Eine Vielzahl physikalischer Größen binärer und mehrkomponentiger Legierungsschmelzen weisen starke und zusammensetzungsabhängige Abweichungen vom idealen Lösungsverhalten auf, die anhand der in der Flüssigkeit eingestellten kurz- und mittelreichweitigen Ordnung plausibel erklärt werden können. Die vorliegende Arbeit hat zum Ziel, anhand von Röntgen- und Neutronenstreuexperimenten die atomare Struktur ausgewählter Legierungsschmelzen zu beschreiben und in Relation zu in der Literatur vorgestellten Untersuchungen der thermophysikalischen Eigenschaften zu diskutieren. Die Resultate der Streuexperimente werden unter anderem durch die Anwendung von Assoziatvorstellungen interpretiert. Weiterführend nimmt die Bestimmung und Analyse partieller Strukturfaktoren und Paarverteilungsfunktionen einen besonderen Stellenwert ein. Binäre Ag-Cu Legierungsschmelzen sind chemisch ungeordnet, die Nahordnung wird praktisch ausschließlich durch topologische Effekte bestimmt. Dem stehen sowohl die halbleitenden As-Se-Legierungen, als auch die metallischen Cu-Ge-, Al-Ni- und Ni-Si- Legierungsschmelzen gegenüber, die eine zum Teil sehr ausgeprägte chemische Ordnung aufzeigen.

This thesis aims to investigate the form-phase diagram of aqueous solutions of the triblock copolymer Pluronic P123 focusing on its high-temperature phases. P123 is based on polyethylene as well as polypropylene oxide blocks and shows a variety of di erent temperaturedependent micelle morphologies or even lyotropic liquid crystal phases in aqueous solutions. Besides the already well-studied spherical aggregates at intermediate temperatures, the size and internal structure of both worm-like and lamellar micelles, which appear near the cloud point, is determined using light, neutron and X-ray scattering. By combining the results of time-resolved dynamic light as well as small-angle neutron and X-ray scattering experiments, the underlying structural changes and kinetics of the sphere-to-worm transition were studied supporting the random fusion process, which is proposed in literature. For temperatures near the cloud point, it was observed that aqueous P123 solutions below the critical crystallization concentration gelate after several hours, which is linked to the presence and structure of polymeric surface layers on the sample container walls as shown by neutron re ectometry measurements. Using a hierarchical model for the lamellar micelles including their periodicity as well as domain and overall size, it is possible to unify the existing results in literature and propose a direct connection between the near-surface and bulk properties of P123 solutions at temperatures near the cloud point. / Ziel dieser Dissertation ist die Untersuchung des Form-Phasendiagrams des Dreiblock-Co- polymers Pluronic P123 mit dem besonderen Fokus auf dessen Phasenverhalten bei hohen Temperaturen. P123 besteht aus Polyethylen- und Polypropylenoxid-Blöcken und zeigt in wässriger Lösung vielfältige, temperaturabhängige Mizellformen oder sogar Flüssigkristallphasen. Neben den bereits intensiv untersuchten sphärischen Aggregaten bei mittleren Temperaturen, werden die Größen und inneren Strukturen der wurmartigen und lamellearen Aggregate mittels Licht-, Neutronen- und Röntgenstreumethoden untersucht, welche nahe des Trübungspunktes der Lösungen auftreten. Durch die Kombination von zeitaufgelösten dynamischen Licht- und Kleinwinkelstreuung-Experimenten wurden die strukturellen Änderungen und kinetischen Prozesse während des Kugel-Wurm-Übergangs untersucht, welche den bereits in der Literatur vorgeschlagenen zufälligen Fusionsprozess weiter bestätigen. Es wurde beobachtet, dass wässrige P123-Lösungen unterhalb der kritischen Kristallisationskonzentration nach mehreren Stunden gelieren, was durch Neutronenreflektometrie mit dem Auftreten und der Struktur von oberflächennahen Monolagen auf den Messzellwänden in Verbindung gebracht wurde. Wenn ein hierarchisches Model für die lamellaren Mizellen verwendet wird, das deren Periodizität, Domänen- und Gesamtgröße berücksichtigt, ist es außerdem möglich, die bisherigen Ergebnisse in der Literatur zu vereinigen und eine direkte Verbindung zwischen dem Aggregationsverhalten von P123 auf Oberflächen und in Lösung bei Temperaturen nahe des Trübungspunktes zu ziehen.

