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Item Open Access Synthesis of new fullerides via the "break-and-seal" approach and their characterization(2009) Kozhemyakina, Nina V.; Jansen, Martin (Prof. Dr.)The present dissertation deals with the synthesis and characterization of fullerides. For the first time the "break-and-seal" technique was applied for fulleride synthesis. The reaction was performed in a completely all-glass apparatus under vacuum, avoiding the use of glass connections and use of grease. Starting from crown-ethers, potassium metal and C60 fullerene, six new fullerides have been synthesized. The modified "temperature difference method" was successfully used for growing single crystals from solution within a few days. In [K(DB24C8)(DME)]2C60*(DME) the fullerene unit has a charge of 2-. The (C60)2- units are arranged in hexagonal layers parallel to the ab plane, forming distorted trigonal prisms. The fullerene anions and potassium cations develop a pseudobinary topology which is reminiscent of the CdI2 structure type. Bond lengths' distribution in (C60)2- was examined. One orientation of the dianion was found to match perfectly the one predicted by calculations. KC60(THF)5*(THF)2 crystallizes in a structure with fully ordered C60 units. C60- anion-radicals and K+ form ion pairs. The ion pairs form corrugated layers in the ac crystallographic plane, the given compound being an example for a low-dimensional fulleride partial structure. For the compound [K(DB24C8)(THF)]2C60*THF the structure solution was complicated by the disorder of crown-ether and solvent molecules which could not be overcome, although the (C60)2- unit was ordered. In [K(DB24C8)(DME)]C60 the fullerene unit exists as a monomeric anion-radical and in [K(DB24C8)(DME)]2[C60]2 - as a dimer-dianion. The latter compound is an example of rather not many fulleride structures, where C60 exists in the form of dimers. The interfullerene C-C bond length is 1.57(3) Å. In [4{K(DB18C6)(C60-)}(THF)6]*[C60]*(THF)6 at temperatures above 220 K each of the four C60- units exists in form of anion-radicals, and at lower temperatures - as a dimer-dianion, the interfullerene bond being 1.63(0) Å. The dimers are fully ordered. In addition, uncharged disordered C60 molecules are found, what follows from the charge balance. The low-temperature phase is a first example of a fulleride structure where fullerene exists in three different bonding states: anion-radical monomer, dianion-dimer, and a neutral C60. In the dimer, the pentagons adjacent to sp3-hybridized carbon atoms, are in trans-conformation. DFT calculations were performed, and it is now for the first time that a localization of the negative charge on a small fragment of the C60 cage was found out. Knowing this, it becomes conclusive, considering the Coulombic repulsion, that the preferred orientation of two bound C60- units is trans-conformation. Magnetic measurements were performed. The method for fulleride synthesis used in the present work has a big potential for broadening by using different metals (e.g. alkali, alkali-earth), varying the complexing agents (crown-ethers, cryptands), as well as the organic solvent (or solvent mixtures).Item Open Access Percolation of oxide networks in nano-composite lithium salt electrolytes(2009) Jarosik, Anna; Maier, Joachim (Prof. Dr.)The starting point for this work was to investigate the network behaviour present in solid-liquid composite electrolytes. Results are obtained using simulations based on real systems. The formation of networks of filler particles that depend sensitively on of volume fraction and particles size, results in interesting influences on electrochemical as well as mechanical properties. In the literature it has been already found that the dispersion of oxides can lead to a conductivity increase in non-aqueous electrolytes (e.g. "soggy sand" electrolytes) as well as in polymer electrolytes. For the latter various explanations are proposed in the literature, many of those concentrating on the change in the mobility owing to segmental motion or variation in the degree of crystallinity as possible causes for the increase in conductivity. In the case of the "soggy sand" electrolytes ion adsorption was proposed as most probable mechanism. Replacing the high polarity solvent with a solvent of low polarity allows one to address the ion adsorption scenario more explicitly. The enhancement of the charge carrier concentration is expected to be due to the selective adsorption of one ion sort on the insulating surface of the introduced oxide particles. The adsorption effect is a special case of the general concept of heterogeneously doped ion conductors - originally developed in Stuttgart for explaining second phase effect on the conductivity of weak solid electrolytes - e.g. AgCl, PbF2. In a covalent matrix, such as polymers and organic molecular crystals, the immobile ground state is the undissociated ion pair and the conductivity effect is attributed due to the adsorption of one of the pair's constituents. This would result in a break-up of the ion pair and the generation of a mobile counter ion, thereby increasing the ion conduction. The enhanced conductivity showed percolation behaviour that is typical for interfacial conductivity. As oxides with sufficiently