Síntese, simulação e estudo teórico-experimental comparativo das fases do NaNbO3

Abstract

Niobium is among the metals for high technological importance and Brazil has the largest reserve, global production and trade in this area. The perovskites (ABO3), such as PZT, exhibits ferroelectric and piezoelectric properties, important to electronic setor. For environmental reasons, lead-free ceramics are used and sodium niobate (NaNbO3) is good candidate in this case, which also has a series of phases as a function of temperature. However, this polymorphism is still little known experimentally and DFT calculations can be a good tool for the structural study. Therefore, the goal of this work was to investigate phases of the NaNbO3, comparing theoretical and experimental results. Sodium niobate was synthesized by Nb2O5, and thermally treated for their characterizations by TG-DTA, solid-state NMR 23Na nuclei and Xray diffraction. Theoretical models were building from literature datas and all the ab initio calculations using DFT were done with program package QUANTUM ESPRESSO. The geometry’s optimizations had good agreement with the parameters of literature. The thermal analysis confirmed the formation of the niobate. After thermal treatment, the peaks of X-ray diffraction were compared with the simulations of the optimized models. The solidstate NMR analysis of the 23Na nuclei showed distinction between chemical sites at the samples according to the convolution of quadrupole signals obtained by calculations, thus serving as a probe for identification of polymorphism. In addition, the NMR parameters calculated of 93Nb nuclei identified the structural distortions of each phases. The both techniques concluded that the B1 and C1 samples suggested to be of S phase; D2 was between R and S; D1 was an intermediate of the P and R phases. By the band’s structure, there was a decreasing of gap for the sequence of phases. Bader charge analysis and pDOS calculations indicated that these electronic properties were not altered by structural differences between the models. The phonon calculations identified an imaginary frequency in the cubic phase and modifications was proposed inside the unit cell. The IR spectra of the phases showed specified couplings of vibrational modes, whose the differences between them are due to structural distortions. The thermodynamic analysis conclude that B phase had the highest total energy value, however, by the variations of energy less than 2 kcal.mol-1 there was no way to predict by DFT which one was the most stable.