Dielectric, Ferroelectric and Impedance Spectroscopic Studies of Agnbo3 and Its Modified Systems

Abstract

This thesis contains some fundamental understandings of mechanism related to the dielectric, ferroelectric and impedance spectroscopic study of AgNbO3 and its modified systems. The results analysis are mostly qualitative in nature which has accounted previous reported facts while explaining as well as added some new concepts that fits to be more appropriate. The quantitative figures are less stressed in order to focus more on the underlying physics for different mechanisms of some particular electric properties in the titled compounds. The modified systems are designed by partially incorporating impurity atoms at the B-site of the ABO3 perovskite compound. The impurity atoms are chosen of isovalent (Ta5+, Sb5+, V5+) as well as alieovalent (Mn4+, W6+, Ti4+) in nature. The alieovalent impurities are chosen to design charge carrier (electron/hole)-doped systems. The effect of Jahn-Teller ions (Mn3+, W5+ and Ti3+) on the ferroelectric properties also understood and properly explained. The role of impurities in bringing structural and microstructural changes and their effect on dielectric, ferroelectric and other electric properties of the compound is extensively analysed and discussed in the subsequent chapters.
This work was carried out to improve the dielectric and ferroelectric properties of AgNbO3 to be an alternative for the hazardous lead (Pb) based ferroelectrics. This is because the dynamics of cations (Ag, Nb) and nature of Ag-O, Nb-O bonds in AgNbO3 features similarly to that of the dynamics of cations in Pb-based compounds. The compound exhibits an exceptionally large spontaneous polarisation(52μC/cm2)in its polycrystalline form at sufficient applied field (220kV/cm) and iscomparable to 50 μC/cm2 of polycrystalline PbTiO3 ceramic, at room temperature. But, the former shows a poor 0.041 μC/cm2 at ~12kV which is quite lower than the latter, which is a matter of concern. Multiple dielectric anomalies also appear in the temperature dependent permittivity of the compound which is related to the complex coupling between NbO6 octahedra and the cations dynamics. Therefore, numbers of modifications were carried out to verify the effects of dopants on dielectric and ferroelectric parameters and to understand the underlying science of several mechanisms.
Solid state synthesis route is chosen to prepare samples for the investigated compound (AgNbO3) in which required oxide materials are taken as raw precursors (Ag2O, Nb2O5). Manual grinding is opted in order to properly mix the oxide powders for their homogeneous presence. Calcination and sintering are two thermal processes which are followed to bring the desired phase of the compound and to densify the pellet type sample for various experiments. The modified systems are titled with the element oxide (Ta2O5, Sb2O5, V2O5, MnO2, WO3, TiO2)-doped AgNbO3 systems. The amount of element oxides is measured in molecular weight fraction of Nb2O5 in order to partially incorporate the metal elements in place of Nb, at the B-site of ABO3 perovskite structure. All the prepared samples are characterised with X-ray diffraction technique (XRD), Scanning electron microscope (SEM)/Field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscopy (XPS) and RAMAN spectroscopy to extract the structure and microstructure information. The frequency dependent electrical parameters (impedance, dielectric loss, phase angle and capacitance) are collected with the help of Impedance analyser within a broad range of temperature from room temperature (RT), 25oC to 450oC. The temperature dependent dielectric constant is extracted from the impedance data to examine the nature of dielectric anomalies, the effect of modification on dielectric constants, Curie temperatures and dielectric losses of the compounds. The polarisation-electric field (P-E) hysteresis loops for all the samples are collected by Radiant precision loop tracer.
The role of precipitated silver particles in AgNbO3 is thoroughly and systematically investigated by complex impedance spectroscopy within frequency and temperature domain of 100Hz to 1MHz and 25 oC to 450 oC respectively. Extensive analysis over relaxation mechanisms verified that only intrinsic (grain) conduction is the major contributing source. The absence of extrinsic (interfacial) conduction indicates that the precipitated silver nano particles has no active role in the electrical properties as these are supposed to be deposited in various interfaces. In the isovalent metal oxides doping, satisfactory results in ferroelectric properties came in Sb2O5-doped and V2O5-doped systems. In the Sb2O5-doped system, the improved P-E loop appeared at higher applied field. MnO2-doped AgNbO3 systems are prepared successfully by retaining the phase of the parent system. Low concentration of MnO2 was substituted in molecular weight fraction (x = 0.02, 0.04) of Nb2O5 to prepare the modified AgNbO3 compound by solid state route, with an object to partially incorporate Mn4+ in place of Nb5+ site. The XPS results identify the presence of manganese in Mn4+ and Mn3+state. The ferroelectric parameters were improved and results were properly discussed. WO3-doped AgNbO3 systems were successfully prepared by retaining the phase of the parent system. Low concentration of WO3 was substituted in molecular weight fraction (x = 0.02, 0.04) of Nb2O5 to prepare the modified AgNbO3 with an object to incorporate W6+ in place of Nb5+ to design the electron doped systems. XPS peaks identify the presence of tungsten in W6+ and W5+ state and hence confirm the possible hole doping in the modified systems. In the AWN2 system, the ferroelectric properties reduced but in AWN4 system, significant increase in the parameters was observed which were analysed with possible reasons. Transition metal oxide TiO2 was selected to dope with the AgNbO3 system in order to partially incorporate Ti4+ in place of Nb5+ cation. XPS results confirmed the presence of Ti4+ and Ti3+ state and hence the possible hole doping in the system was successful. In the TiO2-doped systems, though ferroelectric parameters exhibited high enhanced value but the shape of the loop approaches lossy feature. Apart from this, the effect of modifications on structure, microstructure, dielectric constant, tangent loss, activation energy, resistance of the materials were accurately reported, sincerely analysed and properly discussed.
The thesis provides some qualitative information regarding the titled compound which will benefit the other researchers to go for other transition metal oxide modifications. The properties of the modified systems are also suitable to be used in capacitors and memory applications.