No description available.

[FR] L'énergie photovoltaïque se présente comme une solution primordiale dans la lutte contre le réchauffement climatique. Malgré la maturité et la fiabilité de la technologie des cellules en silicium cristallin, les défis liés à la purification énergivore du silicium restent un obstacle coûteux. Dans cette optique, les pérovskites halogénées, notamment le FAPbI3, se profilent comme des alternatives prometteuses au silicium en raison de leur capacité à être synthétisées à faible coût à température ambiante, tout en présentant des propriétés optiques et électroniques attractives. Cependant, l'instabilité des pérovskites en contact avec l'air ambiant constitue un obstacle majeur à leur utilisation comme couche absorbante. Pour surmonter ce défi, une approche a consisté à modifier la composition chimique des pérovskites en utilisant la technique du spin-coating. L'étude a révélé que les pérovskites mixtes contenant de l'iodure (I) et du brome (Br), tels que le FAPbI2Br et le FAPbBr2I, offrent un compromis intéressant entre stabilité et bande interdite. Contrairement au FAPbI3, qui perd ses propriétés optiques après un certain temps en conditions ambiantes, ces pérovskites mixtes conservent leur capacité d'absorption dans le visible même après vieillissement. De plus, le FAPbBr2I s'est avéré environ trois fois plus photoluminescent que le FAPbI3, suggérant une conversion plus efficace des photons absorbés en paires électron-trou, ce qui en fait un candidat attractif pour les applications photovoltaïques. Cependant, il convient de noter que le FAPbBr2I présente un gap énergétique trop large pour de telles applications. Pour remédier à cette limitation, le dopage du FAPbBr2I avec du bismuth a été étudié, montrant une réduction significative du gap énergétique avec l'augmentation de la concentration en bismuth. Néanmoins, il a été observé que les photons émis par photoluminescence avaient une énergie supérieure à celle des photons absorbés, ce qui pourrait être dû à la dissipation de la chaleur dans le réseau cristallin. Ce décalage anti-Stokes nécessite une enquête plus approfondie. / [ES] La energía fotovoltaica se presenta como una solución clave en la lucha contra el calentamiento global. A pesar de la madurez y confiabilidad de la tecnología de células de silicio cristalino, los desafíos de la purificación de silicio intensiva en energía siguen siendo un obstáculo costoso. Con esto en mente, las perovskitas halogenadas, especialmente FAPbI3, están emergiendo como alternativas prometedoras al silicio debido a su capacidad de ser sintetizadas a bajo costo a temperatura ambiente, aunque tiene propiedades ópticas y electrónicas atractivas. Sin embargo, la inestabilidad de las perovskitas en contacto con el aire ambiente constituye un obstáculo importante para su uso como capa absorbente. Para superar este desafío, un enfoque fue modificar la composición química de las perovskitas utilizando la técnica de recubrimiento por centrifugación. El estudio reveló que las perovskitas mixtas que contienen yoduro (I) y bromo (Br), como FAPbI2Br y FAPbBr2I, ofrecen un compromiso interesante entre la estabilidad y la brecha de banda. A diferencia de FAPbI3, que pierde sus propiedades ópticas después de un cierto tiempo en condiciones ambientales, estas perovskitas mixtas conservan su capacidad de absorción en el visible incluso después del envejecimiento. Además, se encontró que FAPbBr2I era aproximadamente tres veces más fotoluminiscente que FAPbI3, lo que sugiere una conversión más eficiente de los fotones absorbidos en pares electrón-agujero, y, esto lo convierte en un candidato atractivo para aplicaciones fotovoltaicas. Sin embargo, debe tenerse en cuenta que FAPbBr2I tiene una brecha de energía más amplia que la ideal para tales aplicaciones. Para superar esta limitación, se estudió el dopaje de FAPbBr2I con bismuto, mostrando una reducción significativa de la brecha energética con el aumento de la concentración de bismuto. Sin embargo, se ha observado que los fotones emitidos por la fotoluminiscencia tienen una energía superior a la de los fotones absorbidos, y, esto podría deberse a la disipación de calor en la red cristalina. Este retraso anti-Stokes requiere más investigación. / [EN] Photovoltaic energy emerges as a crucial solution in the fight against climate change. Despite the maturity and reliability of crystalline silicon cell technology, challenges related to the energy-intensive purification of silicon remain a costly barrier. In this context, halide perovskites, especially FAPbI3, are emerging as promising alternatives to silicon due to their ability to be synthesized cost-effectively at room temperature while exhibiting attractive optical and electronic properties. However, the instability of perovskites in contact with ambient air poses a major obstacle to their use as an absorbing layer. To overcome this challenge, one approach has been to modify the chemical composition of perovskites using the spin-coating technique. The study revealed that misted perovskites containing iodide (I) and bromide (Br), such as FAPbI2Br and FAPbBr2I, offer an interesting compromise between stability and bandgap. Unlike FAPbI3, which loses its optical properties after some time under ambient conditions, these misted perovskites retain their absorption capacity in the visible range even after aging. Furthermore, FAPbBr2I was found to be approximately three times more photoluminescent than FAPbI3, suggesting a more efficient conversion of absorbed photons into electron-hole pairs, making it an attractive candidate for photovoltaic applications. However, it is worth noting that FAPbBr2I has a wider energy gap than ideal for such applications. To address this limitation, doping FAPbBr2I with bismuth was studied, showing a significant reduction in the energy gap with increasing bismuth concentration. Nevertheless, it was observed that the photons emitted by photoluminescence had higher energy than those absorbed, which could be due to heat dissipation in the crystal lattice. This anti-Stokes shift requires further investigation. / Abdoulaye, T. (2024). Etude de stabilité des pérovskites aux halogénures mixtes plombate de Formamidinium FAPbX3 avec X={ Cl,Br,I} [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/203190