acidic surfaces were used, one could presume that the oxide particles are able to trap the anions on the surface, resulting in the dissociation of ion-pairs in solution. This, in general, leads to an excess of cations in the space charge regions around the oxide particles. It was observed that lithium ion conduction depends on particles size, surface area or volume fraction, on the solvent's dielectric constant and viscosity, as well as on concentration and nature of the different lithium salts. In this thesis 'Monte Carlo Random Walk' was applied taking into account variation of particle size and volume fraction. This 'Random Walk' simulation helps to better understand network formation and stability of these composite electrolytes. At first, the network formation was modelled according to an irreversible 'hits-and-stick' mechanism (diffusion-limited cluster aggregation). Then, once the particle form a network (fractal percolating cluster), they are allowed to coarsen on a longer time scale, which leads to minimization of their surface energy. Coarsening, as was found, rapidly interrupts the percolation pathways resulting in overall decrease of the conductivity with time, which at certain point is lower compared to 'filler-free' electrolytes. The most reasonable explanation suggests that due to coarsening more compact clusters are formed, stronger and faster sedimentation occurs that consequently leads to trapping of the solvent, thereby pronouncedly decreasing the composite's conductivity. The results show that oxide particles form attractive, coherent networks that tend to agglomerate. This formation corresponds to the 'hit-and-stick' mechanism and to the formation of percolating fractal structure. Percolation can lead to a conductivity increase already at small volume fractions. The simulations show that with decreased particle size the percolation threshold appears significantly earlier. On a longer time scale, these networks coarsen, become more compact, and sediment. Because of coarsening the conductivity decreases with time in such composite electrolytes. Owing to the observed packing effect it can be concluded that the stability of the system is greatly enhanced when the particle size is smaller, the volume fraction higher and when more viscous solvents are used. The modelling is accompanied by experiments on model systems based on low molecular weight poly(ethylene glycol).Item Open Access Spectroscopic study of CaMnO3/CaRuO3 superlattices and YTiO3 single crystals(2009) Yordanov, Petar; Keimer, Bernhard (Prof. Dr.)The first two sections of Chapter 1 give a general overview of the research topics and experimental methods discussed in the thesis. Further on, in Chapter 2, some of the most important characteristics and mechanisms underlying the physics of transition metal oxides are presented. As the experimental part of the thesis includes studies on manganites and titanates, these two classes of compounds are exemplified in the exposition of Chapter 2. Several recent works in the emerging research field of transition metal oxide interfaces and superlattices are also discussed along with a brief introduction in x-ray spectroscopic methods with synchrotron radiation. Chapter 3 introduces the principles of optical spectroscopy and the simplest models for dielectric function, i.e., Lorentz oscillator and Drude dielectric function. The following Chapter 4 introduces two of the experimental techniques in optical spectroscopy, reflectance and spectroscopic ellipsometry. Further on, we describe the design of a new home-built apparatus for near-normal reflectance with high magnetic fields. Several critical technical details and findings during the assembling process are also discussed. Chapter 5 represents a comprehensive experimental spectroscopic study of a prototypical superlattice system made from an antiferromagnetic insulator CaMnO3 and a paramagnetic metal CaRuO3. The resulting interface ferromagnetic state was closely investigated by means of optical spectroscopy as well as by soft x-ray scattering and absorption methods. This study led us to the conclusion that magnetic bound states, i.e. magnetic polarons, have to be considered in the description of this SL system. Chapter 6 describes a polarized far infrared reflectance study with high magnetic field on the ferromagnetic Mott insulator YTiO3, single crystals. All 25 infrared-active phonon modes were observed. The temperature and magnetic-field dependence of the phonon modes revealed a weak spin-phonon coupling in YTiO3 and largely extended temperature range (up to TM ~ 80 - 100K), for the field-induced effects on the oscillator parameters. This later observation, uncovered short-range magnetic order state which remains even at temperatures as high as three times the temperature of the actual ferromagnetic transition of Tc ~ 30K. While a quantitative theoretical description of these data is thus far not available, they point to a complex interplay between spin, orbital, and lattice degrees of freedom due to the near-degeneracy of the Ti t2g orbitals in YTiO3.Item Open Access Resonant X-ray scattering studies of ruthenium oxides and ruthenocuprates(2009) Bohnenbuck, Britta; Keimer, Bernhard (Prof. Dr.)The magnetic and orbital properties of the ruthenium oxides