En relation avec le désordre de leur structure, les composés relaxeurs révèlent une dualité entre les échelles locale et globale. L'ordre polaire qui se développe localement ne se manifeste pas à l'échelle globale, la structure globale demeurant cubique. Bien que pertinente au vu de ses capacités à saisir la dualité d'échelle, une incorporation directe de la symétrie locale à l'appareil d'étude des relaxeurs reste absente. C'est autour de cette lacune que s'articule la thèse, en ce qu'elle constitue une implémentation explicite de la symétrie locale dans l'Hamiltonien effectif. Elle porte sur l'analyse du rôle de la symétrie locale à sous-tendre l'ordre locale dont les propriétés macroscopiques émergent. Une question sous-jacente s'adjoint à ces considérations et concerne la possible formulation du problème d'un point de vue topologique / Concomitantly with lattice disorder, there is a discrepancy between local and global scales in relaxor ferroelectrics, in that structural distortions occurring at the local scale are not reflected in the average global structure which remains cubic. There is an absence of direct implementation of the local symmetry in the modeling of relaxors, despite its considerable, but often unacknowledged, ability to encode local features. Central to the thesis is an explicit account for local gauge symmetry within the first-principles-derived effective Hamiltonian approach. The thesis thus aims to consider how an extended symmetry allowing independent transformations at different points in space can effectively bridge local features and macroscopical properties. An underlying question the thesis also seeks to answer is whether the disorder-induced non-trivial interplay between local and global scales can be described from a topological point of view

Concomitantly with lattice disorder, there is a discrepancy between local and global scales in relaxor ferroelectrics, in that structural distortions occurring at the local scale are not reflected in the average global structure which remains cubic. There is an absence of direct implementation of the local symmetry in the modeling of relaxors, despite its considerable, but often unacknowledged, ability to encode local features. Central to the thesis is an explicit account for local gauge symmetry within the first-principles-derived effective Hamiltonian approach. The thesis thus aims to consider how an extended symmetry allowing independent transformations at different points in space can effectively bridge local features and macroscopical properties. An underlying question the thesis also seeks to answer is whether the disorder-induced non-trivial interplay between local and global scales can be described from a topological point of view

The following investigations have been carried out in this thesis 1)Cordierite is already well known for its low thermal expansion behaviour. Chemical substitutions at various octahedral and tetrahedral sites have been done and their thermal expansion characteristics have been studied Synthesis of cordierite in more reactive environment provided by AlF3 used as sintering aid has been attempted 2) Diffuse ferroelectric phase transition of lead based perovskite materials leads to low expansion region. Solid solutions of lead iron niobate with lead titanate is investigated to increase the structural distortion and see it this low expansion region can be extended to wider temperature Preparation of materials with higher tetragonal distortion In PbTi03- BlFeO3 system is undertaken to study the thermal expansion anisotropy. 3) Composites between lead iron niobate(+(x) and lead titanate (-(x below Tc) has been undertaken to prepare low expansion hulk over a wide temperature range 4) Acoustic emission has been employed as a tool to detect the microcracking in solid solutions between PFN1-x, PTx, and PT1-x, ,BFx, It is hoped to understand relation between magnitude of lattice distortion transition temperature and microcracking in ceramics of the class of materials.

Title: Studies of Relaxor Ferroelectrics with Spontaneous Polar Nanoregions Author: Petr Ondrejkovič Institute: Institute of Physics of the Czech Academy of Sciences Supervisor: Ing. Jiří Hlinka, Ph.D., Institute of Physics of the Czech Academy of Sciences Abstract: The thesis is devoted to relaxor ferroelectrics with spontaneous polar nanoregions. We have investigated one of the canonical representatives, uniaxial strontium barium niobate, by means of neutron scattering, and also performed computer simulations with a model of a uniaxial ferroelectric with point defects. Neutron scattering studies of strontium barium niobate single crystals under a defined sequence of thermal and electric field treatments elucidate nature of distinct components of its transverse diffuse scattering. These components are associated mainly with the static ferroelectric nanodomain structure and the dynamic order-parameter (polarization) fluctuations. Moreover, high-resolution neutron backscattering experiments allowed us to resolve characteristic frequencies of the order-parameter fluctuations and prove that this component is caused by the same polar fluctuations that are responsible for the Vogel-Fulcher dielectric relaxation, the hallmark of relaxor ferroelectrics. The model system of a uniaxial ferroelectric with point...