Eine Vielzahl physikalischer Größen binärer und mehrkomponentiger Legierungsschmelzen weisen starke und zusammensetzungsabhängige Abweichungen vom idealen Lösungsverhalten auf, die anhand der in der Flüssigkeit eingestellten kurz- und mittelreichweitigen Ordnung plausibel erklärt werden können. Die vorliegende Arbeit hat zum Ziel, anhand von Röntgen- und Neutronenstreuexperimenten die atomare Struktur ausgewählter Legierungsschmelzen zu beschreiben und in Relation zu in der Literatur vorgestellten Untersuchungen der thermophysikalischen Eigenschaften zu diskutieren. Die Resultate der Streuexperimente werden unter anderem durch die Anwendung von Assoziatvorstellungen interpretiert. Weiterführend nimmt die Bestimmung und Analyse partieller Strukturfaktoren und Paarverteilungsfunktionen einen besonderen Stellenwert ein. Binäre Ag-Cu Legierungsschmelzen sind chemisch ungeordnet, die Nahordnung wird praktisch ausschließlich durch topologische Effekte bestimmt. Dem stehen sowohl die halbleitenden As-Se-Legierungen, als auch die metallischen Cu-Ge-, Al-Ni- und Ni-Si- Legierungsschmelzen gegenüber, die eine zum Teil sehr ausgeprägte chemische Ordnung aufzeigen.

No description available.

In festen azobenzenhaltigen Polymeren wurde bei Bestrahlung mit blauem Licht ein makroskopischer Materialtransport beobachtet. Um die Dynamik der Gitterentstehung zu verfolgen, wurde am Speicherring für Synchrotronstrahlung ein Gitterschreibaufbau errichtet. Damit konnte erstmals in dieser Arbeit die Gitterbildungsgeschwindigkeit in-situ simultan mit Röntgen- und Lichtstreuung untersucht werden. Mit Hilfe einer speziellen Anpassung der Röntgenstreutheorie konnten sehr gute Übereinstimmungen von theoretischen Berechnungen mit den Messergebnissen erzielt werden. Dabei konnte nachgewiesen werden, dass sich zeitgleich mit einem Oberflächengitter auch ein Dichtegitter entwickelt. Durch die Trennung beider Streuanteile ließ sich die Dynamik der Strukturentstehungen bestimmen. Des weiteren konnte erstmals mit Hilfe der Photoelektronenspektroskopie die molekulare Orientierung an der Oberfläche eines Oberflächengitters nachgewiesen werden. Die Bewegungsursache kann auf einen Impulsübertrag während der Isomerisierung zurückgeführt werden, während die Bewegungsrichtung durch den elektrischen Feldvektor festgelegt wird. Die Theorie der Gitterentstehung konnte verbessert werden. / Solid azobenzene containing polymers show a macroscopic material transport under illumination with blue light. A writing setup was constructed at a synchrotron beamline in order to investigate the dynamics of the grating formation. With this setup it was possible to record the grating velocity for the first time simultaneously with x-ray and laser light scattering. <br /> A very good consistency could be achieved between the experiments and a suitable accomodation of the x-ray scattering theory. The theory reveals, that a density grating develops simultaneously with a surface grating. By separation of both parts the dynamics was determined for the density and the surface grating.<br /> The molecular ordering was determined at the surface with photoelectron spectroscopy. A momentum transfer could be identified as the source of the movement while the electric field defines the direction of the movement. The theory of the grating formation was improved.

