In this thesis, the results pertaining to various investigations carried out on Triglycine sulphate (TGS) single crystals, polyvinylidene fluoride (PVDF) films, lithium tantalate (LT)/PVDF nanocomposites and LT thin films are presented with emphasis on the characteristics that are crucial for their use in pyroelectric sensors. TGS single crystals (size 68 x 45 x 42 mm3), which have high pyroelectric coefficients, were grown by slow cooling method using newly designed platform technique based crystal growth work stations. The problem of slow growth rate along c-direction was overcome by placing (010) oriented seeds on the platform. The grown TGS crystals were used for the fabrication of the laser energy meter and temperature sensor. One drawback of TGS is its low Curie temperature (490C). As a consequence when the operating temperature approaches the Curie temperature, the crystals start depolarizing owing to the movement of domains. As a result the linearity of the devices gets affected and restricts the use of TGS. Therefore pyroelectric materials possessing higher Curie temperatures and larger pyroelectric coefficients than that of TGS are desirable. LT in single crystalline form having Curie temperature of ≈6000C has already been in use for pyroelectric device applications. However, growing stoichiometric LT single crystal is very difficult. On the other hand PVDF polymer films (Tc≈1800C) have low pyrolectric coefficients and difficult to pole electrically. Therefore efforts were made to prepare LT/PVDF nanocrystal composites to increase the pyroelectric coefficient of PVDF and to reduce the poling field. Nanoparticles of LT were prepared using sol-gel route. Spherical nanoparticles of size 20-40nm were prepared from sol by adding oleic acid to it. These nanoparticles were characterized using XRD, TEM, DSC and Raman spectroscopy. PVDF films with large percentage of β-phase (ferroelectric phase) were fabricated from solutions prepared using dimethylsulphoxide (DMSO) solvent. PVDF films (30µm thick), embedded with 20-40nm sized nanocrystallites of LT were fabricated to utilize them for pyroelectric sensor applications. The ferroelectric and pyrolectric properties of nano composite films were studied for sensor applications point of view. As a replacement for the single crystals of LT in pyroelectric sensors, investigations were carried out on oriented LT thin films. The studies on LT thin films yielded promising results which could be exploited for pyroelectric sensor applications.

Ferroelectric materials, especially the polycrystalline ceramics, are very promising material for a variety of applications such as high permittivity dielectrics, ferroelectric memories, piezoelectric sensors, piezoelectric/electrostrictive transducers, electrooptic devices and PTC thermistors. Among the ferroelectric based piezoelectric ceramics the lead–zirconate-titanate Pb(Zr1-xTix)O3 (PZT) have dominated transducer and actuator market due to its excellent piezoelectric and dielectric properties, high electromechanical coupling, large piezoelectric anisotropy, ease of processing and low cost. However, the toxicity of lead based compounds has raised serious environmental concerns and therefore has compelled the researchers to look for new lead free alternatives with good piezoelectric and ferroelectric properties. (Na0.5Bi0.5)TiO3 (NBT) and its solid solution is one of the leading lead free piezoceramic ceramics due to their interesting ferroelectric, piezoelectric, electromechanical and dielectric property. The parent compound NBT is a ferroelectric with a moderately high Curie temperature (~250 oC), large ferroelectric polarization (~40µC/cm2) polarization, promising piezoelectric properties with 0.08% strain and longitudinal piezoelectric coefficient (d33) ~ 80 pC/N. X-ray and neutron diffraction studies in the past have shown that NBT exhibits rhombohedral (R3c) at room temperature. Neutron diffraction studies have suggested that NBT undergo a gradual rhombohedral to tetragonal (P4bm) transformation in a temperature region 200-320 ºC. Though the structure and phase transition behavior of NBT has been extensively investigated for over six decades now, this subject has again become debatable in recent few years, with some group reporting formation of orthorhombic phase above room temperature and another group suggesting monoclinic distortion at room temperature using high resolution x-ray diffraction technique. Interestingly the intermediate orthorhombic instability, reported by electron diffraction studies, has never been captured by neutron diffraction method though neutron diffraction is an equally powerful tool for studying (oxygen) octahedral tilts in perovskites. Needless to mention, the understanding of the subtle structural distortions have great significance with regard to the determination of the structure-piezoelectric property correlations in NBT based piezoceramics. The present thesis deals with such subtle structural issues in great detail. The systems investigated in the thesis are Ca and Ba modified NBT. While the Ca modified system was chosen to understand the subtle orthorhombic instability that has been reported above room temperature (only) by detailed electron diffraction work, Ba-modified NBT is the most investigated among the NBT-derived piezoelectric material systems and this thesis attempts to address some of the very complex nature of the structure-piezoelectric property correlation of this system. The first chapter of the thesis provides a brief introduction to the field of ferroelectrics, perovskite structure and their phase transition. A brief exposure to the conventional lead based relaxor ferroelectric and piezoelectric material is provided. A detailed overview of the existing knowledge related to room temperature structure of NBT and its phase transition studies with temperature has been discussed in the later part of this chapter. The second chapter includes various the experimental techniques that have been employed to synthesis and characterize the specimens under investigation. The third chapter deals with the phase transition behaviour of Ca modified NBT as a function of composition and temperature in the dilute concentration region. This work was carried out with the view to obtain a better understanding and compliment the intrinsic high temperature orthorhombic instability in NBT reported by electron diffraction technique. Interestingly, inspite of the fact that neutron diffraction method is a very sensitive tool for investigating subtle change in the nature of octahedral tilt in oxide perovskites, the intermediate orthorhombic distortion proposed by the electron diffraction studies has so far never been captured in any of the neutron diffraction studies. In this work we have verified the genuineness of the intrinsic instability with regard to the