<p dir="ltr">The separation of tellurium from cadmium telluride is examined using a unique combination of mild, anhydrous chlorination and complexation of the subsequent tellurium tetrachloride with 3,5-di-<i>tert</i>-butylcatechol. The resulting tellurium complex, Te(dtbc)₂, is isolated in moderate yield and features a 10³ to 10⁴ reduction in cadmium content, as provided by XRF and ICP-MS analysis. Similar results were obtained from zinc telluride. A significant separation between Te, Se, and S was observed after treating a complex mixture of metal chalcogenides with this protocol. These three tunable steps can be applied for future applications of CdTe photovoltaic waste.</p><p dir="ltr">We report the synthesis and characterization of the first series of tellurium and selenium complexes featuring an η⁵-cyclopentadienyl ligand. Reaction of Ph₃TeX (X = Cl, S₂CNEt₂) with MCp^R (M = Li, K; R = H, Me₄, Me₅) results in high yields of [Cp][TePh₃] (<b>1</b>), [Cp^Me4][TePh₃] (<b>2</b>), and [Cp*][TePh₃] (<b>3</b>), respectively. Similarly, reaction of Ph₃SeCl with LiCp and KCp* furnishes [Cp][SePh₃] (<b>4</b>) and [Cp*][SePh₃] (<b>5</b>). Each was characterized by X-ray crystallography, revealing similar η⁵-coordination with little distortion from an idealized half-sandwich geometry, presumably from the remaining lone pair on tellurium and selenium. The Te–centroid distances are relatively long (<b>1</b>: 2.770(3), <b>2</b>: 2.746(1), and <b>3</b>: 2.733(1) Å), suggesting a mostly ionic interaction. Se–centroid distances (<b>4</b>: 2.748(3), <b>5</b>: 2.707(2), 2.730(2) Å) were found to be surprisingly similar despite its smaller atomic radius. Compounds <b>2</b>, <b>3</b>, and <b>5</b> display rapid decomposition at room temperature, extruding a phenylated cyclopentadiene and the and the respective diphenylchalcogenide. The nature of bonding within these complexes was investigated through DFT methods and found to be primarily ionic in nature.</p><p dir="ltr">Synthesis of homoleptic zirconium and hafnium dithiocarbamate via carbon disulfide insertion into zirconium and hafnium amides were investigated for their utility as soluble molecular precursors for chalcogenide perovskites and binary metal sulfides. Treating M(NEtR)₄ (M= Zr, Hf and R= Me, Et) with CS₂ resulted in quantitative yields of homoleptic Group IV dithiocarbamates. Zr(k²-S₂CNMeEt) (<b>1</b>), Zr(k²-S₂CNEt₂)₄ (<b>2</b>), and Hf(k²-S₂CNEt₂)₄(<b>4</b>), a rare example of a crystal of a homoleptic hafnium CS₂ inserted amide species, were characterized. A computational analysis confirmed assignments for IR spectroscopy.<b> </b>To exemplify the utility of the Group IV dithiocarbamates, a solution-phase nanoparticle synthesis was performed to obtain ZrS₃ via the thermal decomposition of Zr(S₂CNMeEt)₄</p><p dir="ltr">Chalcogenide perovskites have garnered interest for applications in semiconductor devices due to their excellent predicted optoelectronic properties and stability. However, high synthesis temperatures have historically made these materials incompatible with the creation of photovoltaic devices. Here, we demonstrate the solution processed synthesis of luminescent BaZrS₃ and BaHfS₃ chalcogenide perovskite films using single-phase molecular precursors at sulfurization temperatures of 575 °C and sulfurization times as short as one hour. These molecular precursor inks were synthesized using known carbon disulfide insertion chemistry to create Group 4 metal dithiocarbamates, and this chemistry was extended to create species, such as barium dithiocarboxylates, that have never been reported before. These findings, with added future research, have the potential to yield fully solution processed thin films of chalcogenide perovskites for various optoelectronic applications.</p><p dir="ltr">Np(IV) Lewis base adducts were prepared by ligand substitution of NpCl₄(DME)₂. Using acetonitrile and pyridine, NpCl₄(MeCN)₄ (<b>1</b>) and NpCl₄(pyr)₄ (<b>2</b>), were isolated, respectively. All species were fully characterized using spectroscopic and structural analyses.</p>