The relationship between microstructure and mechanical properties of semicrystalline polymer materials has been a hot topic since many years in materials science and engineering. Isotactic polypropylene (iPP) is frequently used as a model material, due to its good mechanical properties and wide applications. In the past few years, numerous studies have been performed in the field of structural evolution during deformation. Previous results revealed that phase transition from crystal to mesophase happens in the crystal scale, lamellae orientation and fragmentation occurs in the lamellae scale, and even cavitation behavior exists in the larger scale. Although abundant work has been done, some problems remain under debate, for instance the relationship between lamellae deformation and cavitation behavior, the role of phase transition on the void formation, et al. In this study, well defined microstructure of iPP is obtained by annealing or adding nucleating agent. Afterward, the structural evolution under three types of mechanical load modes (including uniaxial stretching, creep, and stress relaxation) is in-situ monitored by synchrotron X-ray scattering. During uniaxial stretching, we revealed, for the first time, how lamellae deformation occurs in the time scales of elastic deformation, intra-lamellar slip, and melting-recrystallization, separated by three critical strains which were only rarely found to be influenced by annealing. Strain I (a Hencky strain value of 0.1) marks the end of elastic deformation and the onset of intra-lamellar slip. Strain II (a Hencky strain value of 0.45) signifies the start of the recrystallization process, from where the long period in the stretching direction begins to decrease from its maximum and the polymer chains in the crystal start to orient along the stretching direction. The energy required by melting arises from the friction between the fragmented lamellae. Strain III (a Hencky strain value of 0.95) denotes the end of the recrystallization process. Beyond the strain of 0.95, the long period and the crystal size remain nearly unchanged. During further stretching, the extension of the polymer chains anchored by lamellae triggers the strain hardening behavior. On the other hand, annealing significantly decreases the critical strain for voids formation and increases the voids number, but restricts the void size. For those samples annealed at a temperature lower than 90 oC, voids are formed between strain II and strain III. The voids are oriented in the stretching direction once they are formed. For those samples annealed at a temperature higher than 105 oC, voids are formed between strain I and strain II. The voids are initially oriented with their longitudinal axis perpendicular to the stretching direction and then transferred along stretching direction via voids coalescence. Additionally, the formation of voids influences neither the critical strains for lamellae deformation, nor the final long period, the orientation of polymer chains or the crystal size. β-iPP is a kind of metastable phase which can be induced only under special condition. By adjusting the morphology of N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide (NJS) through self-assembly, the relative content of β-iPP (Kβ) is successfully controlled, under the condition that the weight content of NJS in the composite keeps at 0.3 wt. %. The microstructural evolution of the iPP/NJS composites with different Kβ during uniaxial stretching is studied. The results show that a higher Kβ could increase the number of the voids. However, the size of the voids is similar regardless of the NJS morphology. The β-α phase transition takes place after voids formation. During intralamellar and inter-lamellar slip, no obvious polymer chains orientation can be found for α-iPP. In the strain range of 0.1~0.6, the c-axis of the β-iPP crystal tends to orient perpendicular to the stretching direction due to lamellae twisting, which is a unique deformation mode of β-iPP lamellae. And the lamellae twisting are proposed to be responsible for the intense voids formation of the composite with higher Kβ. During creep, the evolution of the long period can be divided into four stages (primary creep, transition stage, secondary creep, and tertiary creep). This fits quite well with the macroscopic displacement and strain evolution. In primary creep, the long period