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Die Entdeckung von neuen Materialien mit speziellen funktionalen Eigenschaften ist eins der wichtigsten Ziele in den Materialwissenschaften. Das Screening des strukturellen und chemischen Phasenraums nach potentiellen neuen Materialkandidaten wird häufig durch den Einsatz von Hochdurchsatzmethoden erleichtert. Schnelle und genaue Berechnungen sind eins der Hauptwerkzeuge solcher Screenings, deren erster Schritt oft Geometrierelaxationen sind. In Teil I dieser Arbeit wird eine neue Methode der eingeschränkten Geometrierelaxation vorgestellt, welche die perfekte Symmetrie des Kristalls erhält, Resourcen spart sowie Relaxationen von metastabilen Phasen und Systemen mit lokalen Symmetrien und Verzerrungen erlaubt. Neben der Verbesserung solcher Berechnungen um den Materialraum schneller zu durchleuchten ist auch eine bessere Nutzung vorhandener Daten ein wichtiger Pfeiler zur Beschleunigung der Entdeckung neuer Materialien. Obwohl schon viele verschiedene Datenbanken für computerbasierte Materialdaten existieren ist die Nutzbarkeit abhängig von der Darstellung dieser Daten. Hier untersuchen wir inwiefern semantische Technologien und Graphdarstellungen die Annotation von Daten verbessern können. Verschiedene Ontologien und Wissensgraphen werden entwickelt anhand derer die semantische Darstellung von Kristallstrukturen, Materialeigenschaften sowie experimentellen Ergebenissen im Gebiet der heterogenen Katalyse ermöglicht werden. Wir diskutieren, wie der Ansatz Ontologien und Wissensgraphen zu separieren, zusammenbricht wenn neues Wissen mit künstlicher Intelligenz involviert ist. Eine Zwischenebene wird als Lösung vorgeschlagen. Die Ontologien bilden das Hintergrundwissen, welches als Grundlage von zukünftigen autonomen Agenten verwendet werden kann. Zusammenfassend ist es noch ein langer Weg bis Materialdaten für Maschinen verständlich gemacht werden können, so das der direkte Nutzen semantischer Technologien nach aktuellem Stand in den Materialwissenschaften sehr limitiert ist. / The discovery of novel materials with specific functional properties is one of the highest goals in materials science. Screening the structural and chemical space for potential new material candidates is often facilitated by high-throughput methods. Fast and still precise computations are a main tool for such screenings and often start with a geometry relaxation to find the nearest low-energy configuration relative to the input structure. In part I of this work, a new constrained geometry relaxation is presented which maintains the perfect symmetry of a crystal, saves time and resources as well as enables relaxations of meta-stable phases and systems with local symmetries or distortions. Apart from improving such computations for a quicker screening of the materials space, better usage of existing data is another pillar that can accelerate novel materials discovery. While many different databases exists that make computational results accessible, their usability depends largely on how the data is presented. We here investigate how semantic technologies and graph representations can improve data annotation. A number of different ontologies and knowledge graphs are developed enabling the semantic representation of crystal structures, materials properties as well experimental results in the field of heterogeneous catalysis. We discuss the breakdown of the knowledge-graph approach when knowledge is created using artificial intelligence and propose an intermediate information layer. The underlying ontologies can provide background knowledge for possible autonomous intelligent agents in the future. We conclude that making materials science data understandable to machines is still a long way to go and the usefulness of semantic technologies in the domain of materials science is at the moment very limited.

