Universität Stuttgart
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Item Open Access Growth of oxide materials with Ruddlesden-Popper- and garnet-type structures via the optical float zone method: investigations of scintillation, optical, and magnetic properties, and post-growth modifications of single crystals via topochemical routes(2025) Yilmaz, Hasan; Clemens, Oliver (Prof. Dr.)With the continuous progress in materials science, the global market demand for high-performance functional materials, especially those with customized optical and magnetic properties, has increased significantly due to their critical role in next-generation technologies such as photonic devices, data storage systems, quantum computing, and biomedical applications. With this motivation, this dissertation is focused on the growth of high-quality oxide single crystals with Ruddlesden-Popper (RP) and garnet-type structures using the optical floating zone (OFZ) method and the detailed investigation of their scintillation, optical and magnetic properties. Furthermore, the use of chemical doping and post-growth topochemical modifications as strategies to modify material properties and stabilise metastable structures is also covered. With these approaches, we can classify this study into two parts: optical and magnetic properties. In the first part of this work, RP-type n = 1 LaSrGaO4 (LSGO) and Garnet-type Gd3In2Ga3O12 (GIGG) single crystals doped with different Rare Earths (RE) including Eu, Sm, Ho, Nd have been grown via OFZ method to investigate their luminescence, decay time kinetics and scintillation properties. To compare the scintillation behaviour, a well-known commercial scintillator, Gd2.98Ce0.02Al2Ga3O12 (0.02Ce:GAGG) single crystal, was synthesised and characterised using the same methods. This limits deviations in scintillation properties caused by different synthesis methods. Powder X-ray diffraction (XRD) and Single Crystal X-ray diffraction (SC-XRD) measurements together with Rietveld analysis confirmed phase purity and crystallinity, while Scanning Electron Microscopy (SEM-EDX) and backscatter electron imaging (BSE) confirmed the homogeneous distribution of the dopants. In addition to these analytical methods, Photoluminescence Spectroscopy (PL) and luminescence decay measurements have demonstrated that these RE-doped single crystals have strong emission, which is related to the low phonon energy of their host lattices. However, scintillation measurements under 137Cs gamma-ray point source excitation resulted in a limited gamma ray response. This response was significantly weaker than observed for the well-known commercial scintillator 0.02Ce:GAGG. Inefficient absorption of high-energy excitation or inadequate energy transfer to the activator ions in these materials are responsible for this relatively low scintillation performance. The second part of the thesis is focused on bilayer Ruddlesden-Popper nickelates. Single crystals of Sr3Ni2-xAlxO7-δ (SNAO) and its Y-substituted modification YᵧSr3-yNi2-xAlxO7-δ (YSNAO) have been grown by using strategically chemical substitutions and high oxygen pressure conditions to stabilize the RP phase. XRD and SC-XRD analysis confirmed the existence of the n = 2 RP-type structure by doping with Al, while thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) indicated successful oxygen incorporation and corresponding changes in the Ni oxidation state. Magnetic susceptibility measurements (SQUID magnetometry) have demonstrated the antiferromagnetic ordering transitions in these doped nickelate single crystals, and transport measurements have shown enhanced electrical conductivity compared to the undoped parent compound. In addition, a topochemical fluorination process has been investigated as a post-growth modification for n = 1 RP-type La2NiO4+δ single crystals. Fluorine has been introduced into the crystal lattice at moderate temperatures using polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and CuF2 as fluorination reagent. PTFE has been found to be the most effective fluorinating agent, resulting in partially fluorinated La2NiO4+δ with a diffusion-limited penetration pattern. EDX and EDX-mapping analyses have demonstrated an inhomogeneous F distribution, indicating that fluorine incorporation is limited