along loading direction (L_p^∥) increases with time due to the stretching of amorphous phase, whereas the long period perpendicular to loading direction (L_p^⊥) decreases slightly. In secondary creep, strain increases linearly with time. Both L_p^∥ and L_p^⊥ exhibit the same tendency with strain. The increase of the long period is caused by lamellae thickening, which is a kind of cooperative motion of molecular chains with their neighbors onto the lamellae surface. The increasing rate of L_p^∥ is larger than that of L_p^⊥, indicating that the orientation of molecular chains along loading direction decreases the energy barrier for the cooperative motion. In tertiary creep, strain grows dramatically within a limited time. The lamellae are tilted and rotated, and then disaggregated. In addition, fibrillary structure is formed during lamellae breaking. The length of the fibrillary structure increases from 364 nm to 497 nm but its width stays at 102 nm as creep time increases. During stress relaxation, the local deformation behavior of the long period is affine with the macroscopic stress relaxation. However, the evolution of the crystal orientation and the void size lag behind the macroscopic stress relaxation. The decrease of the long period is mainly caused by the relaxation of the strained polymer chains in the amorphous phase. The retardation of the evolution of the crystal orientation is probably caused by the phase transition from stable α-iPP to metastable mesomorphic-iPP. By phase transition, the highly oriented α-iPP is transferred to weakly oriented mesomorphic-iPP. Due to the fact that the void is confined by the network of the strained polymer chains where lamellae blocks serve as the physical anchoring points, the phase transition contributes greatly to the viscoplastic deformation of the network. Consequently, the evolution of the voids size shows a similar trend with that of the phase transition. With this thesis, we gained a deeper insight into the relationship between structure and properties of semicrystalline polymers. The current study will not only benefit the understanding of polymer materials science but also serve as guidance for the processing of semicrystalline polymers for engineering applications.:1 Introduction 1 1.1 Isotactic polypropylene (iPP) 1 1.1.1 Chain structure of PP 1 1.1.2 Crystal forms of iPP 2 1.1.3 Lamellae of iPP 4 1.1.4 The morphology of the supra-structure of iPP 4 1.2 Structural evolution during deformation 5 1.2.1 Deformation process of semicrystalline polymers 5 1.2.2 Cavitation behavior of semicrystalline polymers 7 1.3 Synchrotron X-ray scattering 9 1.3.1 X-ray and its sources 9 1.3.2 The interaction between X-rays and objects 11 1.3.3 Wide angle X-ray scattering 12 1.3.4 Small angle X-ray scattering 13 2 Motivation and objectives 15 3 Samples preparation and basic characterization 17 3.1 Materials and samples preparation 17 3.1.1 Preparation of iPP films with single layer of spherulites and transcrystalline regions 17 3.1.2 Preparation of iPP plates crystallized with different thermal histories 17 3.1.3 Preparation of iPP/NJS plates with different morphologies of NJS 18 3.1.4 Preparation of microinjection molded iPP/NJS sample 18 3.2 Characterization 18 3.2.1 Differential scanning calorimetry (DSC) 18 3.2.2 Dynamic mechanical analysis (DMA) 19 3.2.3 Scanning electron microscopy (SEM) 19 3.2.4 Polarized optical microscopy (POM) 20 3.2.5 Rheology test 20 3.2.6 Gel Permeation Chromatography (GPC) 21 3.2.7 In situ synchrotron X-ray scattering measurements 21 3.2.8 X-ray scattering pattern processing and calculation 24 4 Microstructure characterization in a single iPP spherulite by synchrotron microfocus wide angle X-ray scattering 29 4.1 Introduction 30 4.2 The nucleation efficiency of the carbon fiber on iPP 31 4.3 Morphology of iPP spherulites and transcrystalline region 32 4.4 Defining of the position of the carbon fiber 33 4.5 Microstructure studies of the spherulite 34 4.5.1 Crystallinity in the spherulite 35 4.5.2 The ratio between “daughter” lamellae and “mother” lamellae in the spherulite 36 4.5.3 The orientation of the crystal axis in the spherulite 37 4.6 Conclusion 39 5 Influence of annealing on the mechanical αc-relaxation of iPP: a study from the intermediate phase perspective 41 5.1 Introduction 42 5.2 Crystal form of water cooled and annealed iPP 44 5.3 Microstructure of iPP with