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Item Open Access Ab initio thermodynamic study of defective strontium titanate(2013) Blokhin, Evgeny; Maier, Joachim (Prof. Dr.)In the presented thesis the perfect and defective SrTiO3 bulk crystals and their (001) surfaces are considered on ab initio level. Since the experimental study of the complex defective systems is comparatively expensive and difficult, and the computer performance has been greatly increased in the last years, the ab initio modeling became very efficient tool to be applied in this field. Additionally, there is a significant industrial demand for the investigation and improvements of the performance of the perovskite-based electrochemical devices, e.g. solid oxide fuel cells and permeation membranes. Finally, there is a lack of methodological studies on defect thermodynamics. The oxygen vacancies and iron impurities, as well as their complexes, are the characteristic defects in SrTiO3 perovskite material. The understanding of basic defect properties and defect-induced phenomena under realistic external conditions requires calculation of thermodynamic properties (the free energy and entropy effects). Thus, thesis is focused on atomic vibrations, i.e. phonons, which are necessary to go beyond standard 0°K approximation and to provide a link to the thermodynamic properties at finite temperatures. This is necessary for a realistic treatment of electrochemical devices. The chosen ab initio modeling scheme is found to ensure the most accurate description simultaneously for the structural, electron and phonon properties of the perfect SrTiO3. Namely, the splitting of the phonon frequencies due to the antiferrodistortive phase transition at 105°K is confirmed to be very small (2–12 cm-1). The experimental temperature dependence of the SrTiO3 heat capacity is also successfully reproduced. Further, the modeling scheme is applied for thermodynamic treatment of oxygen vacancies and iron impurities in SrTiO3 at finite temperatures. The calculated phonon densities of states and group-theoretical study of the defect-induced phonon frequencies are used for the experiment analysis. Several defect-induced local phonon modes are identified, and the experimental Raman- and IR-spectroscopy data are interpreted. The Jahn-Teller-type local lattice distortion around both Fe4+ impurity and oxygen vacancy VO is shown to result in Raman- and IR-active phonons. In particular, the experimentally observed Raman frequency near 700 cm-1 is shown to arise for both defects due to a local O ion stretching vibration nearby the Jahn-Teller defect. However, an absence of such a frequency in an experimental phonon spectrum is found to be a manifestation of formation of Fe3+–VO complexes with oxygen vacancies in the first coordination sphere of iron impurities. The Gibbs formation energy calculated for the neutral oxygen vacancies in bulk SrTiO3 taking into account the phonon contribution is found to be in excellent agreement with the experiment. The phonon contribution to the formation energy is shown to increase with temperature, to about 5% above 1000°K. The predicted relative stability of several structural complexes of oxygen vacancy and iron impurity in SrTiO3 is confirmed by known experimental measurements. Several structural models of such Fe3+–VO complexes in SrTiO3 are discriminated according to the XANES and EXAFS experiments. On an example of SrO-terminated SrTiO3 ultrathin films, the one-dimensional confinement effect on the vacancy formation energy is found to be inconsiderable at 0°K. The phonon contribution to the Gibbs free energy of VO formation in such ultrathin films at finite temperatures is shown to be minor. This suggests the further account of anharmonic effects is required. The workflow developed in thesis is proposed for the modeling of wide class of defects in non-metallic solids. Several auxiliary computer tools were designed in order to simplify such possible studies.Item Open Access Modeling the rational synthesis of magnesium difluoride via the low-temperature atom beam deposition method(2013) Neelamraju, Sridhar; Jansen, Martin (Prof. Dr.)A given chemical system, in general, will realize many (meta)stable structures, some of which might be observable on an experimentally viable timescale. Some of these polymorphs could have novel properties waiting to be exploited. However, addressing the problem of directing solid state synthesis towards such unknown polymorphs remains a major challenge. The prediction of new compounds using various theoretical methods is not usually followed up by an actual synthesis and planning the synthesis of novel inorganic solids often requires recourse to theoretical methods that can not only predict the thermodynamic stability of possible structure candidates but also model the kinetic