<p>  </p> <p>Organic-inorganic halide perovskites are at the brink of commercialization as the next generation of light-absorbing materials for solar energy harvesting devices. Perovskites have large absorption coefficients, long charge-carrier lifetimes and diffusion lengths, and a tunable absorption spectrum. Furthermore, these materials can be low-temperature solution-processed, which transfers to low-cost manufacturing and cost-competitive products. The remarkable material properties of perovskites enable a broad product-market fit, encompassing traditional and new applications for solar technology. Perovskites can be deposited on flexible substrates for flexible solar cells, applied in thermochromic windows for power generation and building cooling, or tuned for tandem solar cell application to include in high-performance solar panels. However, perovskites are intrinsically unstable, which has so far prevented their commercialization. Despite large research efforts, including over two thousand publications per year, perovskite solar cells degrade in under one year of operation. In a saturated research field, new ideas are needed to inspire alternative approaches to solve the perovskite stability problem. In this dissertation, we detail research efforts surrounding the concept of a self-healing perovskite solar cell.</p> <p>     A self-healing perovskite solar cell can be classified with two distinctions: mechanically healing and molecularly healing. First, mechanically self-healing involves the material’s ability to recover its intrinsic properties after mechanical damage such as tares, lacerations, or cracking. This type of healing was unique to the organic polymer community and ultra-rare in semiconducting materials. By combining a self-healing polymer with perovskite material, we developed a self-healing semiconducting perovskite composite material which can heal using synergistic grain growth and solid-state diffusion processes at slightly elevated temperatures. The material is demonstrated in flexible solar cells with improved bending durability and a power conversion efficiency reaching 10%. The addition of fluidic polymer enables macroscopic perovskite material movement, which is otherwise brittle and rigid. The results inspire the use of polymer scaffolds for mechanically self-healing solar cells.</p> <p>     The second type of healing, molecular healing, involves healing defects within the rigid crystal domains resulting from ion migration. The same phenomenon which leads to device degradation, also assists the recovery of the device performance after resting the device in the dark. During device operation, perovskite ions diffuse in the perovskite lattice and accumulate at the device interfaces where they undergo chemical reactions or leave the perovskite layer, ultimately consuming the perovskite precursors. The photovoltaic performance can be recovered if irreversible degradation is limited. Ideally, degradation and recovery can match day and night cycling to dramatically extend the lifetime of perovskite solar cells. In this dissertation, we introduce the application of chalcogenide chemistry in the fabrication of perovskite solar cells to control the thin film crystallization process, ultimately to reduce defects in the perovskite bulk and introduce surface functionality which extends the device stability. This new strategy will help improve molecularly self-healing perovskite solar cell by reducing irreversible degradation. Lastly, we present a few other new ideas to inspire future research in perovskite solar cells and assist in the commercialization of the next generation of photovoltaics.</p>