different thermal history 45 5.4 Melting behavior of iPP with different thermal history 50 5.5 Mechanical relaxation behavior of iPP with different thermal history 52 5.6 Conclusion 57 6 Critical strains for lamellae deformation and cavitation during uniaxial stretching of annealed iPP 59 6.1 Introduction 60 6.2 The true stress-strain curves of iPP uniaxial stretched at 75 oC 61 6.3 In Situ SAXS and WAXS Results 63 6.3.1 Synchronize mechanical test and in-situ SAXS/WAXS measurement 66 6.4 Lamellae deformation 67 6.4.1 The evolution of the long period 67 6.4.2 The evolution of the crystal size 69 6.4.3 The orientation of the c-axis of the crystal 71 6.4.4 The evolution of the crystallinity 72 6.5 Cavitation behavior 74 6.5.1 The onset strain of the voids formation and the voids direction transition 74 6.5.2 The evolution of the voids size 75 6.5.3 The scattering invariant (Q) of the voids 76 6.5.4 The morphology of voids 77 6.6 Final discussion 79 6.7 Conclusion 82 7 Accelerating shear-induced crystallization and enhancing crystal orientation of iPP by controlling the morphology of N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide 83 7.1 Introduction 84 7.2 The self-assembly process of N,N'-dicyclohexyl-2,6-naphthalene dicarboxamide 85 7.3 Rheological behavior 88 7.3.1 Frequency sweep test 88 7.3.2 Strain sweep test 88 7.3.3 Steady-state shear test 89 7.4 Shear-induced crystallization 91 7.4.1 Crystallization kinetics studied by rheological method 91 7.4.2 In-situ SAXS measurement 93 7.4.3 Microstructure of iPP after shear-induced crystallization 96 7.4.4 The morphology of the sample 98 7.4.5 The crystallization mechanism 99 7.5 Conclusion 100 8 Influence of nucleating agent self-assembly on structural evolution of iPP during uniaxial stretching 101 8.1 Introduction 102 8.2 The morphology of the NJS in the compression molded iPP 103 8.3 Microstructure of iPP with different NJS morphologies 104 8.4 In-situ SAXS results 105 8.4.1 Cavitation behavior 107 8.4.2 Evolution of the long period 110 8.5 In-situ WAXS results 111 8.5.1 The β-α phase transition behavior 112 8.5.2 The orientation of the crystal 115 8.6 Conclusion 117 9 Microstructural evolution of iPP during creep: an in-situ study by synchrotron SAXS 119 9.1 Introduction 120 9.2 The creep curve 121 9.3 In-situ SAXS results 123 9.3.1 Evolution of long period and domain thickness 125 9.3.2 Lamellae tilting and rotation 128 9.3.3 Lamellae orientation and fibrillary structure formation 129 9.4 Conclusions 132 10 Microstructural evolution of iPP during stress relaxation 133 10.1 Introduction 134 10.1.1 The structural evolution during stress relaxation at 60 oC 135 10.1.2 The structural evolution during stress relaxation at 90 oC 140 10.2 Conclusion 145 11 Conclusion and outlook 146 12 References 148 13 Appendix 158 13.1 List of symbols and abbreviations 158 13.2 List of figures and tables 163 13.3 List of publications 171 14 Acknowledgements 173 15 Eidesstattliche Erklärung 175

Künstliche molekulare Schalter, wie beispielsweise Azobenzole, Diarylethene, Fulgide, Stilbene oder Spiropyrane wurden in den letzten Jahren intensiv erforscht, da sie zur Datenspeicherung, in selbstheilenden Materialien, molekularer Elektronik, Energiespeichern und mikromechanischen Anwendungen eingesetzt werden können. Eine der größten Herausforderungen im Forschungsfeld der molekularen Schalter ist die Frage, wie die Photoreaktion isolierter Moleküle in eine kontrollierte Photoreaktion wohldefinierter supramolekularer Systeme, wie z.B. organischer Dünnfilme oder 3D Nanostrukturen, übersetzt werden kann. Die Integration molekularer Schalter, beispielsweise von Azobenzolen, in supramolekulare Anordnungen kann zu emergenten Phänomenen wie kooperativem Schaltverhalten führen. Kooperatives Schalten bedeutet, dass die energetische Landschaft und daher auch die Isomerisationskinetik eines einzelnen molekularen Schalters von den isomerischen Zuständen benachbarter Schalter beeinflusst werden. Kooperatives Schaltverhalten, oder überhaupt Schaltbarkeit in geordneten Ensembles molekularer Schalter auf Oberflächen bewusst zu erzeugen hat sich allerdings aufgrund von sterischer Behinderung oder Delokalisierung angeregter Zustände als schwierig herausgestellt. Deshalb wäre ein besseres Verständnis der Voraussetzungen für Schaltbarkeit und kooperatives Verhalten molekularer Schalter in supramolekularen Systemen ein großer Schritt