Investigations on Structural, Optical and Multiferroic Properties of Modified BiFeO3 Ceramics

Abstract

Multiferroism is a wide and interdisciplinary area of research that has been growing explosively worldwide in past few years. Multiferroic materials are those materials having two or all primary ferroic orders, i.e. ferromagnetism, ferroelectricity, ferroelasticity much more recently ferrotoroidic, coexist in the same phase. These materials attract now considerable attention due to its applicability in fundamental research and potential devices as information storage, spintronics, magnetically modulated transducers, actuators, sensor devices, ultrafast optoelectronic devices etc. Bismuth ferrite (BFO) is a well known multiferroic material with Néel temperature TN = ~ 643K, Curie temperature TC = ~ 1103 K and coupling phenomena of various ferroic order parameters at room temperature.BFO based ceramics doped with rare earth (La3+, Ho3+) and alkaline earth metal (Ba2+) has been investigated for structural, morphological, dielectric, magnetic and optical properties. Subsequently, the effect of Ti4+ at Fe site for Bi0.9A0.1Fe1-yTiyO3 (A= La3+, Ho3+, Ba2+) ceramics is studied. The present thesis is organized into five chapters: Chapter 1 contains the general introduction to the broad area of multiferroics along with different types of multiferroic materials. A summarized review of single phase BFO with its perovskite structure, origin of ferroelectricity and magnetism are well described. Further, historical journey, literature survey on recent developments and motivation to carry out research are also highlighted. Chapter2 deals with the experimental part of the entire thesis work, wherein the detailed aspects of synthesis and characterization methods of pure, doped and codoped BFO nanostructures are elucidated. Thus, for synthesis process sol-gel auto combustion reaction route and for characterization, the physical principle of X-ray diffraction (XRD), Transmission electron microscopy (TEM), Field emission scanning electron microscopy (FESEM) for the investigation of structural properties, Dielectric measurement, Impedance and ferroelectric analysis for electrical properties, Vibrating samples magnetometer (VSM) for magnetic behaviour, Fourier transform infrared (FTIR) spectroscopy, UV-Visible spectrometer and Photoluminescence (PL) spectroscopy for optical studies are discussed in details.Chapter 3 features the detailed results obtained during La3+, Ho3+ and Ba2+ doping at Bi3+ site in BFO i.e. Bi1-xAxFeO3 (A- La3+, Ho3+ and Ba2+, x=0.0, 0.1 and 0.2) ceramics. Various aspects of structural, microstructural, dielectric, electric, magnetic and optical responses for different concentrations of La3+, Ho3+ and Ba2+ were examined. XRD patterns illustrate single phase formation for all modified BFO ceramics. Refined crystal structure is confirmed rhombohedral symmetry for BFO and Ba2+ doped nanoparticles, whereas structural phase transition from rhombohedral (R3c) to orthorhombic (Pnma) phase has been observed for La3+and Ho3+ substituted nanoparticles at x=0.1. FESEM analysis has revealed the spherical shaped grains together with a grain size<0.1μm. The enhancement in dielectric behavior for La3+and Ho3+ substituted ceramics with very less loss at x=0.1 is observed by analyzing dielectric measurements. The weak ferroelectric behavior of BFO with the effects of La3+, Ho3+ and Ba2+ doping ions are also incorporated in this chapter. However, a large enhancement in magnetization has been observed with increasing La3+, Ho3+ and Ba2+ doping concentration with maximum magnetization of 1.417 emu g-1 for Bi0.8Ba0.2FeO3 ceramic. In addition to multiferroic properties, interesting optical properties along with the possibleapplication of modified BFO ceramics are also summarized at the end of same chapter. In the next Chapter 4, the effect of Titanium (Ti4+) substitution at the Fe3+ site in modified BFO on the structural, morphological, dielectric, magnetic and optical properties were examined. XRD patterns demonstrate single phase formation of co-substituted BFO ceramics and further analyzed by Rietveld refinement. Rietveld refinement of XRD patterns demonstrated the phase transformation in Ba2+-Ti4+ co-doped samples with tetragonal structure (P4mm), whereas, La3+-Ti4+ and Ho3+-Ti4+ substituted samples indicated the existence of orthorhombic phase (Pnma) for all compositions. Room temperature ferromagnetism in co-substituted BFO ceramics has been observed. The substitution of Ti4+ ions at Fe3+ site along with fixed La3+/Ho3+/Ba2+ concentration at Bi3+ site in BFO further improve the magnetic and dielectric properties of BFO ceramics. Optical properties of the samples are also studied, indicating their potential optical applications in visible range and can be utilized in photocatalytic decomposition of organic contaminants in the future. Finally, Chapter 5 summarizes the key results and conclusions obtained from the above mentioned experimental results. In addition, thesis ends with the future scope and related issues of single-site and double-site doped BFO ceramics.