Investigation of relaxor and diffuse dielectric phase transitions of Ba_1-XBi_xTi_0.8Fe_0·2O₃ materials

Different doping elements have been used to enhance the dielectric properties of BaTiO₃ ceramic. In this work, the effect of substitution of Ba by Bi in A site and Ti by Fe in B site on structural, dielectric and electrical properties of Ba_1-xBi_xTi_0.80Fe_0·20O₃ ceramics at (x = 0.00, 0.05, 0.10 and 0.15) was investigated by X-ray diffraction, Raman spectroscopy, Scanning Electron Microscopy (SEM), Mössbauer spectroscopy as well as dielectric measurements. The Rietveld refinement results revealed that the prepared compounds crystallize in both tetragonal (P4mm) and hexagonal (P6₃/mmc) phases for x = 0.00 and 0.05 while at x = 0.10 and 0.15, the hexagonal phase disappears and only the tetragonal phase is fitted. The Raman spectra confirmed the disappearance of hexagonal phase in benefit of tetragonal phase as the Bi³⁺ substitution increases. Based on Mössbauer analyses results, all the samples are in paramagnetic state at room temperature and the Fe is oxidized under Fe³⁺ without the presence of Fe²⁺ or Fe⁴⁺ ions. The dielectric measurements as function of temperature are studied and tree broad and relaxor phase transitions were detected: from rhombohedral to orthorhombic phase T_R-O and to tetragonal ferroelectric phase T_O-T then to cubic paraelectric phase T_m. These phase transitions were displaced to the lower temperature with increasing of Bi³⁺ substitution. The values of ε'_r increase gradually with increasing of Bi³⁺contents which confirmed the enhancement of dielectric properties of BaTi_0·80Fe_0·20O₃ by Bi substitution on Ba site. The diffuse phase transitions were described by fitting the modified Uchino relation. The Cole-Cole analyses showed that both the grain and grain boundaries resistivity values are higher for Bi³⁺ substituted samples which are responsible to the dielectric properties improvement.