Ca3Ru2O7 and Mn doped Sr3Ru2O7 and the ruthenocuprate RuSr2GdCu2O8 were investigated using resonant and high-energy x-ray diffraction. Bilayered Ca3Ru2O7 is a paramagnetic metal at high temperatures and orders antiferromagnetically at T_N=56K. A second phase transition to a less conductive state is observed at T_MI=48K. This transition is accompanied by abrupt structural changes and a reorientation of the magnetic moment. In addition, there is experimental evidence for the existence of orbital order below T_MI. Our resonant x-ray diffraction studies at the Ru L-absorption edges were focused on the investigation of the magnetic reflections (001) and (110). The observation of a magnetic signal at these reciprocal space positions is in full agreement with an A-type antiferromagnetic structure, consisting of ferromagnetic bilayers coupled antiferromagnetically along the c-axis. Based on the azimuthal angle dependence of the signals, the direction of the magnetic moment was determined to lie along the b-axis below T_MI and along the $a$-axis between T_MI and T_N. The origin of the reorientation of the magnetic moment at T_MI is not yet completely understood. However, it might result from the strong spin-orbit coupling which presumably causes an unquenched orbital magnetization. The latter might then induce additional terms in the spin Hamiltonian that are responsible for the reorientation of the magnetic moment. Although various experiments have given indirect evidence of orbital order below T_MI, we did not detect any orbital signal within the experimental sensitivity. This indicates that the orbital ordering parameter is significantly weaker than in the single layered counterpart Ca2RuO4, which is presumably due to residual charge or orbital fluctuations in the insulating state. RuSr2GdCu2O8 exhibits long range magnetic order and superconductivity within a broad coexistence range. Only limited information about the magnetic structure has been available so far, as most studies were performed on powder samples due to the small size of available crystals. In this situation, resonant x-ray diffraction at the Ru L-absorption edges has turned out to be the ideal tool for the investigation of RuSr2GdCu2O8 since it is sensitive to magnetism, but does not depend on a large crystal mass. Our single crystal studies of the magnetic reflections (1/2 1/2 1/2) and (1/2 1/2 3/2) indicate a G-type antiferromagnetic structure, characterized by a doubling of the unit cell along all three crystallographic directions. From the azimuthal angle dependence of the magnetic signal, we deduced a magnetic moment direction along a low symmetry axis with substantial components parallel and perpendicular to the RuO2 planes. These findings are consistent with previous neutron powder diffraction results and magnetization data. A symmetry analysis in conjunction with a recent crystallographic study revealed that the experimentally observed G-type antiferromagnetic structure needs to be accompanied by an additional ferromagnetic in-plane component, which alternates between neighboring RuO2 layers. This ferromagnetic mode corresponds exactly to the one deduced from nuclear and ferromagnetic resonance experiments. Therefore, our resonant x-ray diffraction data reconcile a variety of apparently contradictory results on the magnetic structure of RuSr2GdCu2O8 and thus resolve a big controversy in the experimental literature. Bilayered Sr3Ru2O7 has attracted a lot of interest in the past years due to the observation of the quantum critical behavior which is related to a metamagnetic transition. In the ground state, the material is a paramagnetic metal and shows Fermi liquid behavior below 10K. Upon substituting Mn for Ru, an insulating antiferromagnetic state is induced; its transition temperature varies with the Mn concentration. Using resonant x-ray diffraction at the Ru L-absorption edges, we investigated the antiferromagnetic structure of 10 Mn substituted Sr3Ru2O7. Our studies of the superstructure reflections (1/4 1/4 0) and (3/4 3/4 0) indicate that the magnetic order is essentially two dimensional and that the magnetic moments are aligned along the c-axis. In combination with a previous neutron powder diffraction study, which was carried out on 5% Mn substituted Sr3Ru2O7, our results suggest an up-up-down-down spin arrangement in the RuO2 planes, which is independent of the Mn concentration. This implies that an antiferromagnetic instability is already present in the parent compound Sr3Ru2O7. Interestingly, the anisotropic resistivity behavior, observed in the nematic phase of Sr3Ru2O7, could be explained assuming the same up-up-down-down spin arrangement as in Mn substituted Sr3Ru2O7. If the two phases are in fact identical, has to be checked by a detailed single crystal neutron diffraction study including a complete structure refinement.Item Open Access Computational determination of the low-temperature parts of phase diagrams on ab initio level(2009) Pentin, Ilya; Jansen, Martin (Prof. Dr. Dr. h.c.)From the point of view of thermodynamic, materials are found as thermodynamically stable (equilibrium) or metastable phases. These can be characterized via functions of state that depend uniquely on the given state variables such as temperature, pressure and composition. The