Bismuth layer-structured ferroelectrics have received significant attention recently due to their fairly high TC and good fatigue endurance which make them important candidates for non-volatile ferroelectric random access memories (Fe-RAMs) as well as for the piezoelectric device applications at high temperatures. Structure of these compounds is generally described as the pseudo-perovskite block (An-1BnO3n+1)2- sandwiched between the bismuth oxide layers (Bi2O2)2+ along the c-axis, where n represents the number of corner sharing BO6 octahedra forming the perovskite-like slabs. Only a few compounds belonging to this family show relaxor behavior (frequency dependent diffuse phase transition). Relaxor ferroelectrics are very attractive for a variety of applications, such as capacitors, sensors, actuators, and integrated electromechanical systems. The present work attempts to understand the mechanism of relaxor behavior in Aurivillius oxides as well as to improve the piezoelectric and ferroelectric properties of some of the known phases. Details pertaining to the fabrication and characterization of BaBi4Ti4O15 (n = 4 member of Aurivillius family of oxides) ceramics are presented. X-ray diffraction, Raman spectroscopy, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were employed to probe the structural and microstructural details. The contribution of irreversible domain wall movement to the room temperature dielectric constant and polarization was quantitatively evaluated using the nonlinear dielectric response. Dielectric dispersion and conduction mechanism of these ceramics are also explicated using the complex impedance spectroscopy. The effects of La3+ and Ca2+ doping on the phase transition behavior and other properties of BaBi4Ti4O15 are investigated. La3+ doping for Bi3+ was found to strengthen the relaxor behavior. New compounds such as CaNaBi2Nb3O12, SrNaBi2Nb3O12, Na0.5La0.5Bi4Ti4O12, etc. belonging to the Aurivillius family of oxides have been synthesized and investigations concerning their structural, dielectric and ferroelectric properties are presented. Rietveld refinement of room temperature X-ray powder data suggested that CaNaBi2Nb3O12 and SrNaBi2Nb3O12crystallize in the orthorhombic space group B2cb. SrNaBi2Nb3O12 ceramics exhibited frequency-dependent Tm which follows the Vogel-Fulcher relation implying a relaxor nature. No frequency dependence of Tm was observed for CaNaBi2Nb3O12 ceramics. Polarization - electric field hysteresis loops recorded well above Tm confirmed the coexistence of polar and non-polar domains in SrNaBi2Nb3O12 ceramics. Dielectric anomaly observed around 675 K for CNBN corresponds to the ferroelectric to paraelectric phase transition which is accompanied by the change in crystal structure from orthorhombic to tetragonal. Fe and Nb co-doped Bi4Ti3O12 ceramics were fabricated and characterized for their structural, electrical and magnetic properties.