by diffusion kinetics. This inhomogeneity should be carefully considered in future structural and functional analyses as it may affect the accurate characterisation of intrinsic properties in fluorinated crystals. In summary, the results of this thesis demonstrate the flexibility of the optical floating zone technique for the growth of high-quality functional oxides and show how structural stability, and functional properties can be modified using specific chemical doping approaches. To optimise scintillation performance in novel garnet-type hosts and to improve the magnetic and electronic behaviour of RP-type nickelates, the combined approach of crystal growth and post-growth modification could provide an additional parameter for material modification.Item Open Access Atom probe study on CuNi thin films : miscibility gap and grain boundary segregation(2023) Duran, Rüya; Schmitz, Guido (Prof. Dr. Dr. h. c.)In dieser Arbeit wurde die Lage der Mischungslücke, und die Korngrenzsegregation im Legierungssystem, Kupfer-Nickel, per Atomsondentomographie (APT) analysiert. Zur Untersuchung der Mischungslücke eines binären Systems mit langsamer Diffusion wurde ein neues Verfahren verwendet. Multilagen aus Cu- und Ni- Dünnschichten wurden mittels Ionenstrahlbeschichtung (IBS) auf Wolframpfosten beschichtet und durch fokussierte Ionenstrahlung (FIB) geformt. Bei drei unterschiedlichen Temperaturen, zwischen 573 und 673 K, wurden isotherme Auslagerungssequenzen an einem Ultrahochvakuumofen (UHV) durchgeführt und der Mischungsprozess analysiert. Ein Modell des Diffusionsprozesses wurde mittels mathematischer Überlegungen erstellt. Durch das Fitten der experimentellen Kompositionsprofile mittels dieses Modells konnten die Gleichgewichtskonzentrationen der Schichten auch mit relativ kurzen Auslagerungszeiten ermittelt werden. Darüber hinaus konnten aus den diffusionskontrollierten Zeit- und Temperaturdaten physikalische Eigenschaften wie der effektive Diffusionskoeffizient (Gitterdiffusion einschließlich Defektdiffusion) bestimmt werden. Dieser betrug Deff = 1.86 ∙ 10-10 m2/s ∙ exp(-164 kJ mol-1/RT). Während dem Vermischen wurde die Änderung der multilagigen Mikrostruktur bis zur vollständigen Mischung bei 623 und 673 K beobachtet, wobei Korngrenzen als schneller Diffusionsweg eine wichtige Rolle spielen. Bei 573 K wurde Nichtmischbarkeit experimentell deutlich nachgewiesen, wobei die Phasengrenzen bei cNi=26 at.% und cNi=66 at.% liegen. Mit diesen Phasengrenzen wurde die Mischungslücke über eine Redlich-Kister-Parametrisierung der Gibbs‘schen freien Energie über den gesamten Konzentrationsbereich rekonstruiert. Hierin wurde für die kritische Temperatur, TC, 608 K bei einer Konzentration von 45 at% Ni gefunden. Im zweiten Teil wurde die Korngrenzsegregation durch die FIB/tEBSD- (Transmissions-Elektronen-Rückstreubeugung) Technik, in Korrelation zu APT-Messung charakterisiert. Vier Legierungen mit einem Ni-Anteil zwischen 25 und 85 at.% wurden auf Wolframpfosten per IBS beschichtet, und bei 700 K für 24 h wärmebehandelt. Die Segregation von Cu in die Korngrenzen wurde beobachtet. Durch die Verwendung eines theoretischen Models wurde die Exzess-Kurve über den gesamten Konzentrationsbereich, und die Korngrenz-Formationsenergie auf Basis der experimentellen Daten berechnet. Die tEBSD-Analyse während der FIB-Präparation erlaubt die Identifikation der Körner und deren Orientierung. Ein neues Verfahren wurde entwickelt, um mithilfe der Orientierung benachbarter Körner, Berechnungen zur Ermittlung der Korngrenzorientierung durchzuführen und somit die Orientierung natürlicher Korngrenzen zu bestimmen. Mit diesem Verfahren konnte der zeitliche Aufwand dieser anspruchsvollen Auswertung (verglichen zur herkömmlichen Methode mittels TEM-Untersuchung) stark reduziert werden, so dass eine quantitative Analyse vieler Korngrenzen möglich wurde. Aus den einzelnen Korngrenzorientierungen wurde die Korngrenzrotation, und die jeweiligen Anteile an Kippung und Drehung berechnet. Eine Abhängigkeit der Feststoffsegregation vom Kipp- und Drehanteil der Korngrenze wurde beobachtet, die am kleinsten für die reine Kipp- und Drehrotation war. Die ermittelten Segregationsweiten sind signifikant größer als die strukturellen Korngrenzweiten und bewegen sich zwischen 12 und 85 Å. Dieses Verhalten