non-polar orthorhombic structure using neutron powder diffraction by adopting a special strategy which helped in capturing the characteristic signatures (the superlattice reflections) of the orthorhombic phase in the neutron powder diffraction patterns. It was found that small fraction of Ca-substitution (8-10 mol %) was good enough to amplify the magnitude of the orthorhombic (Pbnm) distortion, without altering the sequence of the structural evolution with temperature of the parent compound (NBT) itself, and stabilizing it at the global length scale at lower temperatures than pure NBT. This chapter presents the innovative approach that was used to extract reliable information about the very complex phase transition behaviour, involving coexistence of the various similar looking but crystallographically different phases in different temperature regimes by Rietveld analysis of temperature dependent neutron powder diffraction pattern in conjunction with temperature dependent dielectric and ferroelectric characterization of the specimens. The detailed study revealed the following sequence of structural evolution with temperature: Cc+Pbnm →Pbnm + P4/mbm → P4/mbm →Pm3 m. The fourth chapter gives a detail account of the structure-property correlations and the phase transition behaviour of (1-x)(Na0.5Bi0.5)TiO3 – (x)BaTiO3 (0≤x≤0.10), the most important solid solution series with NBT as reported in the literature. The phase transformation behaviour of this system has been investigated as a function of composition (0<x≤0.10), temperature, electric field and mechanical-impact by Raman scattering, ferroelectric, piezoelectric measurements, x-ray and neutron powder diffraction methods. The structure of the morphotropic phase boundary (MPB) compositions of this system, which is interesting from the piezoelectric property point of view, has been under controversy for long. While some groups report the structure to be pseudocubic, other groups suggest it to be combination of rhombohedral and tetragonal. A perusal of the literature suggests that the reported nature and composition range of MPB is dependent on the method of synthesis and characterization technique used. In the present study, crystal structure of the NBT-BT solid solution has been investigated at the close interval near the MPB (0.05≤x≤0.10). Though x-ray diffraction study revealed three distinct composition ranges characterizing different structural features in the equilibrium state at room temperature: (i) monoclinic (Cc) + rhombohedral (R3c) for 0≤x≤0.05, (ii) “cubic-like” for 0.06≤x≤0.0675 and (iii) MPB like for 0.07≤x<0.10, Raman and neutron powder diffraction studies revealed identical symmetry for the cubic like and the MPB compositions. Both the cubic like compositions and the MPB compositions exhibit comparatively large d33. In the later part of this chapter this apparent contradiction is resolved by the fact that the cubic like structure transforms irreversibly to MPB after electric poling, a procedure which involves applying high dc electric field (well above the coercive field) to the pellet before carrying out the piezoelectric measurements. The effect of electrical field and mechanical impact has been studied for all the three different composition range, and it was found that electric field and mechanical impact both led to irreversible phase transformation in the same direction, though the transformation with mechanical impact remains incomplete in comparison to electric field. The most pronounced effect was observed for the cubic like compositions 0.06≤x≤0.0675 – they undergo phase separation to rhombohedral and tetragonal phases by electrical and mechanical perturbations. In the non-perturbed state the cubic-like critical compositions mimics features of relaxor ferroelectrics and extremely short coherence length (~ 40-50 Å) of the out-of-phase octahedral tilts. In the poled state this coherence length grows considerably and the system behaves like a normal ferroelectric. This confirmed a strong coupling between the lattice, octahedral tilts and polarization degrees of freedom. Neutron diffraction study of compositions exhibiting cubic-like and the MPB like revealed that the traditional P4bm tetragonal structure model fails to account for the intensity of the superlattice reflections. Thus the tetragonal structure stabilized above room temperature in pure NBT is different from the tetragonal phase observed at room temperature in the NBT-BT system. The results of the effect of mechanical impact and electric field has also been reported in this chapter for the critical composition exhibiting MPB (x=0.07). A detailed structural analysis of the precritical compositions, x≤0.05, revealed coexistence of ferroelectric phases (Cc+R3c) in equilibrium state (annealed specimens). This transforms to single phase (R3c) state after poling. Thus though the precritical (x≤0.05) and critical compositions (0.06≤x<0.10) of NBT-BT exhibits coexistence of ferroelectric phases in the equilibrium state, the fact that the electric poling makes the specimen single phase, R3c, after poling for the precritical compositions and retains the two phase nature of the critical compositions makes the critical compositions exhibit considerably higher piezoelectric response than the precritical compositions. Chapter five is dedicated to phase transition behaviour of the post critical compositions of (1-x)(Na0.5Bi0.5)TiO3–(x)BaTiO3 (0.16≤x≤1) using temperature dependent XRD, dielectric and ferroelectric studies. Though structurally the entire composition range is tetragonal, several notable features were revealed during detailed examination of the structural and dielectric behaviour. This study is also important from the view point that pure BT is a major component of multilayer ceramic capacitors and that an increase in the Curie point would be a welcome step for better temperature stability of the device. NBT does this. The transition temperature increases from 120 ºC for pure BT to 275 ºC for x=0.30 along with simultaneous increase in c/a ratio from 1.009 (pure BT) to 1.02 (x=0.30). Detailed analysis of temperature and frequency dependent dielectric data revealed deviation from Curie-Weiss and suggests a gradual transformation to relaxor-ferroelectric state as the NBT concentration increases in BT. The measure of frequency dispersion ‘γ’ parameter was determined from modified Curie-Weiss law for various compositions in the system. The ferroelectric and piezoelectric properties have also been investigated in detail for this composition range and an attempt has been made to correlate the composition variation of these properties with their structural parameters. This chapter shows a systematic correlation between all physical quantities such as Curie point, piezoelectric coefficient, polarization and tetragonality as a function of composition.