Single self-assembled semiconductor quantum dots (QDs) are able to emit single-photons and entangled-photons pairs. They are therefore considered as potential candidate building blocks for quantum information processing (QIP) and communication. To exploit them fully, the ability to precisely control their optical properties is needed due to several reasons. For example, the stochastic nature of their growth ends up with only little probability of finding any two or more QDs emitting indistinguishable photons. These are required for two-photon quantum interference (partial Bell-state measurement), which lies at the heart of linear optics QIP. Also, most of the as-grown QDs do not fulfil the symmetries required for generation of entangled-photon pairs. Additionally, tuning is required to establish completely new systems, for example, 87Rb atomic-vapors based hybrid semiconductoratomic (HSA) interface or QDs with significant heavy-hole (HH)-light-hole (LH) mixings. The former paves a way towards quantum memories and the latter makes the optical control of hole spins much easier required for spin- based QIP. This work focuses on the optical properties of a new type of QDs optimized for HSA experiments and their broadband tuning using strain. It was created by integrating the membranes, containing QDs, onto relaxor-ferroelectric actuators and was quantified with a spatial resolution of ~1 µm by combining measurements of the µ-photoluminescence of the regions surrounding the QDs and dedicated modeling. The emission of a neutral exciton confined in a QD usually consists of two fine-structure-split lines which are linearly polarized along orthogonal directions. In our QDs we tune the emission energies as large as ~23meV and the fine-structure-splitting by more than 90 µeV. For the first time, we demonstrate that strain is able to tune the angle between the polarization direction of these two lines up to 40° due to increased strain-induced HH-LH mixings up to ~55%. Compared to other quantum emitters, QDs can be easily integrated into optoelectronic devices, which enable, for example, the generation of non-classical light under electrical injection. A novel method to create sub-micrometer sized current-channels to efficiently feed charge carriers into single QDs is presented in this thesis. It is based on focused-laserbeam assisted thermal diffusion of manganese interstitial ions from the top GaMnAs layer into the underlying layer of resonant tunneling diode structures. The combination of the two methods investigated in this thesis may lead to new QDbased devices, where direct laser writing is employed to preselect QDs by creating localized current-channels and strain is used to fine tune their optical properties to match the demanding requirements imposed by QIP concepts.

The present research work mainly focuses on fabrication of compositionally modulated multilayers of (l−x) Pb(Mgi/3N2/3)O3 - x PbTiO3 (PMNPT) through multi target pulsed laser ablation technique. Heterostructures like compositionally varying multilayers; multilayers with graded interface and a ferroelectric [PbTiO3 (PT)] and relaxor (PMN) superlattices of different periodicities were fabricated. Role of artificially enhanced chemical heterogeneity and strain on enhancement of physical property was studied. Dimensional dependent ferroelectric and antiferroelectric type of polarization behavior was observed in the case of both compositionally varying multilayers and the superlattice structures fabricated. The dimensional dependence of various ferroelectric interactions like long-range, short-range and interfacial coupling among the layers was studied. The phase transition behavior and dielectric studies were carried out on these heterostructures. An artificial superlattice of a relaxor ferroelectric with a ferromagnetic layer was also fabricated for magnetoelectric applications. Chapter 1 provides a brief introduction to ferroelectric (FE) heterostructures, their technological applications and the fundamental physics involved in ferroelectric heterostructures. Initially an introduction to the technological importance and advantages of ferroelectric heterostructures is provided. A brief introduction to relaxor ferroelectrics and their characteristic structural features are discussed. A brief review of the ferroelectric heterostructures both from fundamental science and technological point of view is provided. Finally the specific objectives of the current research are outlined. Chapter 2 deals with the various experimental studies carried out in this research work. It gives the details of the experimental set up and the basic operation principles of various structural and physical characterizations of the materials