Impedance and Raman Spectroscopic studies on La modified Bi₃TiNbO₉- Bi₄Ti₃O₁₂ Intergrowth-Aurivillius phase ceramics

Intergrowth Aurivillius structure materials are found to be attractive owing to its complex nature and scientific point of view. Lanthanum (La) modified intergrowth compound was prepared by mixing the two-layered (La_xBi_3-xTiNbO₉: LBTN-x where, x = 0.25, 0.50 and 0.75) and three-layered (La_yBi_4-yTi₃O₁₂: LBT-y, where y = 0.25, 0.50 and 0.75) compounds in a stoichiometric ratio. The resultant novel intergrowth compound, represented as BLTNO-(x + y), where, x + y = 0, 0.50, 1.0 and 1.5), is prepared by solid state route. From the scanning electron microscope pictures, the average grain size is found to increase with the increasing of La substitution, and possessing a plate-like morphology. The X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) images confirm the formation of six-layered phase with orthorhombic crystal structure. Impedance data was measured as a function of frequency at different temperatures. From the combined ac-conductivity and master (scaling) plots it is revealed that the conduction mechanism belongs to thermally activated process. The variation of imaginary part of modulus (complimentary to dielectric) with temperature has shown anomalies near 350^o C and the peak maximum is found to shift towards higher temperature, indicates the dielectric relaxation of the sample. The ac-conductivity data obeys Johnscher’s law and the isotherms appear to fall onto a single curve. The results were corroborated to defect based hopping mechanism. The remnant polarization (Pr) is found to decrease with increasing the lanthanum concentration. The overall result from the structural, Raman and impedance spectroscopic data revealed that the intergrowth is a layered-engineered method and one can improve ferroelectric properties, arises from strain-induced lattice distortions at the interfaces.