behavior of atoms during the experimental synthesis. Here, we strive to fill this gap between knowledge derived from structure prediction methods and performing the actual synthesis of new structures experimentally by using tools available to the theoretical chemist that include energy landscape search algorithms, ab initio spectroscopy calculations and molecular dynamics simulations with the synthesis of MgF2 via the low-temperature atom beam deposition (LT-ABD) method as the model system. Hence, in a first step, in order to understand the possible cluster modifications of MgF2 that can exist in the vapor phase, we perform global optimizations on neutral and charged clusters using Monte-Carlo simulated annealing and find many possible structures. We also explore the energy landscape of (MgF2)3 and (MgF2)4 using the threshold algorithm in order to be able to estimate the stability and dynamics of these clusters. This method allows us to determine not only the stable and metastable isomers but also the barriers separating these isomers and the probability flows among them, yielding estimates of the stability of all the isomers found. We find that there is reasonable qualitative agreement between the ab initio and empirical potential energy landscapes, and important features such as sub-basins and energetic barriers follow similar trends. However, we observe that the energies are systematically different for the less compact clusters, when comparing empirical and ab initio energies. Furthermore, we employ the same procedure to additionally investigate the energy landscape of the tetramer. For this case, however, we use only the empirical potential due to computational limitations. This is followed by the calculation of Raman and IR spectra including the phonon modes and their intensities, for all the clusters found from the above study. We also calculate IR intensities and phonon modes for all bulk polymorphs of MgF2. This way, we provide the synthetic chemist with a means to observe possible (meta)stable phases of this system in both the vapor phase and the deposit while performing a deposition experiment on MgF2. The calculated data are compared with in-situ measurements in the LT-ABD apparatus. The MgF2 vapor and film are characterized via Raman spectroscopy of the MgF2 gas phase species embedded in an Ar-matrix and of the MgF2-films deposited onto a cooled substrate, respectively. We find that, in the vapor phase, there are mostly monomers and dimers of the neutral and charged species present in our experimental setup. Furthermore, the results suggest that in the amorphous bulk MgF2, rutile-like domains are present and MgF2 clusters similar to those in the matrix. Finally, peaks at about 800 cm-1, which are in the same range as the Ag modes of clusters with dangling fluorine atoms connected to three-coordinated Mg atoms, indicate that such dangling bonds are also present in amorphous MgF2 and can be used to track the amorphous to crystalline transition in this system. Finally, we model the growth of solid MgF2 from the gas-phase on an Al2O3 substrate as it occurs in a real LT-ABD experiment, a hypothetical MgF2-anatase substrate and a MgF2-rutile substrate. The process is studied in all its stages, from the dynamics of MgF2 clusters in the gas phase, over their impact on the surface of the cold and hot substrates, and their diffusion on the substrate, to the formation of crystallites. The growth process was analyzed as a function of synthesis parameters including the substrate temperature, deposition rate and types of clusters deposited. Both high and low rates resulted in the formation of amorphous MgF2 deposits. On annealing, we discovered a possible mechanism for the stabilization of the CaCl2-type structure. We find two competing structures in the first few nanoseconds of the deposition related to the CaCl2 and CdI2 structure types and argue that this competition stabilizes the CaCl2-type structure long enough for experimental observations to take place. Furthermore, the atom arrangements found in our simulations are in good agreement with existing experimental observations based on TEM and XRD measurements, for both the amorphous and the partly ordered metastable phase.Item Open Access Fluorine-promoted intramolecular aryl-aryl coupling : toward the isomer-specific fullerene synthesis(2013) Kabdulov, Mikhail; Jansen, Martin (Prof. Dr. Dr. h. c.)Direct synthesis of fullerenes is of considerable interest as a method to access new fullerenes which cannot be obtained in the uncontrolled process of graphite evaporation or form in low yields as a “hard-to-isolate” mixture. The general strategy of the direct approach to fullerenes is based on the synthesis of polycyclic aromatic hydrocarbons (PAH) that already contain the required carbon framework. Such “unrolled” molecules