No description available.

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.

In this dissertation, BaTiO3 nanocrystals, Bi4Ti3O12 nanostructured microspheres, and cosubstituted Bi4Ti3O12 nanoparticles and ceramics were prepared using solvothermal, hydrothermal and citrate-gel methods. The ferroelectric properties of the prepared cosubstituted Bi4Ti3O12 ceramics were studied using P–E hysteresis loop, leakage, and polarization fatigue measurements. A two-phase solvothermal synthesis approach for the preparation of hydrophobic BaTiO3 nanocrystals was developed. The two-phase method is based on the growth of nanocrystals at the oil/water interface by the reaction between metal surfactant complexes in the oil phase and a mineralizer in the water phase. Three kind of organic solvents, hexadecene, toluene, and heptane were used as the oil phase and compared to each other with respect to the product quality. The BaTiO3 particles are crystalline with a mean size of 3.7 nm and can be dispersed in a variety of organic solvents forming highly transparent dispersions. A hydrothermal method was developed for the synthesis of Bi4Ti3O12 nanostructured microspheres consisting of granular nanoparticles and nano-platelets. The precursor powder was prepared using a diethylene glycol mediated coprecipitation method. Tailoring of the morphology was achieved by changing the precursor quantity, sodium hydroxide concentration, and reaction time. The formation mechanism of the nanostructured microspheres probably involves aggregation, followed by dissolution and recrystallization. Bi3.25Pr0.75Ti2.97V0.03O12 (BPTV) and Bi3.25La0.75Ti3-xMxO12, (BLTMx, M = Mo, W, Nb, V, x = 0.0–0.12) ferroelectric nanoparticles and ceramics were synthesized using a modified citrate-gel method that has a crystallization temperature as low as 450 °C. The synthesized nanoparticles were spherical ranging from 30 to 100 nm. Except Nb5+, other donor cations were introduced using the corresponding oxides that have advantages in terms of high purity, low cost, and availability. The Bi3.25Pr0.75Ti2.97V0.03O12 ceramic is orthorhombic and its 2Pr and 2Ec values measured at 300 kV/cm were 35 μC/cm2 and 148 kV/cm respectively. The texture, microstructure, and ferroelectric properties of the prepared Bi3.25La0.75Ti3-xMxO12, (BLTMx, M = Mo, W, Nb, V, x = 0.0–0.12) ceramics depend on x. The maximum 2Pr (30–32 μC cm−2) was achieved at an optimum cosubstitution level (x = 0.025 for M6+, x = 0.03 for M5+). The high remanent polarization, low leakage current, and low polarization fatigue render the prepared ceramics promising for practical applications.