Les principaux objectifs de cette thèse de doctorat ont été de réaliser la synthèse et la caractérisation structurale et dieléctrique des échantillons céramiques ferroélectriques appartenants au système Pb1-xRxZr0,40Ti0,60O3 avec R = Ba et La et x entre 0,00 à 0,50. Ce système a été choisi car il est un matériel ferroélectrique qui a des propriétés physiques intéressantes, comme haute constante diélectrique et piézo-électrique, ce qui les rend candidats potentiels pour des applications telles que les condensateurs à haute densité d'énergie et les actionneurs. Afin d'évaluer le comportement relaxor, les études ont été effectuées avec la variation de la composition, du type de dopage (par des atomes de la même ou différente valence – La ou Ba) et de la taille des particules de céramique, dès l'échelle micrométrique à l'échelle nanométrique. Les échantillons céramiques micrométriques ont été préparées par la méthode de réaction de l'état solide et la frittage dans un four électrique conventionnel. Les données fournies par la technique de diffraction des rayons X de cettes échantillons ont montré une transition de une phase tétragonal pour une phase cubique avec l'augmentation de la concentration de cations substituants. Ces changements ont été attribués à une diminution de distorsion dans le maille cristallographique en raison de l'apparition de défauts causés par l'incorporation de dopage. Les mesures électriques ont été obtenues par spectroscopie d'impédance et ont montré un comportement électrique relaxor à partir de compositions avec plus de 12% at. La et de 30% at. Ba pour les systèmes PLZT et PBZT, respectivement. Les mesures électriques de l'échantillon avec 12%, 13% et 14% at. La et 30% at. Ba présentent un comportement qui, selon la littérature, est liée à une transition de phase spontanée d'un comportement relaxor et au comportement d'un matérial ferroélectrique normal. La technique de diffraction des rayons X a également été utilisé pour surveiller le processus de transition de phase en fonction de la température pour échantillons PLZT et PBZT. Il est possible de voir le changement de structure tétragonal de groupe d'espace P4mm en structure cubique de groupe d'espace Pm-3m. En ce qui concerne la structure locale, nous avons effectué des mesures expérimentales avec la technique de spectroscopie d'absorption des rayons X dans le spectre XANES aux seuils d'absorption de différents éléments pour les échantillons PLZT et PBZT. Dans les cas de seuil d'absorption K du titane, l'intégration de La et Ba atomes de la structure du PZT entraîne une diminution dans le désordre local dans le octaèdre TiO6, vérifié par la réduction du déplacement statique de atome Ti au centre de l'octaèdre TiO6. Cette évolution est plus faible pour les échantillons que montrent le comportement relaxor. Les spectres d'absorption EXAFS au seuil LIII du plomb et seuil K du zirconium ont été effectués aussi et ces mesures indiquent que la structure locale autour des atomes de plomb ou de zirconium est également affectée par l'introduction des atomes de La et Ba dans la structure. Le comportement relaxor a été aussi étudié en fonction de la taille de grain dans une échelle nanométrique. Ainsi les échantillons de compostions PZT, PLZT11 et PBZT10 ont eté préparés en utilisant la méthode de synthèse chimique de polymères précurseurs et le processus de frittage par spark plasma. La caractérisation de ces échantillons par diffraction de rayons X montrent que les paramétres de maille réduisent en comparison avec les échantillons de même composition et taille de grain micrométrique. Pour l'échantillon de composition PLZT11, il est possible de constater le comportement relaxor par les mesures de la constante dieléctrique en fonction de la température. Les changements quand la taille de grain est dans une échelle nanométrique sont attribués à la limitation des frontières de grains, qui provoquent un systéme de tension, responsable de la diminuition des paramétres de maille, et provoquent l'apparition de domaines ferroélectriques nanométriques / The main objectives of this doctoral thesis were the synthesis and structural characterization of Pb1-xRxZr0.40Ti0.60O3 ferroelectric ceramic samples, with R = Ba and La and x between 0.00 to 0.50. This system was chosen because its interesting physical properties such as high dielectric and piezoelectric constant. These characteristics make it potential candidate for applications such as capacitors in high energy density and actuators. To evaluate the relaxor behavior, the studies were carried out with the change in the composition, type of doping (by atoms of the same or different valence – La or Ba) and the particle size of ceramics, from the micrometer to nanometer scale. Micrometric ceramic samples are prepared by the method of reaction of solid state and sintering in a conventional furnace. The characterization with X-ray diffraction technique of these samples showed a transition from tetragonal phase to a cubic phase with increase of the dopping cation concentration. These changes have been attributed to the appearance of defects caused by the incorporation of La or Ba cations. Electrical measurements were obtained by impedance spectroscopy and showed a electric relaxor behavior from compositions with more than 12 at. % of La and the 30 at. % of Ba for PLZT and PBZT systems, respectively. These measurements for the samples with 12 at. %, 13 at. % and 14 at. % of La and 30 at. % of Ba exhibit a behavior that, according to the literature, is related to a spontaneous phase transition from a relaxor behavior to a normal ferroelectric behsvior. The technique of X-ray diffraction also been used to monitor the phse transition phase as a function of the temperature for PLZT and PBZT samples. It is possible to note the change in a tetragonal structure with P4mm space group to a cubic structure with Pm-3m space group. Concerning the local structure, XANES spectra in the absorption edge of various elements in PLZT and PBZT samples were performed. In the cases of Ti K-edge absorption, the dopping of La and Ba atoms in the PZT structure leads to a decrease of the local disorder in the TiO6 octahedron and it is verified the reduction of static displacement of Ti atom in the center of the TiO6 octahedron. This displacement is lower for samples that show relaxor behavior. The EXAFS measurements in Pb LIII-edge and Zr K-edge were performed and also indicate that local structure around lead or zirconium atoms is also affected by the introduction of La and Ba atoms in the PZT structure. The relaxor behavior was also studied depending on the size of particle size in a nanometer scale. Thus samples PZT, PLZT11 and PBZT10 compositions were prepared using the synthesis method of precursor polymers and the process of sintering by spark plasma. Characterization of these samples by X-ray diffraction shows that the lattice parameters are reduced in comparison with samples of the same composition and micrometer particle size. For PLZT11 composition, it is possible to observe a relaxor behavior by measurement of the dielectric permittivity as a function of the temperature. These changes when the grain size is in a nanoscale are attributed to the grain boundaries, that are responsible for the decrease in the lattice parameters and the appearance of ferroelectric nanodomains

Les recherches que nous présentons dans ce mémoire s'inscrivent dans le cadre du développement de nouvelles structures et de l'étude de matériaux accordables en vue d'une intégration industrielle comme radôme actif.Plus particulièrement, ils consistent en la réalisation d'un dispositif micro-onde permettant à la fois un filtrage et une agilité fréquentiels en espace libre. Des structures basées sur des surfaces sélectives en fréquences, pour l'aspect filtrage, et sur des matériaux de type ferroélectrique, pour l'aspect accordabilité, sont développées. Des modélisations et des simulations électromagnétiques montrent que le changement de permittivité du matériau, obtenu par application d'un champ électrique externe, permet le pilotage fréquentiel de la transmission de la structure. Une mise en oeuvre expérimentale complète ces travaux, au cours de laquelle des prototypes ont été fabriqués par des techniques de photolithographie, puis caractérisés en espace libre grâce à un banc ABmm. Les mesures micro-ondes valident ainsi les résultats de simulations menées en amont et montrent les possibilités de contrôler la fréquence de transmission du radôme. / The research we present in this memory registers within the framework to develop new structures and to study tunable materials for an industrial integration as an active radome. Specifically, they consist of achieving a free space microwave device for both a filtering behaviour and a frequency agility behaviour. Structures based on frequency selective surfaces, for the filtering aspect, and on ferroelectric materials for the tuning aspect, are developed. Modeling and simulations show that the change of the material permittivity, obtained by applying an external electric field, enable piloting the transmission frequency of the structure. An experimental implementation complete this work and prototypes have been fabricated by photolithography techniques and then characterized in free space with a bench ABmm. Thus, microwave measurements validate the results of simulations and show the possibility to control the frequency transmission of the radome.