wurde durch eine künstliche Verbreiterung der Korngrenze erklärt, die durch eine Flugbahnabweichung der Korngrenzatome während der Verdampfung verursacht wurde. Eine Korngrenzweite von w0 = (10.1 ± 1.5) Å wurde für eine unverfälschte Korngrenze gefunden.Item Open Access Nanostructured oxide materials as catalyst supports for molecular heterogeneous catalysis under confinement : design, synthesis, and characterization(2025) Kousik, Shravan R.; Bill, Joachim (Prof. Dr.)Item Open Access Quantitative investigation of the cycling behavior and SEI formation via the time-resolved mass spectrometry(2023) Wang, Ke; Schmitz, Guido (Prof. Dr. Dr. h. c.)Item Open Access Bioinspired nanostructuring and eco-friendly synthesis strategies : advancing next-generation metal-ion batteries(2024) Raafat, Leila; Bill, Joachim (Prof. Dr.)Item Open Access Characterization and suppression of side reactions and degradation pathways in all-solid-state fluoride-ion batteries(2025) Aalto, Tommi Hendrik; Clemens, Oliver (Prof. Dr.)In this cumulative dissertation, different degradation mechanisms for all-solid-state fluoride-ion batteries are investigated and approaches for material design are implemented to circumvent those side reactions. First and foremost, parasitic side-reactions associated with conductive carbon are addressed which have been reported as the reason for high overpotentials, low cyclic stability and low discharge capacity. By substituting conductive carbon in anode composites for copper, reductive side reactions between carbon and the metals contained in the solid electrolytes could be avoided. After preparing a composite of copper and barium-doped lanthanum fluoride by ball-milling, the active material La2NiO3F2 could be incorporated with a second ball-milling step. Such anode composites show smaller overpotentials, larger discharge capacity and drastically improved cyclic stability than comparable carbon-based composites. X-ray diffractometry revealed a volume change of only ~0.5 % between the charged and discharged state of La2NiO3F2 explaining the good cyclability in absence of stack pressure. To allow the use of higher-capacity and lower-potential active materials like Sr2TiO3F2 and Sr3Ti2O5F4, a different solid electrolyte composed of CaF2 and BaF2 (referred to as CBF) had to be used, as La0.9Ba0.1F2.9 is not stable in operation with both strontium-titanate oxyfluorides as well as towards alloying reactions with copper at lower potentials. By using this alternative solid electrolyte in combination with copper as the conductive additive, both strontium-titanate oxyfluorides could be electrochemically defluorinated and cycled for the first time. Due to the comparably larger volume change of ~3-5 %, the cycling stability was rather low but could be improved by application of stack pressure. More detailed investigations of the behavior of solid fluoride-ion conductors with applied stack pressure show that fluorite-type Ba0.6La0.4F2.4 benefits from low stack pressure, which helps to densify the microstructure and increase the conductivity. For tysonite-type La0.9Ba0.1F2.9 and fluorite-type CBF, this behavior was not observed. Using scanning electron microscopy, it could be shown that La0.9Ba0.1F2.9 does not tend to form micro-cracks after initial compaction as compared to Ba0.6La0.4F2.4, explaining the good conductivity at low stack pressure. Due to the similarity of the activation volume of CBF to Ba0.6La0.4F2.4 as well as their crystal structures, a conduction mechanism for CBF could be hypothesized. Since side-reactions involving conductive carbon can also lead to accelerated degradation in cathode composites, an alternative was needed here as well. By synthesizing electronically conductive fluorine and antimony co-doped tin(IV)-oxide nanoparticles, cathode composites which don’t rely on conductive carbon could be prepared. It was observed that both, volume change and carbon fluorination were responsible for low cyclic stability in carbon-based cathode composites, as the cyclic performance of conductive oxide-based composites exceeded the performance of carbon-based composites significantly. As volume-change induced contact should not affect the cyclic stability significantly at 50 MPa stack pressure, it can be assumed that carbon fluorination is the