in Hinblick auf die Entwicklung von Bauelementen, die auf der gemeinsamen Bewegung molekularer Schalter basieren. Die in dieser Arbeit erzielten Resultate gewähren neue Einblicke in das Verhältnis zwischen der Photoisomerisierung einzelner Azobenzole und der Photoreaktion supramolekularer Systeme, was dabei helfen wird, neuartige und optimierte stimulireaktive Materialien zu entwickeln. / Synthetic molecular switches, such as azobenzenes, diarylethenes, fulgides, stilbenes or spiropyranes, have been intensively investigated in recent times because of their possible use in data storage, self-healing materials, molecular electronics, energy and information storage and optomechanics. One of the biggest challenges in the research field of molecular switches is the translation of the photoresponse of isolated molecules into a controlled photoresponse of well-defined supramolecular systems, such as organic thin films or functional nanostructures. The main focus of this thesis lies on the photoisomerization of multi-azobenzene compounds in different structural environments. Incorporation of molecular switches, for example azobenzene, into supramolecular assemblies can lead to emergent phenomena like cooperative switching behavior. Cooperative switching means that the energetic landscape and thus also the isomerization kinetics of a single molecular switch is influenced by the isomeric state of adjacent switches. However, it has proven difficult to establish cooperative switching behavior or even switching functionality at all in ordered ensembles of molecular switches on surfaces due to steric hindrance or delocalisation of excited states. Therefore, understanding the prerequisites for switching functionality and cooperative behavior of molecular switches in supramolecular assemblies is a crucial step towards the development of devices that make use of concerted motion of molecular switches. This thesis yields unprecedented insight into the relation between the photoisomerization of isolated azobenzenes and the photoresponse of supramolecular systems, which will ultimately help to build novel and optimized stimuli-responsive materials.

The thesis is dealing with the investigation of charge and orbital order and their behaviour under external pressure. Therefore, a new pressure cell has been developed which allows the observation of superlattice reflections corresponding to the order phenomena under pressure using scattering of high-energy synchrotron radiation. The maximum pressure that can be reached is 1.25 GPa. Until today there has been no possibility to conduct such studies of charge and orbital order superlattice reflections under pressure using x-ray scattering. The intensities of the reflections of the single crystalline samples are quite weak compared to fundamental peaks. Therefore the measurements are strongly affected by the absorption of the radiation in the pressure cell itself. Further difficulties result from the facts that low temperatures are needed and the sample has to be oriented in reciprocal space after being mounted into the cell. Therefore, the design of a compact clamp-type piston pressure cell was chosen here. The cell is made from a copper-beryllium alloy with the wall thickness reduced in the height of the sample volume. This allows the usage inside a closed-cycle cryostat mounted on a three-axis-diffractometer. Absorption effects are minimized due to the combination of reduced wall thickness and the usage of high energy synchrotron radiation (E = 100 keV at the beamline BW5 at HASYLAB/DESY). The new experimental technique was established and used for a study of two representatives of the transition metal oxide compounds, i.e. doped cuprates and manganites, which belong to the class of strongly correlated electron systems. The 1/8-doped cuprate La_{2-x}Ba_{x}CuO_{4} reveals an ordered state at low temperatures. Inside the CuO_{2} planes a combined order of charge stripes and antiferromagnetic spin stripes is observed. The ordering results from the interaction between charge, spin and lattice degrees of freedom. Here the lattice degrees of freedom play a major role. Particularly, a structural transition from an orthorhombic to a tetragonal symmetry is prerequisite for the observation of the ordered state. The cell constructed in this work allows a more exact analysis of the coupling between the crystal lattice and the