Síntese e Propriedades Estruturais, Vibracionais e Dielétricas das Perovskitas Duplas RE2CuMnO6 (RE = Nd, Eu e Gd)

Abstract

In this work, the structural, vibrational, dielectric and electrical properties of polycrystalline RE2CuMnO6 (RE = Nd, Eu, and Gd) ceramics, synthesised by solid-state reaction, were studied. Characterization by X-ray powder diffraction (PXRD) confirmed the presence of a disordered orthorhombic structure (Pbnm) in all the samples. The microstructure and density were evaluated by scanning electron microscopy (SEM) and the Archimedes method, respectively. Vibrational properties were investigated by Raman and infrared spectroscopy, revealing more intense Jahn-Teller effects in Raman. Fits with the four-parameter semi-quantum model allowed polar phonons to be extracted, and the static dielectric constant (0) extrapolated to the microwave region, which proved to be independent of the ionic radius of RE. Atomistic simulations provided structural and dielectric parameters consistent with the experimental data. Impedance spectroscopy revealed non-Debye behaviour, with a predominance of Maxwell Wagner type interfacial polarisation, responsible for the high values of ε′. Analysis of the Nyquist diagrams showed the electrical response centred on the grains, and the results indicated conduction typical of semiconductors. Temperature-dependent structural analysis showed unprecedented negative thermal expansion in the b parameter. Dielectric and electrical measurements as a function of temperature reinforced the non-Debye character and revealed a negative temperature coefficient of resistance for Eu2CuMnO6 and Gd2CuMnO6. The Nd2CuMnO6 sample showed a positive temperature resistance coefficient, an anomalous behaviour associated with a local metallic character and a possible ferroelectric-paraelectric transition. Finally, the analysis of electrical conductivity and scaling behaviour made it possible to identify the dominant charge transport mechanisms, showing that the electrical conduction mechanism in these materials can change with temperature.