Síntese e caracterização de cerâmicas piezoelétricas de niobato de sódio: um estudo teórico e experimental

Abstract

Sodium niobate (NN) is a promising piezoelectric composition for replacing lead zirconate titanate (PZT). Although it is known for its antiferroelectric behavior and is already used in various technological applications, the possibility of ferroelectric phases at room temperature could expand its applicability and has increased the interest of scientists and researchers in this lead-free piezoelectric compound. Based on these considerations, this work presents a systematic study of NN, combining computational simulations based on Density Functional Theory (DFT) and experimental evaluations of samples obtained by the microwave-assisted hydrothermal method. The computational part addressed all NN phases, with a primary focus on the P1m1, P21ma, and Pbcm phases present at room temperature, where the structures were analyzed based on X-ray diffraction patterns, Raman spectra, and profiles of the crystal structure. Based on piezoelectric constants, it was possible to evaluate the potential ferroelectric, antiferroelectric, or paraelectric behavior exhibited by each structure. In the experimental part, the obtained samples were characterized, pelletized, and polarized to verify the piezoelectric behavior. The computational results revealed subtle differences between the monoclinic P1m1 and orthorhombic Pbcm and P21ma phases in the diffraction patterns. In the Raman spectrum, no distinction was found between the P21ma and P1m1 models, suggesting they represent the same structure. The piezoelectric properties of the ferroelectric P21ma and P1m1 models showed nearly identical values; however, the monoclinic model displayed additional positions with non-zero constants. The R3c phase also showed non-zero values for the piezoelectric constants, suggesting that only these three structures are ferroelectric, while the others are antiferroelectric or paraelectric. The arrangement and octahedral characteristics of the Pbcm model suggest that this phase is formed by stacking two P21ma structures, overlaid and inverted by 180°, which explains the antiferroelectric behavior of the Pbcm structure. In the experimental evaluations, both thermally treated and sintered samples exhibited diffraction patterns compatible with the Pbcm or P21ma models, but without conclusively indicating which group they belong to. However, the Raman spectrum indicated that both are compatible with the orthorhombic P21ma structure. The analysis of piezoelectric behavior revealed dipolar alignment after polarization in a low-intensity electric field, confirming that the samples obtained by microwave-assisted hydrothermal synthesis are ferroelectric and compatible with the orthorhombic P21ma model.