graphical representations of all thermodynamically stable phases that exist or co-exist at equilibrium is called the phase diagram of the chemical system as function of the thermodynamic variables. The knowledge of equilibrium phases of chemical compounds as function of thermodynamic parameters and their thermodynamic stability lies at the foundation of our understanding of the properties and processes of modern materials. From the point of view of experimental methods, the determination of the thermodynamic functions and the equilibria between phases is an enormous task, especially at low temperature. For that reason it has become common practice to support the experimental research by a variety of theoretical calculations. Thus, in the seventies the project CALPHAD was started where different phenomenological models and general rules for the analysis and calculation of the phase diagrams were implemented. Over the past few years, calculations of phase diagrams and thermodynamic properties of materials have appeared in the literature, where typically information from experiment, such as the known existence of various ordered crystalline or solid solution-like phases, was combined with quantum mechanical computations. Clearly, such ab initio calculations can be very useful for the validation of existing phase diagrams. However, the reliance on experimental data is often a serious limitation, especially if one attempts to predict a phase diagram or is interested in competing metastable phases that might occur during the synthesis of new materials. Thus, it is necessary to develop a method to compute phase diagrams without experimental information. The general approach to the analysis of the low-temperature part of a (equilibrium) phase diagram without recourse to experimental data proceeds in several stages. First, structure candidates are identified via global explorations of the energy landscape for different compositions for a given chemical system. This is followed by a local optimization of the candidates on ab initio level. Next, one can calculate the enthalpies of formation for selected candidates, using the ideal entropy of mixing write the Gibbs energy function and calculate the low-temperature part of the phase diagram. The goal of this thesis has been to develop a general strategy to analyze and to predict the low-temperature parts of phase diagrams without any input of experimental data, and to apply this method to a number of chemical system. These chemical systems have been selected and investigated with several aims in mind: - For the purpose of validation of our methodology, we have chosen systems where enough thermodynamic data are available for a comparison between theory and experiment, and which are sufficiently simple to be studied systematically while still allowing for the possibility of reasonably complex phase diagrams. - Analysis of systems where the thermodynamic data are incomplete. - Prediction of the low-temperature part of the phase diagram including not-yetsynthesized phases for chemical systems where no solid compounds are known so far or even the whole phase diagram is unknown. Thus, the following chemical systems were investigated in this thesis: quasi-binary alkali metal halides, quasi-binary lanthanum halides, quasi-binary and quasi-ternary semiconductors AIIIBV.Item Open Access Effects of oxide incorporation in proton conducting organic electrolytes(2009) Sörgel, Seniz; Maier, Joachim (Prof. Dr.)In this work, the effects of incorporation of various types of oxide particles (e.g. ZrO2, TiO2, Al2O3) into proton conducting organic electrolytes is investigated. As a weak liquid model electrolyte, moderately proton conducting imidazole is chosen. As a highly proton conducting strong polymer electrolyte, and simultaneously practically very important electrolyte, Nafion® is selected for the second part of the work. In the first part of this work, for the first time, the applicability of the concept of heterogeneous doping to imidazole is demonstrated. Imidazole exhibits moderate proton conductivity due to low intrinsic charge carrier concentration. Therefore, a perceptible conductivity increase by heterogeneously doping imidazole is expected. Ac-impedance spectroscopy measurements of composites of imidazole with various types of nanometer sized oxide particles, which were performed as a function of temperature and oxide concentration show that the composites exhibit significantly enhanced ionic conductivities compared to the pure imidazole. The highest measured composite ionic conductivity is observed for the composite with heated sZrO2, viz. 1.66x10-2 -1 cm-1 at 90 °C corresponding to an enhancement by a factor of 10 compared to the pure ImiH at the same temperature. The composites prepared with the oxides having the highest activity and density of the acidic sites on the surface show the most pronounced improvement in conductivity. These results were quantitatively analyzed in light of the concept of heterogeneous doping. The proton conductivities calculated according to the heterogeneous doping concept are consistent with the experimentally observed conductivities. The results of zeta potential measurements show that the surface charge of the inorganic oxides becomes strongly more negative on the