determining factor for the cyclic stability at this pressure. Surprisingly, at 180 MPa stack pressure, the cyclic stability of carbon-based composites could be improved drastically. Using XPS-spectroscopy, the degree of fluorination of conductive carbon in the cells cycled at 180 MPa was investigated, revealing a significantly lower degree of fluorination than in comparable composites which were cycled without stack pressure. This implies that the application of high stack pressure suppressed the fluorination of carbon. Due to differences in the initial coulombic efficiency in carbon-free anode and cathode composites, it was assumed that certain oxidative side reactions persist and are still taking place in cathode composites. These side reactions are likely to be taking place in the form of gas evolution as few other possible side reactions remain after eliminating carbon fluorination. To be able to detect gas evolution experimentally, a setup for differential electrochemical mass spectroscopy for fluoride ion batteries at an operation temperature of up to 200 °C was developed. While the evolution of CO, NO and CO2 could be observed, possibly explaining the low initial coulombic efficiency, no evolution of oxygen could be detected. Hence, according to the valence of the transition metals in the starting materials and the number of intercalated fluoride ions, tetravalent nickel and cobalt should be present in the fluorinated active materials. The thermal stability of these fluorinated active materials was investigated using high-temperature X-ray diffraction and compared with data from in-situ X-Ray diffraction with coupled mass spectrometry of the fluorination of both active materials using AgF. Consequently, it could be concluded that the decomposition of electrochemically fluorinated active materials above 280 °C or 300 °C for La2NiO4 or La2CoO4 respectively lead to the formation of La2NiO3F2 and La2CoO3F2 as well as the release of oxygen gas. The results of this cumulative dissertation show that the operation of fluoride-ion batteries at high capacity and high cyclic stability was not prohibited by intrinsic material properties but by side reactions in the electrode composites and microstructural degradation. By substitution of materials and application of stack pressure, intercalation-based active materials can be cycled with high coulombic efficiency. According to differential electrochemical mass spectroscopy, the active materials are stable towards oxygen release even at high temperature and high state of charge, implying that they display structural prerequisites for high cyclic stability.Item Open Access Cathode materials for fluoride-ion batteries with Ruddlesden-Popper-type structure : structural and compositional evolution during cycling and the influence of stack pressure on cell performance(2025) Vanita, Vanita; Clemens, Oliver (Prof. Dr.)This cumulative dissertation focuses on the development, structural characterization and electrochemically induced changes of intercalation-type Ruddlesden-Popper (RP) oxides as cathode materials for all-solid-state fluoride ion batteries (FIBs). The study systematically investigates both n = 1 and n = 2 RP-type oxides, examining the impact of transition metal composition, layered crystal structure, and mechanical constraints such as stack pressure and temperature on phase behavior and cycling performance. A combination of solid-state synthesis, ex-situ X-ray diffraction, X-ray absorption spectroscopy, SQUID magnetometry, and electrochemical cell testing was employed to understand the interactions between structure and electrochemical performance that influence fluoride intercalation processes. The study presented in the dissertation is based on four peer- reviewed publications and collectively addresses the major challenges in the development of fluoride ion batteries such as identifying structurally stable and electrochemically active intercalation hosts, elucidating degradation mechanisms that constrain cycling stability and applying engineering strategies such as pressure to improve performance. This study introduced the first multi-transition-metal RP cathode for FIBs, La2Ni0.75Co0.25O4.08. Ex-situ XRD revealed the formation of three fluorinated phases upon charging. XAS measurements demonstrated