formation of the charge and spin ordered phase. The manganite system Pr_{0.7}(Ca_{0.9}Sr_{0.1})_{0.3}MnO_{3} shows a strong magnetoresistive effect, called colossal magnetoresistance (CMR). In this system, several ordered phases can be found, which exhibit charge, spin and - since the orbital degree of freedom is also present in the manganites - additionally orbital ordering phenomena. In particular, an antiferromagnetically spin ordered insulating phase, which is connected to a charge- and orbital ordered state competes with a ferromagnetic metallic phase. This competition leads to a phase separation, which determines the properties of the sample. Both phases are strongly coupled to the lattice degrees of freedom, so that application of external pressure drastically affects the interplay between the different phases and allows a detailed study of the relation between the charge and orbital ordered phase and the crystal structure. / Die vorliegende Arbeit befaßt sich mit dem Studium der Ordnungszustände von Ladungen und Orbitalen und deren Beeinflußung durch externen Druck. Als experimentelle Neuentwicklung wurde dafür eine Druckzelle entworfen, mit deren Hilfe die Beobachtung der jeweiligen Ordnungsphänomene unter Druck mittels der Streuung hochenergetischer Synchtrotronstrahlung möglich ist. Die Zelle erlaubt die Messung der orbitalen und Ladungsüberstrukturreflexe, welche aus den geordneten Zuständen resultieren, in einem Druckbereich bis 1.25 GPa. Die experimentelle Herausforderung ergibt sich hierbei aus der Tatsache, dass die Überstrukturreflexe im Vergleich zu den fundamentalen Reflexen der einkristallinen Proben sehr schwach sind und zusätzlich durch die Absorption im Mantelmaterial der Druckzelle stark beeinträchtigt werden. Darüber hinaus soll die Zelle bei tiefen Temperaturen einsetzbar und die Probe auch innerhalb der Zelle im reziproken Raum orientierbar sein. Bei dem hier realisierten Ansatz wurde für das Design daher der Typ einer kompakten Klemmdruckzelle aus einer Kupfer-Beryllium-Legierung gewählt, deren Zellwände im Bereich des Probenvolumens reduziert wurden. Dadurch ist der Einsatz der Zelle im Inneren eines Closed-Cycle-Kryostaten auf einem Einkristall-Diffraktometer möglich. Aufgrund der geringen Wandstärke der Zelle und der Nutzung von hochenergetischer Röntgenstrahlung (E = 100 keV am Messplatz BW5 des HASYLAB/DESY) werden Absorptionseffekte minimiert. Die neue Messmethode wurde im Rahmen der Arbeit etabliert und zur Untersuchung zweier wichtiger Übergangsmetalloxidverbindungen (dotierte Kuprate, Manganate), die zur Klasse der stark korrelierten Elektronensysteme gehören, eingesetzt. Das 1/8-dotierte Kupratsystem La_{2-x}Ba_{x}CuO_{4}, weist bei tiefen Temperaturen einen statisch geordneten Zustand auf. Innerhalb der CuO_{2}-Schichten des Kristalls ergibt sich eine Ordnung, bei der sich Streifen lokalisierter Löcher und antiferromagnetische Bereiche abwechseln. Ursache dieses Zustands ist das Wechselspiel von Ladungen, Spins und strukturellen Freiheitsgraden. Dabei spielen letztere eine herausgehobene Rolle. So ist insbesondere ein struktureller Übergang von einer orthorhombischen zu einer tetragonalen Phase Voraussetzung für die Beobachtung der Ordnung. Die in dieser Arbeit aufgebaute Druckzelle erlaubt eine genauere Analyse des Zusammenhangs zwischen Struktur des Kristalls und der Ausbildung der ladungs- und spingeordneten Phase. Das Manganatsystem Pr_{0.7}(Ca_{0.9}Sr_{0.1})_{0.3}MnO_{3}, zeichnet sich durch einen sehr starken magnetoresistiven Effekt aus, der auch als kolossaler Magnetowiderstand (CMR) bezeichnet wird. Auch hier kann bei tiefen Temperaturen eine geordnete Phase beobachtet werden. Allerdings spielt in diesem System zusätzlich der orbitale Freiheitsgrad der Elektronen eine entscheidende Rolle, so dass sich eine kombinierte Ladungs- und Orbitalordnung ergibt. Diese Phase, die isolierend und zusätzlich antiferromagnetisch geordnet ist, steht im direkten Wettbewerb zu einer ferromagnetischen Phase. Aus dieser Konkurrenz ergibt sich eine Tendenz zur Phasenseparation, deren Effekte die Eigenschaften des Kristalls dominieren. Da beide Phasen stark an die strukturellen Freiheitsgrade gekoppelt sind, läßt sich das Gleichgewicht zwischen ihnen durch externen Druck beeinflussen und die Abhängigkeit der ladungs- und orbitalgeordneten Phase von den strukturellen Eigenschaften des Kristalls im Detail untersuchen.