O titanato e zirconato de chumbo \'PB\'(\'ZR\'1-y\'TI\'y)\'O IND.3\' é um material ferroelétrico de estrutura perovskita que tem sido aplicado como transdutores, amplificadores, sensores piezoelétricos, piroelétricos e memórias ferroelétricas. É bem estabelecido que a incorporação de íons de \'LA POT.3+\' ou \'BA POT.2+\' nos sítios ocupados pelo \'PB\' no sistema \'PB\'(\'ZR\'1-y\'TI\'y)\'O\' (PZT), formando os sistemas \'PB\'1-x\'LA\'x\'ZR\'1-y\'TI\'y\'O IND.3\' (PLZT) e \'PB\'1-x\'BA\'x\'ZR\'1-y\'TI\'y\'O IND.3\' (PBZT), provoca mudanças significativas nas suas propriedades. No entanto, poucos trabalhos tem sido dedicados a esses sistemas contendo altas concentrações de \'TI\', principalmente no que se refere à estrutura desses materiais. Assim, este trabalho teve por objetivo analisar as propriedades estruturais e suas correlações com as propriedades dielétricas dos sistemas \'PB\'1-x\'LA\'x\'ZR\'0,40\'TI\'0,60\'O IND.3\' (PLZT100x) e \'PB\'1-x\'BA\'x\'ZR\'0,40\'TI\'0,60\'O IND.3\' (PBZT100x) em função da composição e da temperatura. Foram preparadas amostras cerâmicas por meio de sinterização convencional com x variando entre 0,05 e 0,21 para o sistema PLZT e entre 0,10 e 0,50 para o sistema PBZT. Em relação à estrutura a longa distância, medidas de difração de raios X mostraram uma diminuição no grau de tetragonalidade com o aumento da concentração dos cátions substituintes, que foi atribuída à formação de vacâncias no sítio A (caso do \'LA\') e diferença entre o raio iônico (caso do \'BA\'). Estas alterações estruturais em função da composição foram também responsáveis pelo aumento do grau de difusidade das curvas de permissividade dielétrica e pela observação de um estado ferroelétrico relaxor nas amostras contendo altas concentrações de \'LA\' e \'BA\'. Em relação à estrutural local, os resultados obtidos através da técnica de espectroscopia de absorção de raios X (XAS) nas bordas \'K\' do \'TI\' e LIII do \'PB\' mostraram que a incorporação de átomos de \'LA\' ou \'BA\' à estrutura do PZT leva a uma redução no deslocamento do átomo de \'TI\' em relação ao centro do octaedro \'TI\'O IND.6\' e mudanças na ordem local do átomo de \'PB\'. No que tange as composições contendo 21% at. de \'LA\' e 50% at. de \'BA\', diferentemente dos resultados de DRX que mostraram uma simetria cúbica, a técnica de XAS mostrou uma simetria local tetragonal. Em bom acordo com os resultados obtidos pela técnica de espectroscopia Raman, espectros EXAFS medidos em altas temperaturas mostraram também que a estrutura local não é compatível com uma estrutura de simetria cúbica. Espectros XANES medidos na borda \'K\' do oxigênio revelaram uma redução no grau de hibridização entre os estados 2p do \'O\' com 6sp do \'PB\' à medida que a concentração de \'LA\' ou \'BA\' aumenta, que estaria relacionada com o surgimento de comportamento relaxor. Amostras cerâmicas densas nanoestruturadas de composição PZT, PLZT11 e PBZT10 foram preparadas pelo método de spark plasma sintering (SPS) a fim de analisar a influência do tamanho de grão. Foi verificado que as amostras sinterizadas por SPS apresentam tamanho de grão em torno de 60 nm. A caracterização dielétrica destas amostras mostra que a redução do tamanho de grão causa uma redução no valor de máximo da permissividade dielétrica e características difusas da permissividade em função da temperatura devido ao aumento das regiões de contorno de grão. / Lead titanate zirconate (\'PB\'(\'ZR\'1-x\'TI\'x)\'O IND.3\') are ferroelectric materials with perovskite structure which has been used as transducers, capacitors, piezoelectric and pyroelectric sensors and ferroelectric memories. The substitution of \'PB POT.+2\' ions by \'LA POT.+3\' or \'BA POT.+2\' ions in the \'PB\'(\'ZR\'1-x\'TI\'x)\'O IND.3\' (PZT) system, which leads to the formation of the \'PB\'1-x\'LA\'x\'ZR\'1-y\'TI\'y\'O IND.3\' (PLZT) and the \'PB\'1-x\'BA\'x\'ZR\'1-y\'TI\'y\'O IND.3\' (PBZT) systems, induces several changes in the electric and structural properties of these materials. However, PLZT or PBZT systems based on \'TI\'-rich compositions have not been thoroughly investigated and the literature contains few reports concerning their structure. Thus, the main objectives of this doctoral thesis were the synthesis and structural characterization of \'PB\'1-x\'R\'x\'ZR\'0.40\'TI\'0.60\'O IND.3\' ferroelectric ceramic samples, with R = \'BA\' and \'LA\' and x between 0.00 to 0.50 (PLZT100x and PBZT100x). The characterization with X-ray diffraction technique of these samples showed a decrease of the tetragonality degree with increase of the doping cation concentration, which was related to the appearance of defects caused by the incorporation of \'LA\' or \'BA\' cations. These structural modifications were also responsible by the increase of the diffuseness at the dielectric permittivity and a relaxor behavior as a function of the \'LA\' or \'BA\' concentration. Concerning the local structure, XANES spectra in the absorption edge of various elements in PLZT and PBZT samples were performed. In the cases of \'TI\' \'K\'-edge absorption, the doping of \'LA\' and \'BA\' atoms in the PZT structure leads to a reduction of the displacement of \'TI\' atom in the center of the \'TI\'O IND.6\' octahedron. However, even when the crystal structure is cubic, a local octahedron distortion remains. EXAFS measurements in \'PB\' LIII-edge and \'ZR\' \'K\'-edge were performed and also indicate that local structure around lead or zirconium atoms is also affected by the introduction of \'LA\' and \'BA\' atoms in the PZT structure. In addition, XANES spectra measured at \'O\' \'K\'-edge revealed a reduction in the hybridization degree between \'O\' 2p and \'PB\' 6sp states with the addition of \'LA\' or \'BA\' atoms to the structure of PZT. It has been shown that hybridization between these states is essential to ferroelectricity and this reduction would be related to the relaxor behavior. PLZT and PBZT systems were also studied depending on the size of particle size in a nanometer scale. Thus samples PZT, PLZT11 and PBZT10 compositions were prepared using the synthesis method of precursor polymers and the process of sintering by spark plasma. A pronounced decrease in the values of maximum permittivity was observed and the dielectric curve as a function of the temperature exhibits a diffuse behavior. This size-induced diffuse phase transition and the reduction of the permittivity magnitude could be related to the differences between the core grain and the grain boundaries.