prepared. A brief explanation of material fabrication, structural, micro structural and physical property measurements is discussed. Chapter 3 addresses the problem of phase formation of PMNPT over platinum substrates and the role of the template over the phase formation, micro structural evolution and polarization behavior. The surface modifications of bare Pt under the processing conditions used to fabricate PMNPT was also studied. An intermediate roughening mechanism was observed. The role of LSCO over the micro structural evolution of PMNPT, the minimum thickness of LSCO required for phase formation of PMNPT, role of LSCO on phase formation and its effect on the polarization behavior of PMNPT of constant thickness are discussed. Chapter 4 deals with fabrication of different types of relaxor based heterostructures studied in this work. Three different types of PMNPT based heterostructures was fabricated using a multi target laser ablation chamber. The first type of heterostructure is a compositionally modulated multilayer thin film with four different compositions of (1-x) PMN - x PT (x = 0.0, 0.1, 0.2, 0.3 at.%) and is represented as PMNPT multilayer (ML) further in this thesis. PMNPT ML with different individual layer thickness was fabricated (30, 40, 60, 80, 100 and 120 nm). The second type of heterostructure is the PMNPT ML of same dimensions, but associated with a post deposition annealing to achieve a graded interface between the multilayers present and will be named as PMNPT graded or simply graded, further in this thesis. The third type of heterostructure is an artificial superlattice of a simple relaxor ferroelectric (PMN) and a normal ferroelectric (PT), which will be named as PMN-PT superlattice (SL) further in this thesis. The crystallinity, micro structural features and the nature of the interface present in the fabricated heterostructures were studied using various experimental techniques. Chapter 5 deals with the FE studies of compositionally modulated PMNPT ML thin films and PMNPT graded thin films. The ML with individual layer thickness of 120nm exhibited a clear FE behavior but with a reduced remnant polarization and reduced non linear behavior in capacitance - voltage (C-V) characteristics. But on varying the dimensions of the individual layers (30, 40, 60, 80, 100 and 120nm) a large dielectric tunability of around 74% was observed at lOOnm. The polarization behavior of these ML exhibited an interesting size dependent polarization behavior. A FE behavior was observed at low dimensions of 40 and 30nm. An AFE type of loop was observed at 60 and 80nm of individual layer thickness and at lOOnm it showed a clear paraelectric kind of behavior both in polarization hysteresis (P-E) and C-V studies. Graded films exhibited clear FE behavior at all dimensions fabricated and hence the role of interface in developing a critical polarization behavior in the case of ML was confirmed. Apart from the fundamental physics these ML and graded films permits the tunability of their physical properties on just varying the individual layer thickness. The dimensional dependence of dielectric tunability of ML and graded films were studied and it was found that in the case of a ML the dielectric tunability was high at lOOnm individual layer thickness and at 40nm in the case of a graded film. Thus the interfacial strain, interfacial coupling and chemical heterogeneity give an opportunity to engineer the physical property depending on the requirements. Chapter 6 deals with ferroelectric studies (P-E, C-V) of PMN-PT superlattice structures with different periodicities. The dimensional range in which, the interfacial coupling dominates the overall polarization behavior of the system was analyzed. A dimensional dependent FE and AFE behavior was observed in the PMN-PT SL structures. The dimensional dependent tunability of physical properties was achieved. The different interactions like short range, long range and the interfacial coupling and their dimensional dependent behavior was studied. The dimensional dependent tunability of the P-E and C-V behavior was observed both in symmetric and asymmetric SL structures. Chapter 7 deals with the relaxor behavior of the fabricated PMNPT ML, graded and PMN-PT SL structures. The dielectric phase transition of a PMNPT ML exhibited local maxima in the real part of dielectric constant with temperature. The local maxima correspond to the temperature regime at which, the individual layer dielectric maxima dominates the phase transition behavior of the ML structure. In the case of graded films an averaged behavior of all the compositions, with an enhanced diffusivity was observed. All the characteristic features of a relaxor ferroelectric