Structure, Cation Ordering and Phonon studies of Pb(Fe 0.5-x Sc x Nb 0.5 )O 3 , a Multiferroic Relaxor : Bulk and Thin Films

Bandi, Mallesham (2017) Structure, Cation Ordering and Phonon studies of Pb(Fe 0.5-x Sc x Nb 0.5 )O 3 , a Multiferroic Relaxor : Bulk and Thin Films. PhD thesis, Indian Institute of Technology Hyderabad.

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Abstract

A material exhibiting more than one ferroic order (ferromagnetic or antiferromagnetic, ferro/antiferro-electric, ferroelastic) simultaneously is termed as multiferroic. Due to presence of multifunctionality in a single material multiferroics are considered to be useful in many modern device applications such as spintronic, magnetic field sensors, microwave devices etc. Lead iron niobate Pb(Fe 0.5 Nb 0.5 )O 3 (PFN) is an interesting single phase multiferroic material with structural features very similar to relaxor ferroelectrics. Recent investigations on PFN dealing with its room temperature magnetoelectric coupling and relaxor like features have attained great interest to understand the fundamental science behind these structures. Room temperature crystal structure of PFN is ambiguous between rhombohedral (R3m) and monoclinic (Cm) symmetries. Moreover, the reports dealing with effect of elemental substitutions (isovalent/aliovalent) are very limited. Hence, the main focus of thesis is to study the influence of higher ionic radius Scandium (Sc 3+ ) substitution at Fe 3+ site on structural, physical properties of PFN bulk ceramics. Last part of thesis includes investigating the ferroelectric domains of polycrystalline and epitaxial thin films. Chapter 1 begins with introduction of multiferroics and importance of multifunctionality in device applications. A brief overview of Multiferroic research and classification of multiferroic materials are discussed. Further, relaxor ferroelectrics and their differences from normal ferroelectrics is discussed. A brief summary of various phenomenological models describing the peculiar dielectric properties of relaxors and a brief literature review of lead iron niobate with objectives of the thesis are detailed. Chapter 2 deals with experimental methods such as synthesis and characterization tools used in this work. Mainly the steps in synthesis of Pb[(Fe 0.5-x Sc x )Nb 0.5 ]O 3 (PFSN) ceramic powders via wolframite precursor route have been detailed with underlined chemical reactions. Basic principle of several characterization techniques and purpose of using such techniques in this work is discussed. Moreover, pulsed laser deposition technique and its advantage in fabricating thin films of complex oxides is discussed. Chapter 3 deals with structural analysis of PFSN ceramics. The phase purity of synthesized compounds has been verified through X-ray diffraction analysis. Rietveld analysis of x-ray diffraction pattern confirms that with increasing Sc content structural viiitransformation from a monoclinic (Cm) to rhombohedral (R3m) structure at x = 0.3 at % of Sc. The structural transformation from monoclinic to rhombohedral at x = 0.3 at % Sc is associated with volume increment of 34-36%. Further, Selected area electron diffraction analysis reveal the presence of superlattice reflections due to additional local symmetry corresponding to cation ordered regions in x = 0.5 composition i.e., Pb(Sc 0.5 Nb 0.5 )O 3 (PSN). Optical properties of studied compounds reveal with increasing Sc concentration band gap of PFSN ceramics increases due to diminishing of sp-d exchange interaction between the valance band and localized d-electrons. The considerable increment in band gap is found in compounds with high Sc content (≥ 0.3). To verify the spin state and oxidation state of iron Mössbauer spectra has been recorded, which reveal iron exists in Fe 3+ state with low spin configuration. Chapter 4 deals with dielectric and magnetic phase transition behavior of PFSN ceramics. Ferroelectric phase transition temperature (T m ) of PFSN solid solutions shows unconventional compositions dependence. T m increases with increasing Sc concentration until x = 0.25 at% and decreases beyond. Such anomalous behavior in T m is due to the onset of B', B" local cation ordering beyond Sc content x = 0.25. The dielectric phase transition behavior of all compounds is diffused and the diffusivity parameter (γ) was calculated from modified Curie-Weiss law. Dielectric loss decreases with increasing Sc content due to increment in bad gap. Moreover the key observations in frequency and temperature dependence of dielectric permittivity and dielectric loss have been discussed. Finally, Magnetic properties of PFSN ceramics were discussed. Chapter 5 deals with high temperature phonon studies of PFSN ceramics. Raman spectra of PFSN ceramics shows shift in frequency of phonon mode with Sc content due to lower atomic mass. Evolution of Pb-O coupled vibrational mode around 350 cm -1 (F 2u ) at and beyond x= 0.3 is attributed to presence of local B-site cation ordering. Temperature dependence Raman scattering studies reveal B-localized mode [F 1u , ~ 250 cm -1 ] and BO 6 octahedral rotational mode [F 1g , ~ 200 cm -1 ], both originating from polar nanoregions (PNRs) behave like coupled phonon modes in compounds with rhombohedral symmetry. The presence of correlation between the B-localized and BO 6 -rotational modes introduces a weak relaxor feature for systems with rhombohedral symmetry. ixChapter 6 deals with fabrication of polycrystalline and epitaxial thin films of PFN and investigation of ferroelectric domains using piezoresponse force microscopy (PFM). PFM studies of polycrystalline and epitaxial film reveals that polycrystalline film shows in- plane and out-of-plane piezoresponse, whereas epitaxial film show only in-plane piezoresponse. Finally the chapter 7 deals with summary of thesis work and future scope.

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