Colorless crystals of the new hydroxometalates Ba[RE(OH)5] with the rare-earth elements RE=Tb, Dy, Ho were synthesized under ultra-alkaline conditions in a KOH hydroflux at 200 °C. Single-crystal X-ray diffraction revealed that the three compounds crystallize isostructural in the monoclinic space group P21/n (no. 14). In the crystal structure, the rare-earth cations are coordinated by the oxygen atoms of seven hydroxide anions, which define a distorted pentagonal bipyramid. These polyhedra share edges of their basal ring forming infinite chains that run parallel to the [010] direction. Hydrogen bonds connect the chains into layers parallel to the (101) plane. The Ba2+ cations are located between these layers and surrounded by nine oxygen atoms. Ba[Dy(OH)5] is paramagnetic and shows no luminescence under UV light. When heated in synthetic air or argon, water is released in well-defined steps. Ba[Dy(OH)5] decomposes via DyOOH to Dy2O3, which then reacts with the remaining Ba(OH)2 to form BaDy2O4. Thus, the hydroxometalates can be used as carbon-free precursors for oxides.

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No description available.

Hf₁₋ₓZrₓO₂ (x ∼ 0.5–0.7) has been the leading candidate of ferroelectric materials with a fluorite crystal structure showing highly promising compatibility with complementary metal oxide semiconductor devices. Despite the notable improvement in device performance and processing techniques, the origin of its ferroelectric crystalline phase (space group: Pca2₁) formation has not been clearly elucidated. Several recent experimental and theoretical studies evidently showed that the interface and grain boundary energies of the higher symmetry phases (orthorhombic and tetragonal) contribute to the stabilization of the metastable non-centrosymmetric orthorhombic phase or tetragonal phase. However, there was a clear quantitative discrepancy between the theoretical expectation and experiment results, suggesting that the thermodynamic model may not provide the full explanation. This work, therefore, focuses on the phase transition kinetics during the cooling step after the crystallization annealing. It was found that the large activation barrier for the transition from the tetragonal/orthorhombic to the monoclinic phase, which is the stable phase at room temperature, suppresses the phase transition, and thus, plays a critical role in the emergence of ferroelectricity.

<p>Continuum atomistic and mesoscopic models are developed and utilized in the context of studying microstructural evolution and phase selection in materials systems. Numerous phenomena are examined, ranging from defect-solute interaction in solid state systems to microstructural evolution under external driving conditions. Emphasis is placed on the derivation and development of models capable of self consistently describing the intricate mechanisms at work in the systems undergoing these phenomena.</p> <p>Namely, grain growth dynamics are studied in nanocrystalline systems under external driving conditions using a newly developed phase-field-crystal model, which couples an additional free energy source term to the standard phase-field-crystal model. Such external driving can be attributed to incident energetic particles. The nanocrystalline system is found to be susceptible to enhanced grain growth as a function of the intensity/flux associated with the external driving and the energy of driving. Static kinetic phase diagram calculations also seem to confirm that systems under external driving conditions can be forced into long metastable states.</p> <p>Early stage solute clustering and precipitation in Al alloys is also examined with a variant of the phase-field-crystal method, so-called structural phase-field-crystal models for multi-component alloys developed as part of this thesis. We find that clustering is aided by quenched-in defects (dislocations), whereby the nucleation barrier is reduced and at times eliminated, a mechanism proposed by Cahn for a single dislocation for spinodal systems. In a three-component system, we predict a multi-step mechanism for clustering, where the nature and amount of the third species plays an important role in relieving stresses caused by the quenched-in dislocations before clustering, i.e., segregation as predicted by the equilibrium phase diagram, can occur.</p> <p>Finally, we present a new coarse-graining procedure for generating complex amplitude models, i.e., complex order-parameter phase-field models, derived from phase-field-crystal models. They retain many salient atomistic features and behaviours of the original phase-field-crystal model, however is now capable of describing mesoscopic length scales like the phase-field model. We demonstrate the scheme by generating an amplitude model of the two-dimensional structural phase-fieldcrystal model, which allows multiple crystal structures to be stable in equilibrium, a crucial aspect of proper multi-scale modelling of materials systems. The dynamics are demonstrated by examining solidification and coarsening, peritectic growth, along with grain growth and the emergence of secondary phases.</p> / Doctor of Science (PhD)

The rare mineral cyanochroite, K2Cu(SO4)2·6H2O, features isolated Cu2+ ions in distorted octahedral coordination, linked via a hydrogen-bond network. We have grown single crystals of cyanochroite as large as ∼0.5 cm3 and investigated structural and magnetic aspects of this material. The positions of hydrogen atoms deviate significantly from those reported previously based on X-ray diffraction data, whereas the magnetic response is fully consistent with free Cu2+ spins. The structure is not changed by deuteration. Density functional theory calculations support our refined hydrogen positions.