Patch antennas are fundamental elements in many microwave communications systems. However, patch antennas receive/transmit signals over a very narrow bandwidth (typically a maximum of 3% bandwidth). Design modifications directed toward bandwidth expansion generally yield 10% to 40% bandwidth. The series varactor tuned patch antenna configuration was the bandwidth enhancement method explored in this research; this configuration is implemented by dividing a patch antenna into multiple sections and placing varactors across the resultant gaps. In addition to yielding a large bandwidth, the configuration has a number of ancillary benefits, including straightforward integration and design flexibility. Through the research represented by this work, the properties of the series varactor tuned patch antenna, herein referred to as the Fragmented Patch Antenna (or FPA), were explored and optimized. As a result, an innovative patch antenna was produced that yielded 63.4% frequency tuning bandwidth and covered a frequency range between 2.8 and 5.4 GHz. The wide bandwidth was achieved through a detailed parametric study. The products of this study were the discovery of multiple tuning resonances that were used to expand the tuning bandwidth and the understanding/documentation of the significance of specific antenna dimensions. Measurement results were obtained through the fabrication of a prototype antenna using semiconductor varactors. In the second research phase, the construction of capacitors using the tunable permittivity material Barium Strontium Titanate (BST) was investigated. Using this material in conjunction with nano-fabrication techniques, varactors were developed that had good estimated performance characteristics and were considered appropriate for integration into adaptive microwave circuitry, such as the tunable antenna system. The varactors were constructed by using Focused Ion Beam (FIB) milling to create a nano-scale capacitive gap in a transmission line. A combination of end-point current detection (EPD) and cross-section scanning electron (SEM) and ion beam (FIB) microscope images were used to optimize the milling procedure. The future extensions of this work include the integration of the BST varactors with the antenna design; the configuration of the developed BST varactors lends itself to a straightforward integration with the FPA antenna.

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.

Fabrication of artificially structured superlattices, when controlled on a nanoscale level, can exhibit enhanced dielectric properties over a wide temperature range. Possible fabrication of new functional devices based on the parametric values of dielectric constants of these heterostructures was the major motivation behind the work. Chapter 1 gives a brief overview of ferroelectrics; their defining features and their commercial importance to electronic industry. An introduction to ferroelectric superlattices, their technological application and fundamental physics that influence the behavior of superlattices are provided. 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 involves fabrication of two-component ferroelectric superlattices consisting of Barium Titanate (BTO), and Strontium Titanate (STO) with nanoscale control of superlattice periodicities by high-pressure multi target pulsed laser deposition on Pt (111)/Ti/SiO2/Si (100) substrate. Superlattices with varying periodicities were fabricated and their compositional variation across the thin film and the interface width were studied using Secondary Ion Mass Spectrometry (SIMS). Fabrications of superlattice structure were supported by observation of satellite peaks in XRD corresponding to the coherent heterostructures. The microstructural analysis was carried out using cross-sectional scanning electron microscopy (SEM), and contact mode-AFM was used to image surface morphology and root-mean-square (rms) roughness of the thin film heterostructure. Chapter 4 deals with ferroelectric studies of BTO/STO superlattices. The size dependent polarization behaviors of the superlattices are shown. The experimental realization of the dimensional range in which, the long-range coupling interaction dominates the overall polarization behavior of the system was studied. The dependence of average spontaneous polarization on the individual layer thickness, temperature and the dimensional range of interaction are discussed. The enhanced non-linear behaviors of the films were measured in terms of tunability. The dielectric phase transition behavior of superlattice structures of different periodicities was studied. Chapter 5 focuses on fabrication of three-component ferroelectric superlattices consisting of Barium Titanate (BTO), Calcium Titanate (CTO) and Strontium Titanate (STO). The fabrications of superlattice structures were confirmed by the presence of satellite reflections in XRD analysis and a periodic concentration of Sr, Ba and Ca throughout the film in Depth profile of SIMS analysis. The microstructural analysis was carried out using cross-sectional scanning electron microscopy (SEM), and contact mode-AFM was used to image surface morphology and root-mean-square (rms) roughness of the thin film heterostructure. The dielectric characteristic and polarization properties of the system are discussed. Large variations of lattice distortion in the consisting layers were achieved by varying the stacking sequence and superlattice periodicity. The influence of interfacial strain on enhancement of ferroelectric polarization was studied. The size dependence and the role of interfaces in the observed enhancements of the dielectric behaviors were highlighted. The tunability of about 55% was achieved in these systems and was higher than any of the single polycrystalline thin film of the constituent materials reported till date. The enhanced dielectric properties were thus discussed in terms of the interfacial strain driven polar region due to high lattice mismatch and electrostatic coupling due to polarization mismatch between individual layers. Chapter 6 deals with the dielectric response, impedance spectroscopy and the DC leakage characteristics of the superlattice structures. All the heterostructures fabricated, exhibited low frequency dispersion, similar to that of the Jonscher’s universal type of relaxation behavior. The anomalous dispersion was observed in the imaginary dielectric constant at high frequencies. A Debye type relaxation behavior was observed in the impedance analysis at low temperatures, whereas, a departure from ideal ‘Debye’ type was noticed as the temperature was increased. The leakage currents of all the heterostructures were found to be a few orders less than the homogeneous single layer thin films. A space charge limited conduction was observed in al the superlattice structures fabricated. Chapter 7 summarizes the present study and discusses about the future work that could give more insight into the understanding of the ferroelectric perovskite heterostructures.