can be “rolled up” to form fullerenes through intramolecular Aryl-Aryl condensation under flash vacuum pyrolysis (FVP) conditions. The presence of chlorine or bromine in the initial precursor is essential for effective Aryl-Aryl condensation via free radical mechanism. On the other hand the use of chlorine and bromine functionalizations reaches its limits in the case of large molecules such as fullerene precursors. Availability of alternative promoters which do not have these disadvantages is a key prerequisite for successful direct fullerene synthesis. In this work various functional groups have been tested as alternative promoters of Aryl-Aryl intramolecular condensation under FVP conditions. Methyl and fluorine functionalization has been found to be promising approaches. Unexpected high selectivity in cyclization was observed for fluorine derivatives. It was found that HF elimination is a synchronous process leading directly to the target molecule without any intermediates, thus producing no side products. The small size and low molecular weight of fluorine as well as high thermostability of the C-F bond, makes fluorine a “perfect” activating group for rational fullerene synthesis. Since fluorine can promote the desired ring closure only if hydrogen is placed neighboring in space in the precursor structure, full control in the direction of the condensation can be achieved. It was shown that the use of fluorine, as an activating group, solves the problem of selectivity in FVP and provides an effective conversion of the respective PAH precursors. Several fullerene precursors containing fluorine atoms in key positions have been synthesized and investigated as a precursor for direct fullerene synthesis. Furthermore optimization has led to the discovery of a highly effective alternative solid-state strategy for intramolecular Aryl-Aryl coupling via HF elimination. The efficiency of the approach has been demonstrated by quantitative transformation of the precursor molecules to the desired PAHs and buckybowl structures. The quantitative conversion to the extended C46 buckybowl, representing more than 75% of the C60 fullerene connectivity, demonstrates the high potential of the technique for construction of extended non-planar carbon based nanostructures, including higher fullerenes, giant buckybowls and nanotubes. The results obtained point the way to the fabrication fullerenes as well as other carbon based nanostructures such as single walled nanotubes and nanoribbons in a fully controllable manner.Item Open Access Size effects on lithium storage and phase transition in LiFePO4/FePO4 system(2013) Zhu, Changbao; Maier, Joachim (Prof. Dr.)LiFePO4 is one of the most promising cathode materials, especially for its great potential to be applied in electric vehicles (EVs) and hybrid electric vehicles (HEVs), and has attracted great interest due to its appealing advantages, such as high theoretical capacity (170 mAhg-1), high safety, environmental benignity and low cost. Although a great improvement has already been reached in terms of electrochemical performance of LiFePO4 by doping, size-reduction, and network formation, several intrinsic properties of LiFePO4 are still not clear and need further investigation. One of the most important unresolved issues is the effect of size on lithium storage and phase transition in the LiFePO4/FePO4 system, which is not only crucial for fundamental understanding of LiFePO4 behavior, but also relevant to the application of such materials. In this thesis, morphology and size controlled synthesis of LiFePO4 and related electrochemical performance are discussed at first. Afterwards, size effects on miscibility gap, lithium potential variations and phase transition process are investigated systematically. The main results of this thesis are the following: Carbon-coated single-crystalline LiFePO4 thin nanowires are successfully prepared by the electrospinning method, which show good rate performance and excellent cycling stability due to the unique morphology. Small LiFePO4 nanoparticles (the thickness only around 10 nm) can be prepared by the oleylamine-assisted polyol method and the particle sizes can be controlled by adjusting experimental parameters, such as the ratio of oleyamine to tetraethylene glycol (TEG), the precursor concentration, the reaction time and the addition of carbon nanotubes. After sintering at 700 °C for 2 hours the material displays excellent electrochemical performance. The shrinking of the miscibility gap with reduction of the particle size is observed by the potentiostatic intermittent titration technique (PITT). Lithium potential variations for nanocrystalline and amorphous LiFePO4 are investigated thermodynamically and experimentally by considering the lithium intercalation regime (single phase regime and two phase regime). For