Di-tert-butyl N,N′-{[13,15,28,30,31,33-hexa­ethyl-3,10,18,25,32,34-hexa­aza­penta­cyclo­[25.3.1.15,8.112,16.120,23]tetra­triaconta-1(31),3,5,7,9,12(33),13,15,18,20,22,24,27,29-tetra­deca­ene-14,29-di­yl]bis­(methyl­ene)}dicarbamate methanol disolvate, C52H72N8O4·2CH3OH, was found to crystallize in the space group P21/c with one half of the macrocycle (host) and one mol­ecule of solvent (guest) in the asymmetric unit of the cell, i.e. the host mol­ecule is located on a crystallographic symmetry center. Within the 1:2 host–guest complex, the solvent mol­ecules are accommodated in the host cavity and held in their positions by O—H⋯N and N—H⋯O bonds, thus forming ring synthons of graph set R22(7). The connection of the 1:2 host-guest complexes is accomplished by C—H⋯O, C—H⋯N and C—H⋯π inter­actions, which create a three-dimensional supra­molecular network.

Mechanisch flexible Kristalle können gebogen werden, ohne dabei zu zerbrechen. Je nach Art der Verformung können sie als mechanisch plastisch (irreversibel) oder elastisch (reversibel) biegbar eingestuft werden. Die verbesserten mechanischen Eigenschaften machen sie zu einer faszinierenden neuen Klasse von Materialien. Sie sind vielversprechend für die Entwicklung neuer funktioneller Bauelemente wie zum Beispiel fortschrittliche optoelektronische Elemente, intelligente Sensoren und künstliche Muskeln. Die derzeitigen Herausforderungen auf dem Forschungsgebiet der mechanisch flexiblen Kristalle umfassen ihre eingeschränkte Verfügbarkeit, ein fehlendes Verständnis für den Biegemechanismus und die Erforschung möglicher Anwendungen. Die vorliegende Arbeit befasst sich mit allen drei Bereichen und zielt darauf ab, 1) robuste Strategien zu entwickeln, um den Zugang zu mechanisch flexiblen Kristallen zu erleichtern, 2) ein atomistisches Biegemodell für einen mechanisch elastischen Kristall auf Grundlage von μ-fokussierter Einkristall-Röntgenbeugung und Dichtefunktionaltheorie-Berechnungen zu entwickeln und 3) die Lichtleitereigenschaften mechanisch flexibler Kristalle zu untersuchen, einschließlich der Verwendung elastischer und plastischer Kristalle und der Herstellung einer miniaturisierten photonisch integrierten Schaltung. / Mechanically flexible crystals can be bent without breaking. Depending on the nature of the deformation, they can be classified as mechanically plastic (irreversible) or elastic (reversible) bendable. The improved mechanical properties make them a fascinating new class of materials. They are promising materials for the development of new functional materials, including advanced optoelectronics, smart sensors, and artificial muscles. Current challenges in the research field of mechanically flexible crystals are the limited accessibility of these materials, the lack of mechanistic understanding of the bending mechanism, and the investigation of potential applications. The present work addresses all three areas with the aim of 1) developing robust strategies to facilitate access to mechanically flexible crystals, 2) developing an atomistic bending model for mechanically elastic crystals based on μ-focused single crystal X-ray diffraction (SCXRD) and density functional theory (DFT) calculations, and 3) investigating the waveguide properties of mechanically flexible crystals, including the use of elastic and plastic crystals and the fabrication of a micro-scale photonic integrated circuit.