were observed in the phase transition behavior of a graded thin film. The dielectric maxima exhibited a Vogel-Fulcher type of behavior with frequency, A similar averaged behavior was observed in the phase transition behavior of PMNPT ML at low dimensions (< 40 nm) of the individual layer. The dielectric phase transition behavior of PMN-PT SL structures of different periodicities was studied. No characteristic of a relaxor ferroelectric was observed for the periodicities in the range of 10 to 50 nm. At 60 nm periodicity the individual layer dominance was observed in the phase transition behavior of the SL structure. The phase transition behavior was found to be insensitive to the interfacial coupling in both the PMNPT ML and PMN-PT SL. Chapter 8 deals with the dielectric response, impedance spectroscopy and the DC leakage characteristics of the relaxor heterostructures. All the relaxor heterostructures fabricated, exhibited low frequency dispersion, similar to that of the Jonscher's universal type of relaxation behavior. The anomalous dispersion common of a relaxor ferroelectric was observed in the imaginary dielectric constant at high frequencies. A.multi debye type of relaxation behavior was observed in the impedance analysis and the relaxation time was found to obey Vogel-Fulcher type of relation with temperature. The leakage current of all the heterostructures were found to be few orders less than the homogeneous single layer thin films. A space charge limited conduction was observed in all the heterostructures fabricated. Chapter 9 deals with an attempt of realizing the magnetoelectric effect in an artificial superlattice structure consisting ferromagnetic [Lao.6Sro.4Mn03 (LSMO)] and ferroelectric (PMNPT 70-30) layers. Both symmetric and asymmetric SL structures were fabricated and the asymmetric SL exhibited both room temperature ferromagnetic and ferroelectric behavior. A weak influence of magnetic field over the polarization behavior was observed. The magnetic behavior and its influence over electrical behavior were found to be dominated by the interface and were confirmed from the Maxwell-Wagner type of relaxation. Chapter 10 gives the summary and conclusions of the present study and also discusses about the future work that could give more insight into the understanding of the relaxor heterostructures.

Relaxor ferroelectrics have been a subject of intense research owing to their interesting physical properties such as high dielectric constant and giant electro-striction. Unlike the conventional lead based relaxors, the relaxors belonging to Aurivillius family of oxides have received much less attention because of the poor understanding of the origin of the relaxor behavior and high processing temperatures involved. In the present investigations, an attempt has been made to understand the origin of relaxor behavior of the materials belonging to Aurivillius family of oxides. The structure and relaxor behavior of BaBi2Nb2O9 (BBN) has been established via the XRD, electron diffraction and dielectric spectroscopy. The results are compared with that of a normal ferroelectric like SrBi2Nb2O9 belonging to the same family as well with that of a conventional relaxor like PMN. The results indicate that the dielectric behavior of BBN is significantly different from that of the conventional relaxors like BBN with very slow broadening of relaxation times and was attributed to the absence of significant polar ordering. To substantiate the existing understanding, studies have been carried out by adopting different strategies such as B-site and A-site cationic substitutions and texturing of the ceramics. Vanadium doping on B-site was found to decrease the sintering temperatures significantly. Aliovalent La3+ doping was found to affect the dielectric behavior strongly with substantial decrease of the freezing temperature and dielectric constants which shows that the relaxor behavior of BBN is highly sensitive to A-site order-disorder. The (00l) textured ceramic of pure and vanadium doped BBN was fabricated via a simple melt-quenching technique and was found to exhibit a significant dielectric and pyroelectric anisotropy. A new class of relaxor compositions (K0.5La0.5Bi2Nb2O9 & K0.5La0.5Bi2Ta2O9) have been synthesized and characterized. These new compounds exhibited interesting physical properties which are akin to that of the conventional lead based relaxors. The presence of superlattice reflections in the electron diffractin patterns recorded on these compounds establish the presence of polar nano regions of significant size. These relaxor crystallites at nano/micro level embedded in a glass matrix have been found to be very promising from their physical properties view point.