nanocrystalline LiFePO4, the reversible open-circuit voltage (OCV) values decrease with reduction of particle sizes. Surface chemistry (γ) plays a crucial role in the OCV variations. For amorphous LiFePO4, compared with crystalline LixFePO4, the excess OCV can be either negative or positive, which can be explained by the signs of the ionic part and the electronic part of the excess chemical potential of lithium. Phase transition of large LiFePO4 single crystal is investigated by chemical delithiation. FePO4 layers with high porosity and cracks are observed at the surface of LiFePO4. The kinetics is governed by a parabolic growth law that indicates diffusion limitation. The pore/crack network provides fast diffusion channels and enhances the kinetics pronouncedly. With the help of the advanced scanning transmission electron microscopy with annular bright field imaging (STEM-ABF) performed in Sendai (Japan), a first order lithium staging structure is directly observed in the partially delithiated Li1-xFePO4 (x~0.5) nanowires for the first time. Size-dependent staging structure is also found. For large crystals, staging structures form an intermediate phase between LiFePO4 and FePO4, and the staging area narrows with increasing size. For small crystals, the staging structure appears throughout the whole particle.Item Open Access Oxygen exchange kinetics of the potential solid oxide fuel cell cathode material (Bi,Sr)(Co,Fe)O3-delta(2013) Wedig, Anja; Maier, Joachim (Prof. Dr.)In this thesis, the oxygen exchange kinetics of Bi-based perovskites was investigated. The investigations were conducted on morphologically well-defined thin-film model electrodes by means of electrochemical impedance spectroscopy. Alongside, a range of other methods was applied to further characterize the structural and transport properties which possibly influence the oxygen exchange performance. In particular, electrochemical polarization experiments were carried out to examine the relationship between ionic conductivity and oxygen exchange rate. Thus, it was aimed to gain a deeper understanding of the mechanistic principles governing the oxygen exchange reaction on mixed-conducting perovskite-type oxides.Item Open Access Synthese und Charakterisierung von Fluorooxoboraten, Fluoroboratosulfaten und Fluoroboratophosphaten(2013) Pilz, Thomas; Jansen, Martin (Prof. Dr. Dr. h.c.)Li2B6O9F2 wurde erstmals durch Umsetzung verschiedener Borate (LiBO2, LiB3O5, Li2B4O7) sowie B2O3 mit LiBF4 bei 400 °C erhalten. Phasenreines Produkt gewinnt man aus den Gemengen von LiB3O5 mit LiBF4 im Verhältnis 2 : 1 sowie LiBF4 mit B2O3 im Verhältnis 2 : 3. Neues Li2B3O4F3 wurde durch Umsetzung von LiBO2 mit LiBF4 bei 280 °C und anschließende Entfernung des Nebenprodukts LiF durch Lösen in BF3.THF reinphasig gewonnen. Während des Temperns bei 350 °C wandelt sich Li2B3O4F3 in Li2B6O9F2 um. Eine Pulverprobe von Na3B3O3F6 in ausreichender Kristallinität wurde durch Synthese aus den wasserfreien Edukten NaBO2 und NaBF4 bei 400 °C gewonnen. Das entsprechende Pulverdiffraktogramm bestätigt, im Gegensatz zu bisherigen Pulveraufnahmen die Einkristallstrukturlösung. Das Fluorooxoborat kristallisiert in der Raumgruppe C2/c. Na2SO4BF3 entsteht durch Lewis-Säure-Base-Reaktion von Na2SO4 mit BF3 bei 330 °C in geschlossener Quarzglasampulle. Von vorher unbekanntem Na3B2PO5F4 wurden zwei Modifikationen erhalten. Sie kristallisieren in den Raumgruppen P21/n (Na3B2PO5F4-I) und Cmcm (Na3B2PO5F4-II). Aufgrund der Synthesebedingungen lässt sich darauf schließen, dass ersteres die Tieftemperaturmodifikation und letzteres die Hochtemperaturmodifikation ist. Na3B2PO5F4 bildet sich durch Festkörperreaktion von Na3PO4 mit NaBF4 bei 350 bis 400 °C oder durch Begasung des Phosphates mit BF3 bei 350 °C. Li2(BF4)F entstand bei der thermischen Behandlung von LiBF4 bei 300 °C in geschlossener Quarzglasampulle.Item Open Access Influence of extended defects on the electrical properties of TiO2 (rutile)(2013) Adepalli, Kiran Kumar; Maier, Joachim (Prof. Dr.)TiO2 is a promising material for many technological applications such as solar cells, water splitting, memory devices and Li-ion batteries. Even though the functionalities are diverse depending on their applications, the point defects in TiO2 play a decisive role in all these technologies. The point defect chemistry of TiO2 was intensively studied in the last few decades and so far modication of point defect concentrations was attained by either aliovalent doping or by nano size effects. Both methods are widely applied in the eld of solid state ionics, however, with limitations such as limited solubility of dopants grain growth effects already at moderate temperatures. Another adjusting screw for altering point defect concentrations is by incorporating mircostructural modications in the material such as one