The group of materials with ABO3 type perovskite structure are very important due to their attractive electrical and magnetic properties for technological applications and have been studied in the form of single crystals, bulk polycrystalline materials and thin films. Recently, efforts have been made to synthesize and understand the growth of ABO3 type perovskite nanostructures because of their distinctive physical properties and potential applications in the nanodevices. The primary aim of the present thesis is to synthesize the perovskites at nano-scale, with zero-dimension (0D), and one-dimension (1D) configurations. Basic work was carried in terms of synthesis – structure – composition correlation. Due to the small nature of the synthesized materials, few attempts were done to examine the physical properties, but to a limited extant. Efforts were also done to emphasize the structural behavior of nano perovskite in comparison with their bulk counterparts. Chapter 1 provides a brief introduction to perovskite materials and nanostructures, their technological applications and the fundamental physics involved. A brief review of the perovskite nanostructures 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 experimental studies carried out in this thesis. It describes the methods used for the synthesis, experimental set up and the basic operation principles of various structural and physical characterizations such as X-ray diffraction (XRD), thermal analysis, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), compositional analysis (EDX), focused ion beam (FIB), electrical and magnetic studies of the materials prepared. Chapter 3 describes the fabrication of porous anodic aluminum oxide (AAO) templates with different pore size, basic steps for synthesis of nanotubes and the possible growth mechanism of nanotubes in the AAO template. In chapter 4, we report the synthesis of ferroelectric Ba1-xSrxTiO3 (x = 0.0, 0.3) nanoparticles (diameter range: 20-40nm) and Ba1-xSrxTiO3 (x = 0.0, 0.4) nanotubes with diameter about 200nm by the sol-gel method. The Ba1-xSrxTiO3 nanostructures so obtained were characterized by number of techniques, including FE-SEM, XRD, DTA/TGA, FTIR spectroscopy, TEM, HRTEM as well as EDX and SAED. Formation of Y-junctions and multi-branches in Ba1-xSrxTiO3 nanotubes were also observed. The wall of the nanotubes were found to be made of randomly oriented nanoparticles which were confirmed from the HRTEM image. The average thickness of the wall of the nanotubes was found around 15(±5) nm and nanoparticles consisting the wall were found to be in the range of 5-10nm. Diffused phase transition (cubic to tetragonal), shifted to lower temperature side and leaky ferroelectric P–E loops were observed in Ba1-xSrxTiO3 (x = 0.0) ceramic prepared from nanoparticles. Curie temperature was observed at 120oC in the BT nanotube array as confirmed by the dielectric study. The P–E loops of as-prepared Ba1-xSrxTiO3 (x = 0.0) nanotube array were also measured and the hysteresis clearly demonstrates the room temperature ferroelectricity in the as prepared nanotubes, indicating these nanotube array is potential media as ferroelectric information storage. In chapter 5, we report the synthesis of single crystalline nanoparticles and polycrystalline nanotubes of Pb0.76Ca0.24TiO3 (PCT24) by sol-gel processing and characterized by various techniques. The crystallinity and phase purity of the PCT24 nanoparticles and nanotubes were confirmed by the XRD and SAED pattern. Compositional homogeneity and their crystalline structure confirms the formation of the tetragonal perovskite phase. The wall of the nanotubes was found to be made of nanoparticles which were confirmed from the HRTEM analysis. The average thickness of the wall of the nanotubes was found around 20nm and nanoparticles consisting the wall were found to be in the range of 5-8nm. Formation of some single crystalline PCT24 nanorods was also observed as confirmed by SAED and HRTEM analysis. Formations of Y-junctions and multi-branches in this complex functional oxide were observed. Dielectric measurements shows the diffuse phase transition and frequency dependence of Tm (temperature at which real part of dielectric constant shows maxima) suggesting the relaxor type behavior in the PCT24 ceramic prepared from nanoparticles. Polarization study was carried out on PCT24 nanotube array, which shows the ferroelectric nature at room temperature. Chapter 6 reports the synthesis and studies of PbZrO3 (PZ) nanoparticles and PbZr1-xTixO3 for x = 0.0, 0.48 and 1.0 nanotubes. PZ nanoparticles were prepared by a novel sol-gel method based on diol-based solution. Initially, PZ was crystallized with some intermediate m-Z and t-Z phases at 400-550oC and start transforming to orthorhombic at around 600oC and then finally transformed into pure orthorhombic PZ phase at about 700oC. XRD and TEM confirmed the nanocrystalline nature of PZ particles. Curie temperature in the PZ ceramic prepared from PZ nanoparticles was observed around at 205oC, which is lower as compared to the bulk (233oC). P–E hysteresis loops of PZ ceramic prepared from nanoparticles were measured at different applied voltages and single ferroelectric loops of leaky nature were observed rather than antiferroelectrics. The lead zirconate nanoparticles produced may have potential applications as materials used in microelectronics and microelectromechanical systems. PbZr1-xTixO3 for x = 0.0 (PZ), 0.48 (PZT48) and 1 (PT) nanotubes were fabricated by sol-gel method within the closely packed porous alumina templates and characterized by various techniques. The crystallinity of the PZ, PZT48 and PT nanotubes were confirmed via XRD and SAED studies. EDX analysis demonstrated that stoichiometry was formed. Formation of Y-junctions in this complex functional oxide was also observed. The wall of the nanotubes was found to be made up of randomly oriented nanoparticles, which were confirmed by the HRTEM studies and also by a typical SEM image. The average thickness of the wall of the nanotubes was found to be around 10-20nm and nanoparticles consisting the wall was found to be in the range of 3 – 8nm. The Curie temperature was observed at 220oC in the PZ nanotube array. For the first time, PLD has been employed for the synthesis of lead zirconate nanotubes using AAO template. Well-registered arrays of these nanotubes could function as three dimensional (3D) device elements in