic calculations based on Density Functional Theory (DFT) indicated an electronic band gap of approximately 3.0 eV, mainly associated with the contributions of the Co2+ ion's d orbitals. Vibrational characterization allowed for the suitable assignment of Raman and FT-IR modes based on the structural units [Co(H2O)6] 2+, [SO4] 2- , and Rb+ . TG-DSC analyses revealed a dehydration event at approximately 111 °C, with the associated enthalpy for the dehydration process. The optical study showed absorption bands in the UV region associated with ligand-to-metal charge transfer transitions and bands in the Vis region attributed to d-d electronic transitions of the Co2+ ion, evidencing the Jahn-Teller effect. The observed absorption and transmittance windows indicate the material's intrinsic spectral selectivity, reinforcing its potential for applications in optical filters and selective photonic systems. Instituição: Universidade Federal do Maranhão Tipo do documento: Dissertação 2025-12-22T00:00:00Z