dimensional line defects (dislocations), or two dimensional planar defects (grain boundaries). In ionic solids, these extended defects should be charged, and in order to maintain global charge-neutrality, they will locally modify the point defect concentration in space charge zones. The influence of dislocations on point defect concentrations in oxides, and thus the electrical properties, has not been studied much in TiO2 for far. The present thesis gives such a detailed investigation using TiO2 in the rutile structure as model material. Dislocations were created in TiO2 single crystals as well as polycrystallinematerials by uniaxial compression at elevated temperature. Based on the creep deformation map for TiO2, a suitable combination of temperature and pressure was chosen to activate dislocation creep. The dislocation formation and migration strongly depends on the material properties; in case of TiO2 various factors { such as comparable cation (Ti interstitials) and anion (oxygen vacancies) mobilities, comparably low melting temperature and shear modulus when compared to other wide band gap oxides { allows for their formation at moderate conditions (typically at 1000 C, 40 MPa for single crystals and 925 C, 400 MPa for polycrystalline materials). In the temperature range (350 C to 550 C) in which the conductivity measurements have preformed, dislocations are typically immobile and therefore measured properties well reproducible. Dislocations generated by this process are characterized by transmission electron microscopy and it is found that the dislocations favorably lie on f110g slip planes. Based on the slip planes, electrical measurement axis is chosen to be [001] and [110], directions parallel and perpendicular to the dislocations respectively. Electrical properties are also studied as a function of dislocation density, temperature and oxygen partial pressure. With increasing density of dislocations the conductivity type of TiO2 at moderate temperatures (550 C) and high oxygen partial pressures (1 bar to 1e-5 bar) changes from usual hole conductivity (p-type semiconductor) to predominant ionic conductivity. Further, to discriminate between oxygen vacancy and titanium interstitial transport in the partial ionic conductivity, oxygen isotope exchange and SIMS analysis were performed. The comparison of the results from these characterization techniques shows that the enhanced ionic conductivity is due to negatively charged dislocation cores and adjacent space charge accumulation zones of positive carriers. This interpretation is further supported by the fact that the concentrations of ionic defects with their higher charge are influenced to a much greater degree than electronic defects. Similar effects of dislocations on the electrical properties are observed for polycrystalline TiO2. Dislocation creation have a persistent effect on the electrical properties with the ionic conductivity of the samples increased more strongly than the electron hole conductivity. The partial ionic conductivity of the sample is measured by Hebb-Wagner type electrodes, and an unusually high ionic transference number is observed for TiO2 at high oxygen partial pressures. Further, effects of acceptor dopant (0.1 mol % Y) on the dislocation generation is studied by either homogeneously doping or by selectively decorating grain boundaries. No dislocations were observed in homogeneously doped samples due to solid solution strengthening, which therefore resulted in a regular defect chemistry as expected for an acceptor doped TiO2. In case of decorated samples, very similar to the undoped samples, dislocations are generated throughout the sample and once again a oxygen partial pressure independent ionic conductivity is observed, typically in the range of 1 bar to 1e-7 bar. The effect of dislocations is very persistent - even a high temperature treatment at 1300 C for 5 h did not anneal much of the dislocation density. Hence, the observed changes in electrical properties are very stable and reproducible over a wide temperature range. Grain boundary cores of perovskite and fluorite structured oxides with large band gaps such as SrTiO3, CeO2, ZrO2 (Y stabilized) are typically positively charged due to the presence of excess anion vacancies. Positive grain boundary cores impede the transport of positively charged defects such as holes, cation interstitials and anion vacancies by formation of depletion layers. However, in case of TiO2 also negative grain boundary or dislocation core charges may form. For this reason, grain boundaries of TiO2 bicrystals with symmetric tilt boundaries are investigated in the second part of the study. Two main orientations viz. ∑5 (210)[001] and 6 [001] symmetric tilt boundaries are investigated with a focus on the boundary electrical properties. High-resolution transmission electron