Rosivaldo Xavier Silva

We report a temperature-dependent Raman scattering study of the hybrid organic–inorganic azide perovskite of formula [(CH3)2NH2][Mn(N3)3]([DMA][Mn(N3)3]). The Raman spectra reveal distinct shifts in phonon frequencies, particularly in the νsCNC and νs(ν1)N3- modes, demonstrating that spin-phonon interactions play a critical role in their thermal and magnetic responses. Differential scanning calorimetry confirmed a first-order structural phase transition, from an orthorhombic high-temperature HT(α) phase, belonging to the Cmca space group, to a monoclinic low-temperature LT(β) phase, with P21 symmetry. The associated entropy change was found to be |∆S| ~37.2 J kg-1 K-1, and the barocaloric (BC) coefficient (|δTt/δP|) was ~2.94 K kbar-1, estimated by using the Clausius-Clapeyron method. Room-temperature FTIR reflectivity spectra revealed the polar phonon modes and their corresponding damping coefficients, showing minimal contributions from the DMA+ cation to the intrinsic dielectric constant (~4.3). These results provide new insights into the coupling between lattice dynamics, spin interactions, and barocaloric behavior in azide-based hybrid perovskites.

In recent years, the frequency of extreme weather events on Earth has significantly increased. This phenomenon is driven by the intensification of the greenhouse effect caused by anthropogenic activities, which leads to temperature variations in urban environments that affect thermal comfort and quality of life. Given this context, the present study investigates temperature mapping in urban biomes using an infrared thermometer, conducted as part of a hands-on workshop offered during the 21st National Science and Technology Week. The initiative involved students from the public school system, and was grounded in physics education, aiming to foster scientific enculturation. Participants engaged in a problem-based learning experience, actively contributing to all stages of the knowledge-construction process. The objective of this study was to examine the relationship between the presence of vegetation and its impact on temperature in urban environments. A qualitative and quantitative methodological approach was adopted, enabling the identification of scientific literacy indicators, such as information sequencing, data organisation, logical reasoning, hypothesis formulation, justification, and explanation of observed phenomena. The analysis of the students’ statements and activity guidelines provided insights into their critical thinking development. The findings indicated that students developed essential skills for understanding physical and environmental phenomena, effectively linking collected data to scientific concepts, and proposing well-supported interpretations. Moreover, experience reinforced the perception of science as a dynamic and investigative process, fostering curiosity and enhancing students’ argumentative abilities. Thus, the workshop proved to be an effective strategy for promoting scientific literacy and engaging participants in the study of environmental impact in urban contexts.

The [(CH₃)₂NH₂]PbBr₃ perovskite exhibits an unusual thermal expansion, and we propose a novel thermomechanical mechanism that differs from the typical “wine-rack”. Also, we probed the presence of an H-bond between the DMA cation and the framework.

Weak antiferromagnetic (AF) interaction in the ferromagnetic (FM) partially ordered La2CoMnO6 (LCMO) was detected by Raman spectroscopy by monitoring spin-phonon coupling. Because of the sensibility to probe short-range disorder and lattice modifications, the Raman spectroscopy showed to be an useful tool to indicate less remarkable magnetic transitions in LCMO compound. Apart from the expected spin-phonon coupling due to the long-range FM superexchange (Tc~230 K), phonons parameters pointed out an additional spin-phonon coupling related to the short-range AF interaction at Tc~135 K, which was not detected from the magnetic bulk response. These results reinforce the Raman spectroscopy as a powerful technique to detect antisite disorder into A2B'B"O6 magnetic double perovskites, whose magnetic properties are driven by superexchange interactions.

Temperature-dependent Raman scattering and differential scanning calorimetry were applied to the study of the hybrid organic-inorganic azide-perovskite [(CH₃)₄N][Cd(N₃)₃], a compound with multiple structural phase transitions as a function of temperature. A significant entropy variation was observed associated to such phase transitions, |∆S| ~ 62.09 J·kg⁻¹ K⁻¹, together with both a positive high barocaloric (BC) coefficient |δT_t/δP| ~ 12.39 K kbar⁻¹ and an inverse barocaloric (BC) coefficient |δT_t/δP| ~ −6.52 kbar⁻¹, features that render this compound interesting for barocaloric applications. As for the obtained Raman spectra, they revealed that molecular vibrations associated to the NC_4, N₃^– and CH₃ molecular groups exhibit clear anomalies during the phase transitions, which include splits and discontinuity in the phonon wavenumber and lifetime. Furthermore, variation of the TMA⁺ and N₃^– modes with temperature revealed that while some modes follow the conventional red shift upon heating, others exhibit an unconventional blue shift, a result which was related to the weakening of the intermolecular interactions between the TMA (tetramethylammonium) cations and the azide ligands and the concomitant strengthening of the intramolecular bondings. Therefore, these studies show that Raman spectroscopy is a powerful tool to gain information about phase transitions, structures and intermolecular interactions between the A-cation and the framework, even in complex hybrid organic-inorganic perovskites with highly disordered phases.