Bi3R2Ti3FeO15 (R=Bi, Gd, and Nd): from structural properties to microwave device applications

Abstract

This work comprises the study of bismuth layer-structured ferroelectric material Bi 3 R 2 Ti 3 FeO 15 (R = Bi, Nd, and Gd) and the investigation of their properties, aiming to evaluate these materials as alternative for application in electronic devices, such as ceramic capacitors and cylindrical dielectric resonator antenna (CDRA). The ceramic materials were obtained by conventional solid-state reaction, and the characterizations were performed through the techniques of PXRD, Raman Spectroscopy, FC-ZFC magnetization, Mössbauer spectroscopy, and Impedance spectroscopy, in order to better understand the relations between the morphological and structural properties of these ceramic materials and their electrical, magnetic, and dielectric properties. X-ray diffraction and Rietveld refinement reveal that all samples presented orthorhombic structure with A2 1 am space group. Bi 3 Nd 2 Ti 3 FeO 15 (BNFTO) and Bi 3 Gd 2 Ti 3 FeO 15 (BGFTO) presented a reduction in the orthorhombicity when compared to Bi 5 Ti 3 FeO 15 (BFTO). For the Raman analysis, significant changes observed in the intensity of some modes, as well as the shift of modes to higher frequencies, occurred due to the replacement of Bi atoms in the perovskite layers by the lighter Gd and Nd atoms. The magnetic susceptibility of all samples followed the Curie–Weiss law, with negative values of the Curie–Weiss temperature, demonstrating that the magnetic interactions are antiferromagnetic in nature. The magnetization curves suggested a weak canted ferromagnetic behavior for temperatures below 25 K, followed by a linear behavior in the curves at high temperatures. Mössbauer spectroscopy measurements revealed an increase of the quadrupole splitting values as the temperature decreases, indicating that the samples present local distortions, favoring the existence of weak ferromagnetic phase via the antisymmetric Dzyaloshinskii–Moriya interaction. Impedance spectroscopy was performed to establish a correlation between the electrical properties and the microstructure of the ceramics. A non-Debye relaxation induced by a thermally activated mechanism can be observed in all samples. In an innovative way, these materials were applied as ceramic capacitors and CDRA. The thermal stability and operating performance parameters, obtained by experimental and simulated analysis, are contrasted in detail with other published works. The results suggest that the three phases are new candidates for X4D capacitors, with excellent temperature stability (29–150 ◦ C, TCC ≤ ±3.3%). Furthermore, CDRA showed great potential for microwave application, operating in the S band. A change in the value of τ f of the BFTO phase (-428.48 ppm ◦ C −1 ) for BGFTO (+ 59.17 ppm ◦ C −1 ) and BNFTO (+ 57.69 ppm ◦ C −1 ) is observed. This result opens a great opportunity for future work on CDRA with temperature coefficients close to zero (τ f ∼ 0).