microscopy revealed that the symmetric tilt grain boundaries correspond to a periodic array of dislocations with a spacing according to Frank's rule. It is also observed that the electrical conductivity in two boundary orientations are similar and the boundaries are not blocking for the transport of holes. This indicates that the symmetric tilt boundaries in TiO2 are not similar to other wide band gap oxides, but are usually positively charged. To summarize, it is quite obvious from this work on TiO2 that dislocations can be used as a means of modifying defect transport of ionic solids locally and globally, hence, allowing additional degrees of freedom for tuning the ionic/electronic properties of various functional oxides.Item Open Access Synthesis and characterization of new alkali-metal oxometalates obtained via the azide-nitrate route(2013) Djuris, Katarina; Jansen, Martin (Prof. Dr. Dr. h. c.)During this PhD thesis, azide/nitrate route has been confirmed to be an efficient tool in the synthesis of alkali-metal oxometalates. As a particularly beneficial feature, azide/nitrate route enables to approach such metastable oxometalates, which are not in reach via classical oxide-acid-base reactions. Especially, with this method it was possible to synthesize new ternary oxides containing low valences and uncommon coordination numbers (CN) of the respective alkali ions (K, Rb, Cs) and transition metal (Ni). Most significantly, this method allows to precisely fix the oxygen content of the target compound, and thus the valence state of transition metal, by the starting azide/nitrate ratio. Along this approach, we have been able to synthesize two families of low dimensional, intrinsically doped oxocuprates(II/III) and oxonickelates(II/III). In addition, the influence of the kind of alkalimetal on the secondary structure as well as physical properties of the one dimensional polyoxometalate anion was examined. Also, introduction of the alkaline earth metal azides in the azide/nitrate route has resulted in a the new phase displaying unusual structural features.Item Open Access Boundary effects on the electrical conductivity of cerium oxide thin films(2013) Göbel, Marcus; Maier, Joachim (Prof. Dr.)Boundary effects in mixed and ionic conductors and in particular space charge layer (SCL) effects are a major field of research in solid state ionics since they are known to strongly affect the electrical transport properties of the materials. In particular, in this study the boundary effects of cerium oxide were investigated which due to its high ionic conductivity is a material of large relevance for a wide range of applications such as solid oxide fuel cells (SOFCs), oxygen membranes and catalysis. Here the thin film geometry was applied which offers the advantage of a usually well defined microstructure allowing to access effects at both the film-substrate interface (FSI) and the grain boundaries (GBs). The ceria thin films were grown with pulsed laser deposition (PLD) on various substrates. Thin films were prepared of (1) different doping contents (nominally pure, acceptor doped and donor doped), (2) different microstructures (epitaxial and nanocrystalline) and (3) different thicknesses (between 20 and 450 nm). Their microstructure was characterized with XRD, SEM, TEM and electron diffraction. The defect chemistry and conductivity properties of the samples were investigated with impedance spectroscopy. Additionally, a software to numerically compute the SCL profiles and conductivity effects in ceria was developed. The numerical approach allowed for the determination of all relevant SCL profile characteristics and for the correlation of them with both the material properties and the resulting conductivity effects. Notably, the numerical approach was observed to yield precise results without the use of further assumptions also for asymmetric and mixed cases in contrast to the well known analytical solutions. This allowed for a test of the assumptions made in the analytical solutions which resulted in the development of improved analytical relationships. Remarkably, the improved analytical approach which is not restricted to ceria but generally applicable was observed to yield very reliable outcomes even for complex situations. In most cases the boundary effects were found to dominate the conductivity of the investigated ceria thin films. Effects at both the film-substrate interface and at the grain boundaries were observed. As expected in the framework of the SCL theory no significant FSI effect was detected in strongly acceptor doped films. In epitaxial, nominally pure ceria films grown on Al2O3 <0001> the conductivity was observed to be reduced at the FSI in agreement with the SCL theory. In nanocrystalline, acceptor doped samples grown on SiO2 <0001> a significant decrease of the