The present research work mainly focuses on the fabrication of 0.85PMN-0.15PT thin film relaxor ferroelectrics for energy harvesting applications. Chapter 1 gives a brief review about why energy harvesting is required and the different ways it can be scavenged. An introduction to relaxor ferroelectrics and their characteristics structural features are discussed. A brief introduction is given about charge storage, electrocaloric effect , DC-EFM and integration over Si substrate is discussed. Finally, the specific objectives of the current research are outlined. Chapter 2 deals with the various experimental studies carried out in this research work. It gives the details of the experimental set up and the basic operation principles of various structural and physical characterizations of the materials prepared. A brief explanation of material fabrication, Microstructural and physical property measurements is discussed. Chapter3 involves the optimization process carried out to contain a phase pure PMN-PT structure without any pyrochlore phase. The optimization process is an important step in the fabrication of a thin film as the quality of any device is determined by their structural and Microstructural features. XRD, SEM, AFM were used to characterize the observed phase formation in these films. The optimizing domain images of polycrystalline 0.85PMN-0.15PT thin films on La0.5Sr0.5CoO3/ (111) Pt/TiO2/SiO2/Si substrates deposited at different oxygen partial pressures are presented. The oxygen pressure has a drastic influence on the film growth and grain morphology which are revealed through XRD and SEM characterization techniques. The presence of oxygen vacancies have found to influence the distribution of polar nanoregions and their dynamics which are visualized using domain images acquired by DC-EFM In Chapter 7 the piezoelectric response of 0.85PMN-0.15PT thin films are studied due to the electric field induced bias. From this the d33 value is calculated. d33 value is an important parameter which determines whether a material is suitable for device application (PZT). But, for a device fabrication it is important to integrate them with Si wafer which is not a straightforward work .Hence, buffer layers are used to obtain a pure perovskite PMN-PT film. We have deposited 0.85PMN-0.15PT thin films of 500 nm on a SOI wafer and tried to investigate their piezoelectric application. Chapter 8 summarizes the present study and discusses about the future work that could give more insight into the understanding of the0.85PMN-0.15 PT relaxor ferroelectric thin film.

Single self-assembled semiconductor quantum dots (QDs) are able to emit single-photons and entangled-photons pairs. They are therefore considered as potential candidate building blocks for quantum information processing (QIP) and communication. To exploit them fully, the ability to precisely control their optical properties is needed due to several reasons. For example, the stochastic nature of their growth ends up with only little probability of finding any two or more QDs emitting indistinguishable photons. These are required for two-photon quantum interference (partial Bell-state measurement), which lies at the heart of linear optics QIP. Also, most of the as-grown QDs do not fulfil the symmetries required for generation of entangled-photon pairs. Additionally, tuning is required to establish completely new systems, for example, 87Rb atomic-vapors based hybrid semiconductoratomic (HSA) interface or QDs with significant heavy-hole (HH)-light-hole (LH) mixings. The former paves a way towards quantum memories and the latter makes the optical control of hole spins much easier required for spin- based QIP. This work focuses on the optical properties of a new type of QDs optimized for HSA experiments and their broadband tuning using strain. It was created by integrating the membranes, containing QDs, onto relaxor-ferroelectric actuators and was quantified with a spatial resolution of ~1 µm by combining measurements of the µ-photoluminescence of the regions surrounding the QDs and dedicated modeling. The emission of a neutral exciton confined in a QD usually consists of two fine-structure-split lines which are linearly polarized along orthogonal directions. In our QDs we tune the emission energies as large as ~23meV and the fine-structure-splitting by more than 90 µeV. For the first time, we demonstrate that strain is able to tune the angle between the polarization direction of these two lines up to 40° due to increased strain-induced HH-LH mixings up to ~55%. Compared to other quantum emitters, QDs can be easily integrated into optoelectronic devices, which enable, for example, the generation of non-classical light under electrical injection. A novel method to create sub-micrometer sized current-channels to efficiently feed charge carriers into single QDs is presented in this thesis. It is based on focused-laserbeam assisted thermal diffusion of manganese interstitial ions from the top GaMnAs layer into the underlying layer of resonant tunneling diode structures. The combination of the two methods investigated in this thesis may lead to new QDbased devices, where direct laser writing is employed to preselect QDs by creating localized current-channels and strain is used to fine tune their optical properties to match the demanding requirements imposed by QIP concepts.