addition of imidazole. This is consistent with the formation of a space-charge layer on the oxide surface as a consequence of an adsorptive interaction: trapping of imidazolate anions (Imi-) on the oxide surface results in an increased concentration of imidazolium cations (ImiH2+) in the space charge region at the interface of oxide and conductor. The second part of this work focuses on the investigation of the effects of inorganic oxide admixture on proton conductivity, microstructure and mechanical properties of a strong polymer electrolyte, namely Nafion®. Various composite and respective bare membranes were investigated for which performance improvements had been proven in literature before. Thermal and hydrothermal treatments were applied to the membranes in order to get an insight into the properties of the materials at high temperature and low humidity conditions. According to the attenuated total reflection infrared (ATR-IR) spectroscopy results, upon hydrothermal treatments a condensation reaction and consequently an anhydride formation (R-O2S-O-SO2-R) is suggested to occur in the membranes. The thermal treatment above Tg may also lead to the same kind of products. In addition, sulphur formation (aging) is proposed to occur in such conditions which can be derived by X-ray powder diffractometry and energy dispersive microanalysis. These reactions (condensation and sulphur formation) result in an increase of the equivalent weight (EW) and local ordering between polymer crystallites which were detected by acid-base titrimetry and small-angle X-ray scattering (SAXS) measurements, respectively. The conductivity of the membranes is observed to decrease upon thermal and hydrothermal treatments. At high water contents, the decay of conductivity can be explained by the equivalent weight increase. However, at low water contents the mobility of the charge carriers is observed to be slightly suppressed which can explain the conductivity behavior. The lower mobility at low water contents can be due to the less favorable microstructure of the membranes for proton conduction. The proposed condensation reaction and/or sulphur formation (aging) lead to a decrease of hydrophilicity of the side chains. This negatively affects the nanophase separated morphology since hydration of the ionic clusters decreases. Thereby, the water content in the membranes decreases. It is observed by dynamic mechanical analysis (DMA) measurements that the lower amount of water in the membranes is unfavorable for the mechanical properties of the membranes at high temperatures as water acts as a stiffener in such conditions. The above explained effects of thermal and hydrothermal treatments on EW, proton conductivity, activation enthalpy, mobility and microstructure of the membranes without oxide particles are more severe than they are for the composite membranes. A probable condensation reaction and/or aging and therefore changes in microstructure and transport properties of the material are suppressed in the presence of oxide particles. DMA measurement results show that the composite membranes also keep a higher amount of water at elevated conditions and they are thermally and mechanically slightly more stable compared to the respective bare membranes. The incorporation of the oxide particles also increases the glass transition temperature about 10 °C which indicates that the composites have slightly higher thermal stability. In conclusion, in this work it is shown that the oxide incorporation has a positive effect on both weak and strong proton conducting electrolytes: while in the former the proton conductivity is improved by charge carrier concentration increase in the space charge layer, in the latter one it is the structural, thermal and mechanical stability of the material that is beneficially affected at elevated conditions. This study may encourage further developments of electrolyte materials for alternative energy conversion devices.Item Open Access Synthesis and characterization of ion conducting solid polyelectrolytes and alkali fluorooxoborates(2009) Cakmak, Güliz; Jansen, Martin (Prof. Dr. Dr. h.c.)The polyelectrolytes studied in the present work are divided into two general groups, namely poly(lithium organylborylborates) and poly[lithium tris(tetraethylenesilyl)organylborates]. The representative polymers have been synthesized for each group, which have further been modified with incorporation of various organyl lithium compounds. Such representative polymer for poly(lithium organylborylborates) is poly[lithium tetrakis(ethyleneboryl)borate], PLEB. This polymer has been reacted with RLi reagents, where R denotes for methyl or phenyl. All the polymers of poly(lithium organylborylborates) have been further annealed and the effect of annealing on the structure and the ionic conductivity have additionally been investigated. Analogously, the representative polymer for poly[lithium tris(tetraethylenesilyl)organylborates] is poly[tris(tetraethylenesilyl)borane], PESB, which has further been reacted with RLi reagents, where R is phenyl, n-butyl or t-butyl. The PLEB polymer has been synthesized through a hydroboration reaction between lithium tetravinylborate and BH3· THF, whereas the PESB polymer has been prepared through a hydroboration reaction of