Ce mémoire concerne l'influence d'un degré d'oxydation inusuel d' un element de transition (Cu(III), Cu(IV), Fe(IV)) sur les proprietes physico-chimiques des réseaux oxygènes dérivés de structure perovskite. Les différentes structures du compose du cuivre trivalent LaCuO3 (formes basse et haute pression) ont été étudiées. Le role de plusieurs facteurs sur les propriétés physiques de ce matériau a pu etre mis en évidence, en particulier: 1) l'accroissement de la covalence moyenne de la liaison Cu-O du a la stabilisation de la valence mixte Cu(III)/Cu(IV) obtenue par substitution du Sr au La; 2) la distorsion structurale induite par substitution de l'yttrium au lanthane; 3) la pression d'oxygène de synthèse. La seconde partie de ce travail permet, au travers d' une étude Mössbauer par sonde diamagnétique locale (119Sn), d' expliquer le mécanisme de dismutation du fer tétravalent au sein d'un réseau perovskite AFeO3 (A=Ca, Sr)

Dans les multiferroïques coexistent au moins deux ordres ferroïques différents (ferromagnétique, ferroélectrique, ferroélasticité et ferrotoroïdicité) ou anti-ferroïques. Ces différentes propriétés peuvent être couplées ou non. Parmi ces matériaux, les plus étudiés sont ceux présentant un ordre magnétique et un ordre ferroélectrique. La présence d’un couplage magnétoélectrique entre ces deux ordres, permet de contrôler la polarisation par l’application d’un champ magnétique et inversement. Cependant très peu de ces matériaux ont des températures de transition supérieures à la température ambiante. Ces matériaux multiferoïques peuvent être séparés en deux catégories : la première regroupe les matériaux où les transitions des deux ordres sont indépendantes ; la deuxième regroupe les matériaux dont la transition ferroélectrique est liée à la mise en ordre magnétique. Dans cette thèse nous nous sommes intéressés à deux types d’oxyde multiferroïques dont l’un appartient à la première catégorie (YMnO3) et l’autre à la deuxième (RCrO3). / In multiferroics, at least two different ferroic orders coexist (ferromagnetic, ferroelectric, ferroelastici and ferrotoroidici) or anti-ferroic. These different properties can be coupled or not. Among these materials, the most studied are those with magnetic and ferroelectric orders. The presence of magnetoelectric coupling between these two orders, allows one to control the polarization by the application of a magnetic field and vice versa. However very few of these materials have transition temperatures above room temperature. These multiferoics materials can be separated into two categories : the first one includes the materials where the transitions of both orders are independent ; the second comprises the materials the ferroelectric transition of which is related to magnetic ordering. In this thesis we have studied two types of multiferroic oxides, one belongs to the first category (YMnO3) and the other to the second (RCrO3 ).

Die Arbeit behandelt die vielseitigen Möglichkeiten im Bereich der Kristallstrukturanalyse mit Röntgenstrahlung, welche sich zusätzlich bei resonanter Anregung von Elektronenübergängen ergeben. Existierende resonante Methoden aus diesem Bereich werden im materialwissenschaftlichen Kontext neu dargelegt und ausgebaut. Zudem werden neue Methoden zur Strukturverfeinerung vorgestellt, welche darauf zielen, mithilfe resonanter Anregung kleine Abweichungen von der Idealstruktur oder aber Phasenumwandlungen zu beschreiben. Im Vordergrund steht dabei die hier erstmals ausgearbeitete Methode der Unterdrückung von Beugungsintensität durch Variation der atomaren Streufaktoren über gezieltes Einstellen der Röntgenenergie. Dies ist stark abhängig von internen Strukturparametern und ermöglichte so eine pikometergenaue Bestimmung von Atompositionen in einer neuen, polaren Oberflächenschicht des Strontiumtitanats. Weitere Anwendungen auf verschiedene Klassen kristalliner Materialien werden vorgestellt und basieren auf unterschiedlichen Aspekten resonanter Beugung wie zum Beispiel verbotenen Reflexen.