conductivity at the GBs was observed in accordance with a SCL potential of 0.32 ± 0.05 V. For this set of samples a thickness dependence of the grain size was detected resulting in a considerable change of the conductivity with film thickness. Therefore, it could be demonstrated that a thickness dependent conductivity in polycrystalline samples can not always be assigned to FSI effects; a finding of relevance for a number of similar studies on interface effects. Nanocrystalline, acceptor doped films grown on Al2O3 <1-102> and MgO <100> substrates were found to exhibit a less significant decrease of the ionic conductivity and, hence, a smaller SCL potential of 0.19± 0.05 V. This is most likely the result of the smaller lattice mismatch between substrate and CeO2 film resulting in less pronounced GB core charges. Thin films prepared at room temperature, characterized by a nanocrystalline microstructure with very small grains, were measured to exhibit very pronounced SCL effects (decrease of the ionic conductivity by 3 orders of magnitude). Most likely here the Mott-Schottky assumption is fulfilled much better resulting in a particularly strong depletion of the oxygen vacancies. Remarkably, for these samples the pO2 of the electrolytic domain boundary, was found to be shifted by 29 orders of magnitude compared with what is expected from an extrapolation of the literature bulk data. Two superimposing effects were found to be the origin of this drastic shift: In addition to (1) the pronounced SCL effects, (2) already in the bulk the electronic conductivity was measured to be strongly increased. Both effects also affected the activation energies. Remarkably, in this case the electronic activation energy fell even below the ionic value resulting in an increase of the electronic conductivity contribution for decreasing temperatures in marked contrast to the properties typically observed in ceria. Notably, in donor doped ceria the electronic conductivity was observed to be decreased at the GBs. The recorded conductivity data indicates that either a negative SCL potential or a change of the electron mobility at the GBs is the cause of this effect. The investigation could also confirm the presence of oxygen interstitial defects in donor doped cerium oxide which was suggested in earlier studies. Also the oxygen insertion enthalpy was measured.Item Open Access Synthese und Charakterisierung neuer Übergangsmetallkomplexe(2013) Aydin-Cantürk, Dilahan; Jansen, Martin (Prof. Dr. Dr. h.c.)Die vorliegende Arbeit befasst sich hauptsächlich mit der Synthese und Charakterisierung von mehrkernigen linearen Metallkomplexen, die ausschließlich Übergangsmetalle ent-halten. Neben linearen Metallkomplexen wurden oxo-verbrückte und schwefelhaltige einkernige Verbindungen des Eisens untersucht. Die neue heterometallische Kettenverbindung CrCrNi(dpa)4Cl2•Et2O (1) kristallisiert monoklin in der Raumgruppe P21/c (Nr. 14). Die Suszeptibilität folgt im Temperaturbereich zwischen 2 bis 300 K dem Curie-Weiss-Gesetz. Der homometallische Komplex [Cr3(dpa)4Cl2]•[CuCl2] (2) kristallisiert orthorhombisch in der Raumgruppe Pccn (Nr. 56) Die Kristallstruktur ist aus diskreten [Cr3(dpa)4Cl2]+-Kationen und Dichlorocuprat(I)anionen, [CuCl2]– aufgebaut. Wie Komplex 1, zeigt auch 2 im gesamten Temperaturbereich von 2 bis 300 K annähernd idealen Curie-Paramagnetismus. Wie es in vielen Verbindungen dieses Typs vorkommt, beinhalten die Komplexe 1 und 2 eine lineare unsymmetrische Metallkette, die aus einer vierfach gebundenen dia-magnetischen (Cr2)4+-Dimereinheit und einem paramagnetischen 3d-Metallion mit high-spin-Konfiguration (Ni2+ bzw. Cr3+), aufgebaut ist. Die Kristallstrukturen beider Komplexe weisen eine intrinsische Orientierungsfehlordnung mit zwei kristallographisch unabhängigen Orientierungen auf. Jedes Metallatom ist pseudo-oktaedrisch koordiniert, wobei jeweils 4 N-Atome der dpa–-Liganden, eine quadratisch-planare Anordnung um die Metallatome bilden. Ferner werden die axialen Koordinationsstellen durch ein Metallatom bzw. durch die einzähnigen Chloratome eingenommen. Der gemischtvalente vierkernige Eisenkomplex [FeII FeIII(µ3-O)(dpa)2Cl]2 (3) kristallisiert monoklin in der Raumgruppe P21/c (Nr.14). Die Kristallstruktur von 3 beinhaltet einen [FeII2FeIII2(µ3-O)2]6+-Kern, bestehend aus zwei Fe(II)- und Fe(III)-Ionen in einer „Schmetterlingsanordnung“. Zwei (µ3-O)2–-Ionen ver-brücken jeweils drei der vier Eisenatome in einer Dreiecks-Anordnung. Jedes der zwei Eisen(III)-Atome ist pseudo-oktaedrisch koordiniert. Die Eisen(II)-Atome befinden sich hingegen in einer tetraedrischen Koordinationsumgebung und formen die „Flügel“ des „Schmetterlings“. Das Ergebnis der magnetischen Messung liefert Hinweise auf antiferromagnetische Wechselwirkung zwischen vier Eisenzentren und deutet auf einen diamagnetischen S = 0 Grundzustand hin.