tetravinylsilane with BH3·S(CH3)2. Amorphous nature of all these polyelectrolytes has been deduced by powder X-ray analysis. Structural properties have been studied via FT-IR and solid-state NMR spectroscopy techniques. Elemental analyses have been performed to characterize the chemical compositions. Thermal properties have been investigated by DTA/TG/MS analysis. Ionic conductivities of these polyelectrolytes have been analysed by impedance spectroscopy. Novel alkylborane-type single ionic solid polyelectrolytes have been prepared with immobilized anions, reduced ion trapping and a lithium transference number close to one in this thesis. Even though poly[lithium tetrakis(ethyleneboryl)borate] (PLEB) is a polyelectrolyte, it has additionally been served as a starting material for the other poly(lithium organylborylborates). According to results from FT-IR, solid-state NMR and DTA/TG/MS analyses, the main structural building blocks of the poly[lithium tetrakis(ethyleneboryl)borate] (PLEB) has been determined to contain mainly tetraethylene borate and hydrogen-bridged borane units, which are connected through ethylene linkages. In these three dimensional network, lithium atoms are compensated by the negative charge on the borate units. The ionic conductivities and structures have been analyzed along with the effect of annealing, which has been performed at 275 °C. Annealing of the PLEB polymer reveals that the main building blocks and connectivity of the structure for the polymer have been remained the same but minor amount of vinyl groups have additionally been observed. The activation energy of the ionic conduction was lower for the annealed PLEB polymer (70 kJ/mol) as compared to the as-synthesized PLEB polymer (145 kJ/mol). The temperature induced crosslinking by annealing modifies the structure along with increasing the charge carrier concentration. Most salts and linear or crosslinked networks which were investigated in the literature have the cationic transference number in the range of 0.3 to 0.5. In contrast, poly[lithium tetrakis(ethyleneboryl)borate] is a pure lithium conductor having t value of 1.0(2) at 137 °C. Incorporation of phenyllithium into PLEB polymer (which is poly[lithium tetrakis(triethylenephenylboryl)borate], PLEPB) increases the ionic conductivity values (i.e. 3x10-7 S/cm for PLEB and 2.2x10-6 S/cm for PLEPB at 80 °C) both for the as-synthesized and annealed PLEB polymers, while no significant improvement in activation energies (145 kJ/mol for PLEB and 146 kJ/mol for PLEPB) is emerged. In addition to the main structural units of the PLEB polymer, a new building block has been identified from the combination of performed analyses, which is a four coordinated borate unit substituted by three ethylene and one phenyl in PLEPB polymer. Moreover, incorporation of methyllithium into PLEB polymer (which is poly[lithium tetrakis(triethylenemethylboryl)borate], PLEMB) has also been examined, and among these polyelectrolytes, the highest conductivity has been obtained for the annealed PLEMB as 3.2x10-5 S/cm at 350 °C. In all cases, annealing decreases the activation energies. Analogous to polyelectrolytes based on poly[lithium tetrakis(ethyleneboryl)borates], various lithium conducting polyelectrolytes have been synthesized by reacting poly[tris(tetraethylenesilyl)borane] (PESB) polymer with phenyl- , n-butyl- and t-butyllithium, which are poly[lithium tris(tetraethylenesilyl)phenylborate] (PLESPB), poly[lithium tris(tetraethylenesilyl)n-butylborate] (PLESnBB) and poly[lithium tris(tetraethylenesilyl)t-butylborate] (PLEStBB), respectively. Compared to the polyelectrolytes based on poly[lithium tetrakis(ethyleneboryl)borates], all of the polyelectrolytes based on poly[tris(tetraethylenesilyl)borane] have lower ionic conductivities, which is most probably resulted from increase in hopping distances and decrease in charge carrier concentration of lithium ions due to incorporation of tetraalkylsilane groups between borate groups in the polymer networks. An all-solid state reaction between LiF and B2O3 at 400 °C has been applied for the synthesis of crystalline LiB6O9F. This work presents the first crystal structure analysis of an alkali fluorooxoborate. LiB6O9F crystallizes in the orthorhombic space group Pna21 with lattice parameters a = 7.6555(1) Å, b = 8.5318(1) Å and c = 10.7894(2) Å. The structure of LiB6O9F contains a pair of boroxine rings, which are connected via a bridging oxygen atom constituting the basic building unit of the fluorooxoborate anion. Moreover, fluoride ion is bonded to one of the boroxine rings. The fluorooxoborate anions form two-dimensional corrugated sheets, whereas lithium cations are embedded between the layers. DTA/TG/MS analysis has shown that LiB6O9F has almost no weight loss up to 470 °C and only 8 % of the initial mass is exhausted at 1000 °C under argon atmosphere. The activation energy for ionic conduction amounts to 160 kJ/mol and according to the ionic conductivity measurements deduced by impedance spectroscopy, this material can be classified as a solid electrolyte with a lithium ion conductivity of 6.6x10-9 S/cm at 400 °C. Crystalline Na3B3O3F6 has been synthesized by reacting H3BO3 and NaBF4 at 350 °C. The crystal structure of this compound is a unique example of a fluorooxoborate