RESUMO
Double perovskites RBaFeTiO6 (R = La, Eu) were successfully synthesized using a solid state reaction route. Structural refinement analysis has been performed to investigate the details of the crystalline structure which was found to be a cubic double perovskite structure at room temperature (space group Pm3Ìm, No. 221). Crystallite size, lattice strain, density, and porosity parameters were also calculated. The morphology and the elemental composition were analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and elemental mapping. FTIR and Raman scattering spectroscopy have been performed in order to study the vibrational modes and the various bond formations of the synthesized samples. The optical properties investigated by means of the optical absorbance measurements highlight that both materials present large band gap energies, and are thus potential candidates for various technological applications. The electrical behavior of these double perovskites was also studied by frequency-dependent dielectric measurements and impedance spectroscopy. The electrical conduction follows Jonscher's power law and the conduction mechanisms are identified.
RESUMO
The influence of Zn substitution on the magnetic and magnetocaloric properties of Cd1-xZnxCr2Se4 (0.35 ≤ x ≤ 0.45) spinel was investigated. All the samples exhibited two successive magnetic transitions associated with the reentrant spin glass at a freezing temperature (Tf) and Curie temperature (TC). It was demonstrated that Tf changed slightly with the Zn content; however, the TC value decreased with increasing Zn content due to the decrease of lattice constant. The magnetic entropy variation (-ΔSM) was found to exhibit two maxima at TC and Tf, which have an opposite trend as a function of Zn content. With increasing Zn content, (-ΔSM) was found to decrease at TC, whereas at Tf, (-ΔSM) increased. It was shown that the change of (-ΔSM) is closely correlated to the local exponent factor n at both transitions. The results showed that materials with double magnetic transitions could be integrated in a new class of magnetic refrigeration working in different temperature windows.