miniaturized ferroelectric random access memory (FRAM). In chapter 7, we report the synthesis of single crystalline 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (PMN-PT) nanoparticles. PMN-PT nanoparticles were developed by a novel sol-gel method based on diol route. After partial calcination at 450oC/1h, PMN-PT powder morphology started transforming from pyrochlore to perovskite phase. It is interesting to note that this partially crystallized PMN-PT powder was unstable under electron beam and generated freestanding lead nanoparticles after absorbing energy from a focused electron beam. PMN-PT powder annealed at 700°C was fully transformed to perovskite phase and was stable under electron beam. XRD calculations and TEM imaging confirmed the nanocrystalline nature of PMN-PT particles. Magnetic measurements on PMN-PT nanoparticles prepared at 650 and 750oC show room temperature ferromagnetic hysteresis, whereas the bulk or the agglomerated particles show diamagnetic behavior. With an increase of annealing temperature or the particle size the magnetic moment decreases. PMN-PT nanotubes with diameter about 200nm were fabricated successfully by the sol-gel method based on diol route within the closely packed porous nanochannel alumina templates. Phase purity and crystalline perovskite phase formation of PMN-PT nanotubes were confirmed by the XRD and SAED pattern. EDX analysis demonstrated that stoichiometry was formed within accepted limit. The wall of the nanotubes was found to be made of nanoparticles which were confirmed from the HRTEM analysis. The average thickness of the wall of the nanotube was found around 20 nm and nanoparticles consisting the wall were found to be in the range of 10-20 nm. Since electroceramic materials are following a similar trend to miniaturization as conventional semiconductors, the synthesis of nanosized oxidic building blocks is moving into the focus of scientific and technological interest. Ferroelectrics are promising class of materials for the fabrication of electronic devices, as they are already an integral part of modern nanotechnological operations. Chapter 8 deals with the synthesis and properties of BiFeO3 (BFO) nanoparticles and nanotubes. Single crystalline BFO nanoparticles of different size and polycrystalline BFO nanotubes were prepared by sol-gel method. As prepared nanostructures were characterized by various techniques such as XRD, TGA-DTA, FTIR, scanning electron microscope (SEM), transmission electron microscope (TEM), selected-area electron diffraction (SAED), high resolution TEM and energy-dispersive X-ray spectroscopy (EDX). The crystallinity and phase purity of the BFO nanoparticles and nanotubes were confirmed by the XRD, SAED pattern and HRTEM analysis. Compositional homogeneity and their crystalline structure confirms the formation of the rhombohedrally distorted perovskite phase. EDX analysis demonstrated that stoichiometric BiFeO3 was formed within accepted limit. The HRTEM analysis confirmed that wall of the BFO nanotubes was made of nanoparticles, which were randomly oriented in the wall. The average thickness of the wall of the nanotubes was found to be around 15 nm and nanoparticles consisting the wall were found to be in the range of 3-6nm. Formation of Y-junctions in this complex functional oxide was observed. Magnetic measurements show clearly the enhancement of ferromagnetism in BFO nanotubes and ferroelectric loops were also observed in these nanotubes, that indicates the multiferroic nature of these nanotubes. BFO nanostructures at a large scale might be important for many applications such as memory elements in nanoscale devices in future. Chapter 9 reports the synthesis of a series of crystalline La1-xCaxMnO3 (x = 0, 0.3, 0.5, 0.7) nanoparticles with average diameter about 20 nm by an improved sol-gel method. The crystallinity and phase formation of as prepared nanoparticles was confirmed via XRD, SAED and HRTEM studies. EDX analysis demonstrated that desired stoichiometric was formed. Magnetic characterization reveals that the PM-FM transitions (Tc) occurs around at 205, 235, 235 and 230 K for x = 0, 0.3, 0.5, 0.7, respectively. The strong irreversibility between zero field cooling (ZFC) and field cooling (FC) magnetization curves, a cusplike peak in ZFC curve and unusual shape of M versus H loop at T = 5 K gives strong support for surface spin glass behavior. The highly stable charge ordering state in bulk manganites is suppressed, while the ferromagnetism is enhanced in these nanoparticles (x = 0.5 and 0.7). La0.7Ca0.3MnO3 were fabricated by sol-gel method within the closely packed porous alumina templates. The wall of the nanotubes was found to be made up of randomly oriented nanoparticles (8-12nm) as confirmed by HRTEM studies. The strong irreversibility between ZFC and FC magnetization curves as well as a cusplike peak in ZFC curve gives strong support for surface spin glass behavior. Magnetization value as obtained from M-H loop was about 28.5% of expected value, suggesting the existence of a magnetic dead layer, which avoids the propagation of exchange interaction between magnetic grains. The PM-FM transition was observed at 235 K. Chapter 10 gives the summary and conclusions of the present study and also discusses the possible future work that could after more insights into the understanding of the perovskite nanostructures. Highlight of the present work (i) Successful growth of nanostructures in both particles and tube forms, and study of their structure – composition correlations. (ii) Present work could optimize the necessary chemistry to successfully grow nanoparticles and nanotubes of various perovskite compositions. (iii) Successful studies of physical properties of nanoparticles and nanotubes, ofcourse, to a limited extent. However the properties observed in the present nanostructures have a strong indication of nonlinear phenomena similar to their bulk counterparts. (iv) It was reported in the literature, the observation of ferromagnetic behavior in several nonmagnetic compositions at nano-scale. Surprisingly, similar ferroelectric behavior was noticed even in our perovskite complex oxides such as relaxors (PMN-PT). A clear interaction of magnetic spin and an electric dipole was evident in these oxides such as relaxors and also multiferroics at nano-scale (~10-20 nm). (v) In ferromagnetic compositions such as LCMO, a very interesting spin-glass type behavior was observed.