anion with alkali ions (besides LiB6O9F). As well, it is the first crystal structure known which is composed solely from BO2F2 tetrahedra. Herein, the hexafluoro derivation of the boroxine ring, namely B3O3F6(3-) anion, is also manifested. Na3B3O3F6 crystallizes in the monoclinic space group C2/c with lattice parameters a = 11.866(7) Å, b = 6.901(4) Å, c = 9.367(6) Å, = 113.724(9)°. Two fluorine atoms are coordinated with each boron atom forming BO2F2 tetrahedral units. The basic building unit of the structure is a six-membered hexafluorotriborate ring with constitution of B3O3F6(3-). The B3O3F6(3-) anions form a layered structure and sodium cations are located in space between these layers. Na3B3O3F6 has been characterized by single crystal X-ray analysis, FT-IR, Raman, DTA/TG/MS, DSC, direct current measurements and impedance spectroscopy techniques. Direct current measurements at 55 °C yield t(+) as 1, and at 287 °C reveal t(el) as 0.0016; in turn confirming that Na3B3O3F6 is a pure sodium conductor. The compound has a sodium ion conductivity of 3.6x10-3 S/cm at 350 °C and an activation energy for ion conduction as 107 kJ/mol between 200 °C and 350 °C. Hence, Na3B3O3F6 can be classified as a fast ion conductor. Crystalline K3B3O3F6 has been synthesized by a solid-state reaction between anhydrous KBO2 and KBF4 at 400 °C. Based on the full characterization of Na3B3O3F6 (which is analogous to the titled compound) in this work by single crystal analysis and spectroscopic techniques, the complete similarity in the spectroscopic analyses between these two analogous compounds manifests the structure of K3B3O3F6. Conclusively, the structure of K3B3O3F6 is proven to compose of six-membered hexafluorotriborate rings of the BO2F2 anions, in which two fluorine atoms are coordinated to each boron atom, forming BO2F2 tetrahedral units in a similar manner to the Na3B3O3F6 analog. K3B3O3F6 can also be classified as a fast ion conductor since it exhibits an ionic conductivity of 1.2x10-4 S/cm at 350 °C, and a negligible electronic contribution is observed.Item Open Access Moleküle als Bausteine zur Synthese von Festkörpern : ein Beitrag zur Entwicklung neuer Reaktionspfade in der anorganischen Materialforschung(2009) Schmidt, Carsten Ludwig; Jansen, Martin (Prof. Dr. Dr. h.c.)Der Einsatz molekularer elementorganischer Ausgangsstoffe (sog. Prekursorkonzept) ist im oxidischen Bereich seit langem ein etablierter Reaktionspfad zur gezielten Adressierung (metastabiler) Festkörper. Insbesondere im Rahmen des wohlbekannten Sol-Gel-Prozesses sind homo- und heterometallische Alkoxide bewährte Startmoleküle. Zur Realisierung eines Ammono-Sol-Gel-Prozesses wurde in der vorliegenden Arbeit ausgehend von verschiedenen molekularen Vorstufen (Butylamide, Methylate, Isocyanate) in geeigneten Lösemitteln polymere amido-, imido oder nitrido-Netzwerke synthetisiert, analysiert und Ihr thermisches Verhalten studiert. Es ist gelungen, ausgehend von molekularen Edukten über polymere Zwischenstufen poröse Xerogele zu erhalten, die unter geeigneten Bedingungen in kristalline Nitridkeramiken überführbar sind. Der neue Syntheseweg, welcher neben einer belegten signifikanten Reduktion der Prozesstemperatur auch noch den Vorteil der Darstellung von geometrischen Grünkörpern aufweist, wurde in allen Einzelheiten beschrieben. Dieser neue Reaktionspfad wurde den Ergebnissen der klassischen Synthese von Nitriden (am Beispiel der Nitride von Re und Os) gegenübergestellt. Neben der Untersuchung geeigneter Einkomponentenvorläufer zur Darstellung ternärer Nitride wurde unter Verwendung von Exotemplaten nitridische Nanopartikel hergestellt und untersucht. Im Rahmen dieser Arbeiten wurden ebenfalls ternäre Amide des Silbers analysiert und Ihre Verwendung zur Darstellung explosiver Feststoffe des Silbers untersucht. Es wurde ein neuer Vorschlag zur Interpretation des sogenannten "Knallsilbers" vorgestellt. Als ein weiterer Schwerpunkt der Arbeit wurde ein Reaktionspfad zur Darstellung eines wasserstofffreien C3N4 (Kohlenstoffnitrid) untersucht. Ausgehend von molekularen und polymeren Isocyanaten (Cyanisocyanaten) wurde unter kontrollierter CO2-Abstraktion (Polykondensation) ein reines binäres C-N-Netzwerk synthetisiert und untersucht.Item Open Access Ab initio study of point defects in the bulk and on surfaces of an SrTiO3 crystal(2009) Alexandrov, Vitaly; Maier, Joachim (Prof. Dr.)The main goal of the present thesis was the theoretical study of substitutional iron impurities and oxygen vacancies present in different charge states in the bulk and on the SrTiO$_{3}$ (001) surface by means of first-principles simulations. Our first step was to examine basic properties of perfect perovskite crystals. We found for SrTiO$_{3}$ and SrFeO$_{3}$ parent compounds that the best agreement with experimental data on lattice constant, bulk modulus, cohesive energy and optical band gap is provided by the hybrid Hartree-Fock and the density functional theory (DFT) approach in the linear combination of atomic orbitals approximation with the so-called B3PW functional which was therefore chosen as a main tool in the study. In order to study SrFe$_{x}$Ti$_{1-x}$O$_{3}$ solid solutions, we carried out a series of calculations for various iron contents (50, 6.25, 3.70, 3.125, and 1.85 at.