Owing to their versatile orbital character with both local and highly dispersive degrees of freedom, transition metal oxides span the range of ionic, covalent and metallic bonding. They exhibit a vast diversity of electronic phenomena such as high dielectric, piezoelectric, pyroelectric, ferroelectric, magnetic, multiferroic, catalytic, redox, and superconductive properties. The nature of these properties arises from sensitive details in the electronic structure, e.g. orbital mixing and orbital hybridization, due to non-stoichiometry, atomic displacements, broken symmetries etc., and their coupling with external perturbations. In the work presented here, these variations of the electronic structure of crystals due to structural and electronic defects have been investigated, exemplarily for the quasi-binary system SrO - SrTiO3 - TiO2. A number of binary and ternary structures have been studied, both experimentally as well as by means of electronic modeling. The applied methods comprise Resonant X-ray Scattering techniques like Diffraction Anomalous Fine Structure, Anisotropy of Anomalous Scattering and X-ray Absorption Fine Structure, and simultaneously extensive electronic calculations by means of Density Functional Theory and Finite Difference Method Near-Edge Structure to gain a thorough physical understanding of the underlying processes, interactions and dynamics. It is analyzed in detail how compositional variations, e.g. manifesting as oxygen vacancies or ordered stacking faults, alter the short-range order and affect the electronic structure, and how the severe changes in mechanical, optical, electrical as well as electrochemical properties evolve. Various symmetry-property relations have been concluded and interpreted on the basis of these modifications in electronic structure for the orbital structure in rutile TiO2, for distorted TiO6 octahedra and related switching mechanisms of the Ti valence, for elasticity and resistivity in strontium titanate, and for surface relaxations in Ruddlesden-Popper phases. Highlights of the thesis include in particular the methodical development regarding Resonant X-Ray Diffraction, such as the first use of partially forbidden reflections to get the complete phase information not only of the tensorial structure factor but of each individual atomic scattering tensor for a whole spectrum of energies, as well as the determination of orbital degrees of freedom and details of the partial local density of states from these tensors. On the material side, the most prominent results are the identification of the migration-induced field-stabilized polar phase and the exergonic redox behavior in SrTiO3 caused by defect migration and defect separation.

Owing to their versatile orbital character with both local and highly dispersive degrees of freedom, transition metal oxides span the range of ionic, covalent and metallic bonding. They exhibit a vast diversity of electronic phenomena such as high dielectric, piezoelectric, pyroelectric, ferroelectric, magnetic, multiferroic, catalytic, redox, and superconductive properties. The nature of these properties arises from sensitive details in the electronic structure, e.g. orbital mixing and orbital hybridization, due to non-stoichiometry, atomic displacements, broken symmetries etc., and their coupling with external perturbations. In the work presented here, these variations of the electronic structure of crystals due to structural and electronic defects have been investigated, exemplarily for the quasi-binary system SrO - SrTiO3 - TiO2. A number of binary and ternary structures have been studied, both experimentally as well as by means of electronic modeling. The applied methods comprise Resonant X-ray Scattering techniques like Diffraction Anomalous Fine Structure, Anisotropy of Anomalous Scattering and X-ray Absorption Fine Structure, and simultaneously extensive electronic calculations by means of Density Functional Theory and Finite Difference Method Near-Edge Structure to gain a thorough physical understanding of the underlying processes, interactions and dynamics. It is analyzed in detail how compositional variations, e.g. manifesting as oxygen vacancies or ordered stacking faults, alter the short-range order and affect the electronic structure, and how the severe changes in mechanical, optical, electrical as well as electrochemical properties evolve. Various symmetry-property relations have been concluded and interpreted on the basis of these modifications in electronic structure for the orbital structure in rutile TiO2, for distorted TiO6 octahedra and related switching mechanisms of the Ti valence, for elasticity and resistivity in strontium titanate, and for surface relaxations in Ruddlesden-Popper phases. Highlights of the thesis include in particular the methodical development regarding Resonant X-Ray Diffraction, such as the first use of partially forbidden reflections to get the complete phase information not only of the tensorial structure factor but of each individual atomic scattering tensor for a whole spectrum of energies, as well as the determination of orbital degrees of freedom and details of the partial local density of states from these tensors. On the material side, the most prominent results are the identification of the migration-induced field-stabilized polar phase and the exergonic redox behavior in SrTiO3 caused by defect migration and defect separation.