that both Ni and Co undergo oxidation during the charging process as indicated by the shifts of their respective absorption edges to higher energies. This combined redox activity of both Ni and Co contributes to the observed structural stability and capacity retention. The material showed a stable reversible capacity of ~ 40 mAh g⁻¹ over 120 cycles with coulombic efficiencies between 85 % and 90 % making it a stable intercalation-type FIB cathode. Taking this into account, the impact of mechanical stack pressure on the cycling stability and electrochemical efficiency of the intercalation-type cathode material La2Ni0.75Co0.25O4.08 in fluoride-ion batteries was studied in detail. Testing was performed at 170 °C and high stack pressure (450 MPa). The application of high stack pressure notably enhances the cell performance compared to ambient pressure cycling. Under pressure, the cell rapidly attains the coulombic efficiency surpassing 95% within the first 10 cycles. Furthermore, the energy efficiency of the pressurized cell stabilizes around 60% and remains steady up to 420 cycles over a one-year test period. The main effect is a reduction of overpotentials during charge-transfer processes which suppresses carbon fluorination, a side reaction in fluoride-ion batteries. This suppression helps to preserve conductive pathways leading to improved cycling longevity and stability. The study demonstrates that mechanical pressure is a critical parameter in enabling extended cycling of La2Ni0.75Co0.25O4.08 cathodes achieving stable performance over 420 cycles. The work was then extended to investigate LaSrMnO4, an n = 1 RP type oxide as a cathode material in FIBs. The material demonstrated reversible phase transitions during cycling with interstitial sites accommodating fluoride ions without irreversible degradation. Notably, LaSrMnO4 exhibited fluoride ion diffusivity which facilitated efficient ion transport. The electrochemical performance of LaSrMnO4 was evaluated at 140 °C and 170 °C. At 170 °C, the material achieved stable cycling over 100 cycles with no degradation observed, as confirmed by X-ray diffraction analysis. Additionally, LaSrMnO4 demonstrated successful operation at a lower temperature of 140 °C, outperforming other materials in low-temperature cycling. DFT-based thermodynamic calculations and molecular dynamic simulations identified stable intermediate phases and two-dimensional fluoride-ion migration pathways, offering mechanistic insight into the ion mobility. This was the first experimental and theoretical confirmation of 2D anion transport in RP-type oxides. Increasing structural complexity, associated with high order of RP phases motivates the investigation of LaSr2Mn2O6.96, a bilayer RP-type oxide (n = 2) with reduced La content. Upon fluoride intercalation, ex-situ XRD revealed the sequential formation of three distinct tetragonal phases, each associated with an expanded c-axis and progressive Mn oxidation. The lattice parameter c increased from ~19.98 Å (pristine) to ~23.50 Å in the fully fluorinated phase. XAS confirmed reversible Mn oxidation and the structural retention of the RP host framework. Under stack pressure (20 kN), electrochemical cycling of LaSr2Mn2O6.96 in the potential range of 1 V to -1 V exhibited a continuous increase in specific capacity from ~ 30 mAh g⁻¹ to ~ 68 mAh g⁻¹ over 200 cycles with ~ 99 % coulombic efficiency and no signs of capacity fading whereas the absence of stack pressure results in a continuous capacity fading. These results confirm the structural reversibility and fluoride intercalation capacity of LaSr2Mn2O6.96, making it as a cycling stable cathode for all-solid-state FIBs, especially under the application of stack pressure. Overall, this dissertation establishes an extended understanding of intercalation mechanisms in RP-type oxides for FIBs. It demonstrates that phase stability, redox reversibility and ion transport can be tailored through compositional tuning (Ni/Co vs Mn), structural dimensionality (n = 1 vs n = 2) and mechanical optimization (stack pressure). The findings provide a roadmap for the rational design of next-generation solid-state FIB cathodes, offering a pathway toward safe, efficient battery systems that rely on alternative resources and go beyond lithium-ion technology.