\%) in doped SrTiO$_{3}$. We found that the Jahn-Teller (JT) distortion around an Fe$^{4+}$ ion is the largest for the most dilute solid solution, becomes less pronounced for 50\% iron doping, and disappears in a pure SrFeO$_{3}$. This tendency in changing the magnitude of the JT distortion agrees well with the EXAFS experimenatal data extrapolated to the dilute defect limit. Also, the electronic structure calculations indicate that SrFe$_{x}$Ti$_{1-x}$O$_{3}$ containing more than 50\% iron is metallic, and that its conductivity is caused by a strong mixing of O $2p$ and Fe $3d$ ($e_{g}$) states in the pre-Fermi energy region. We found that the iron impurity insertion energy of 1.79 eV (1.85\% iron content) is very close to that for 6.25\% iron, and this energy decreases down to 1.59 eV for 50\% and lastly to 1.57 eV for a pure SrFeO$_{3}$. Calculations of iron impurities on the SrTiO$_{3}$ (001) surfaces revealed that the Fe$^{4+}$ ion has the propensity to segregate from the bulk to both SrO and TiO$_{2}$ facets, with the segregation energies of 0.32 and 0.48 eV, respectively. The Mulliken population analysis indicates that the presence of Fe$^{4+}$ ion in the subsurface plane significantly reduces the charge of the nearest oxygen atom at the topmost SrO surface (in a comparison to the substituted Ti$^{4+}$), thus severely diminishing its basic properties. We also examined the neutral and positively single-charged oxygen vacancies ($F$ and $F^{+}$ centers), both in the bulk and on the SrTiO$_{3}$ (001) surfaces. It was found that the neutral vacancy has an even higher tendency than Fe$^{4+}$ ions to surface segregation, being $\thicksim$1.0 eV for SrO and 1.4 eV for TiO$_{2}$ surfaces. The defect energy level becomes much more shallow when going from the bulk (0.77 eV below the bottom of the conduction band at the $\Gamma$-point of the BZ) to SrO (0.27 eV) and TiO$_{2}$ surfaces (very shallow level, almost degenerate with the CB within an accuracy of the method). Our simulations of the bulk charged $F^{+}$ center show that it has a deeper energy level (1.20 eV) than the neutral defect revealing also a lower vacancy formation energy. The charged nature of the center results in a more pronounced relaxation around the defect with a repulsive interaction with neighboring titanium atoms. This relaxation becomes even stronger at the surface. Thus, a common feature of both types of vacancies is the more shallow energy levels on the surfaces compared to the bulk, particularly, for the TiO$_{2}$ facet, while the effect is less pronounced for the $F^{+}$ center. We simulated additionally the diffusion of oxygen species by means of the density functional theory combined with the nudged elastic band (NEB) method. We have shown that the calculated activation barrier for diffusion of oxygen vacancy along the TiO$_{2}$ surface is almost by a factor of three smaller than in the bulk (0.14 vs. 0.38 eV). Adsorption energy of oxygen atom atop Ti ion for the TiO$_{2}$ facet is as large as 2.13 eV being considerably higher than that atop Sr ion on SrO facet (0.57 eV). Moreover, the creation of surface oxygen vacancy nearby the O atom adsorbed atop Ti ion leads to a significant increase in the O$_{ads}$-Ti binding energy. Because of such a strong adsorbate-adsorbent bonding, penetration of the adsorbed O atom into the surface layer could occur predominantly when the very mobile surface oxygen vacancy meets adsorbed oxygen atom. Simulation of a drop of the adsorbed O atom into the oxygen vacancy nearby reveals a distinguishable but extremely small activation barrier, $\thicksim$0.01 eV. Thus, we predict almost no-barrier soaking of the adsorbed O atom into the surface layer, fast surface diffusion of the oxygen vacancies and a much slower diffusion in the bulk. Considerable part of our study was devoted to the examination of oxygen vacancies (neutral and charged) as important ionic charge carriers under confinement conditions. This aspect is of paramount importance in nanoionic systems in which the boundary zones overlap and not only the density but in addition the nano-size spacing at interfaces becomes a key factor.Item Open Access Darstellung und Charakterisierung neuer leerer und endohedraler Fullerene(2009) Epple, Lars; Jansen, Martin (Prof. Dr. Dr. h. c.)Mit Hilfe einer Hochfrequenzofenanlage wurden neue leere und endohedrale Fullerene dargestellt. Die vorhandene Hochfrequenzofenanlage (60 kW, 420 KHz) wurde modifiziert und ein Onlinemassenspektrometer adaptiert. Das Feld der Hochfrequenzspule wurde durch einen konischen Induktor homogenisiert und dadurch längere Verdampfungszeiten erreicht. Zum ersten Mal wurden Samarium-, Thulium- und Ytterbium-Fullerene mit dem Hochfrequenzofenverfahren dargestellt und die Verbindungen massenspektrometrisch nachgewiesen. Die endohedralen Fullerene mit Ytterbium wurden weiter gehend untersucht. Durch einen modifizierten Aufbau zur Probenentnahme während laufendem Versuch, wurde das Darstellungsverhalten der Fullerene im Hochfrequenzofen untersucht. Die Untersuchungen wurden sowohl bei leeren als auch bei endohedralen Barium-Fullerenen durchgeführt. Neue Kristallstrukturen von leeren Fullerenen wurden dargestellt und eine Methode zur Kristallisation von Fullerenen entwickelt.
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