This work deals with the structural and dielectric properties of Bariumtitanate (BTO) / Strontiumtitanate (STO) superlattices. The investigations were carried during the research for a doctoral thesis on the IFW Dresden, Institute for Metallic Materials (under supervision of Prof. Schulz). These multilayers have been prepared on single crystalline STO of (100) and (111) orientated substrates. All films where grown in an epitaxial mode. Additional superlattices and Bariumstrontiumtitanate (BSTO) thin films on silicon substrates with platinum bottom electrodes have been prepared. Thereby, (111) fibre-textured polycrystalline superlattices were produced. According to our knowledge this result was achieved for the first time (is unique in the world at the moment). According to high resolution TEM investigations of (001) oriented superlattices multilayers with atomically thin interfaces without noticeable interdiffusion have been prepared. XRD pattern of a multilayer consisting of BTO and STO monolayers that have only a thickness adequate one unit cell of BTO respective STO confirm this assumption. Multilayers on (111) oriented STO substrates show a much higher interface roughness than (001) orientated films. Regarding to the examinations in this thesis it is suggested that the roughness is correlated with the reduction of internal stresses by deformation of the stack and not with interdiffusion between the monolayers. For electrical measurements the film thickness has been varied from 30 nm to 300 nm and the periodicity in the range from 0.8 nm to 20 nm. Additionally, BSTO films of equivalent thickness and integral chemical composition were produced. Dielectical measurements were carried out in the temperature range from 20 K to 600 K and hysteresis measurements were done. It has to be pointed out, that multilayers have always lower dielectrical performances then BSTO films. In all cases the dielectric constant (DC) decreases with decreasing film thickness. Multilayers of a small periodicity show the highest DC?s, decreasing with increasing monolayer thickness in all cases. The maximum of DC shifted with decreasing film thickness to higher temperatures thus correlating with an increase of the out of plane lattice parameter. In this paper the mismatch between the stack respectivly the BSTO layers and the substrate has widely been discussed. In the case of BSTO the dielectric data can be qualitatively explained with the theory of strained films, developed mainly by Pertsev, under the assumption of a strain gradient in the thin film. Strain effects do also play an important role in ferroelectric multilayers as well as size and coupling effects between the monolayers. An adequate theory for the description of the dielectric behaviour of the ferroelectric superlattice produced during this research does yet not exist. Some thesis where pointed out, which effects have to be essentially included in to a consistent theory of ferroelectric multilayer. Some practical tips are also given, how to prepare monolayers and superlattices with very high DC and exellent hysteretic behaviour. / Es wurden (001) und (111) orientierte symmetrische BTO/STO-Multilagen auf niobdotierten STO-Einkristallen abgeschieden. Hierbei wurde sowohl die Gesamtschichtdicke, als auch deren Periodizität variiert. Zum Vergleich wurden weiterhin Ba0.5Sr0.5TiO3-Mischschichten unterschiedlicher Dicke präpariert. Aus den HRTEM und XRD Untersuchungen kann geschlossen werden, dass alle erhaltenen Schichten sowohl phasenrein als auch perfekt biaxial texturiert sind. Im Falle der (001) orientierten Multilagen konnten atomar scharfe Grenzflächen zwischen Einzellagen erhalten werden, wobei sich die Einzellagendicke bis auf eine Monolage (0.4 nm) reduzieren lässt. Aus der Schichtdickenabhängigkeit von d(001), dem mittleren out-of-plane Gitterparameter der Schicht, wird geschlossen, dass die Schichten auf den STO-Einkristallen Spannungsgradienten in den Schicht-normalen besitzen und an der Grenzfläche zum Substrat am stärksten verspannt sind. Die (111) orientierten Multilagen auf den STO-Einkristallen zeigen gegenüber den Schichten auf den (100) orientierten STO-Einkristallen eine deutlich erhöhte Interfacerauhigkeit. Vermutet wird, dass dies einerseits durch die andere kristallographische Orientierung der Wachstumsnormalen bedingt ist, weil damit jeweils keine geschlossenen SrO- bzw. BaO- und TiO3-Lagen ausgebildet werden. Andererseits zeigen die TEM-Aufnahmen eine deutliche Zunahme der Welligkeit der Einzellagen mit wachsendem Abstand vom Substrat, die rein mechanischen Effekten zugeschrieben wird. Die Verwölbung der Einzellagen könnte damit der Reduzierung der mechanischen Energie innerhalb des Systems dienen, wobei die Netzebenen dem Verlauf der Einzellagen folgen. Auf platinbeschichteten Siliziumsubstraten konnten erstmals phasenreine (111) fasertexturierte Mischschichten und BTO/STO-Multilagen abgeschieden werden. Grundlage hierfür war die Optimierung des Pt/Ti/SiO2/Si Schichtsystems hinsichtlich seiner thermischen Stabilität bis zu 800°C. Die Textur der Schichten wird von der Platingrundelektrode übernommen und deren Rauhigkeit teilweise verstärkt. Eine mechanische Verwölbung der Einzellagen konnte hier nicht beobachtet werden. Für die elektrischen Messungen wurden auf allen Schichten etwa 50 nm dicke Platinelektroden durch eine Hartmaske mittels Elektronenstrahlverdampfung im Hochvakuum bei etwa 300°C aufgebracht. Anschließend wurden die Schichten an Luft getempert, um das Sauerstoffdefizit, dass sich bei der Elektrodenabscheidung einstellt, auszugleichen. Die elektrischen Messungen zeichnen sich durch den sehr großen untersuchten Temperaturbereich aus. Temperaturabhängige Messungen im Bereich von 30-600 K finden sich für ferroelektrische Dünnschichten sehr selten in der Literatur und stellen für BTO/STO-Multilagen ein Novum dar. Auch die biasabhängige und teilweise auch temperaturabhängige Messung der Kapazität der Multilagen (C-V-Messungen) ist bisher einmalig. Durch die temperaturabhängigen Hysteresemessungen wurden Einblicke in den elektrischen Polungszustand der Schichten erhalten. Dadurch wird eine sinnvolle Interpretation der &amp;amp;#949;(T)-Kurven erst möglich. Der Vorteil der Integration des Polarisationsstromes unter Verwendung einer Dreieckspannung als Messsignal besteht in der direkten physikalischen Aussage der Strom-Spannungskurven über die Schaltspannung der Schichten.