Literatur

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Characterization of TiO<Subscript>2</Subscript> powders and thin films prepared by non-aqueous sol–gel techniques

Stable TiO₂ sols were prepared using two non-aqueous sol–gel processes with titanium n-butoxide and titanium isopropoxide, respectively. Crystallization and phase transitions of powders and thin films were studied by ex situ and in situ X-ray diffraction. For both methods, TiO₂ began to crystallize around 320 °C in air. Using helium instead of air during heat treatment slowed down the crystallization and substoichiometric powders were formed. TiO₂ thin films were obtained by spin coating. The morphology of the films was evaluated using scanning electron microscopy. The films were homogeneous and transparent in the visible range. The effect of the heating atmosphere and the type of substrate was investigated.     

EPR and speciation simulation study of Cu2+ complexes in an amine-based aqueous precursor system used for preparation of superconducting YBCO coatings

In this work, we investigate the chemistry for an aqueous acetate-triethanolamine-ammonia based YBa(2)Cu(3)O(7-δ) (YBCO) precursor system. These precursor solutions are suited for the chemical solution deposition of superconducting YBCO layers on top of single crystal SrTiO(3) or buffered NiW tapes. The development of this kind of precursor inks often involves trial-and-error experimenting and thus is very time-consuming. To reduce labwork to the minimum, the theoretical prediction of pH stability limits and the complexation behaviour of the different metal ions and complexants in the inks are very important. For this purpose, we simulated, based on literature values, the complexation behaviour of Cu(2+) in the aqueous precursor solutions as a function of pH. To validate the used model, we performed potentiometric pH titrations for solutions with similar composition and checked the correctness of fit between experiment and model. The generated complexometric results are coupled with X-band EPR spectra to further confirm the results. EPR spectra for fully prepared precursor solutions as well as for Cu(2+) reference solutions containing only one type of ligand (acetate, triethanolamine or ammonia) were investigated as a function of pH. We find that, in line with speciation simulation, only acetates are actively complexing the Cu(2+) ions at pH values below 7, while when reaching higher pH levels mainly triethanolamine complexes are formed. Over the entire pH range, no trace of free Cu(2+)or Cu(OH)(2), possibly creating precipitation during gelation and thus complicating further processing, could be found.     

Ternary nitride GaFe(3)N: an experimental and quantum-theoretical study

The recently published two-step ammonolysis reaction giving access to phase-pure GaFe(3)N has been reinvestigated. Thermochemical calculations show that a high-temperature route is necessary to avoid the formation of the competing GaN phase. Compared to the prior study showing a Vegard-like behavior (that is, a linear correlation between lattice parameter and elemental composition), improved X-ray analysis using Mo Kα(1) radiation in combination with density-functional theory calculations reveal a more complicated behavior of the lattice parameter within the entire Ga(x)Fe(4-x)N series. The new finding originates from the magnetic properties, and the change in the magnetic ordering with increasing Ga content from ferromagnetic γ'-Fe(4)N to antiferromagnetically ordered GaFe(3)N, as observed from susceptibility measurements, is reproduced by different theoretical spin-alignment models, that is, a systematic evaluation of several antiferromagnetic spin orientations. Nonetheless, all structural models are based on the favored atomic ordering for GaFe(3)N, explainable by the strong affinity between iron and nitrogen.     

Spatial resolution and radiation damage in quantitative high-resolution STEM-EEL spectroscopy in oxides

The chemical analysis on the atomic scale in a scanning transmission electron microscope bears a number of challenges. These are an unambiguous assignment of a spectroscopic signal to a sample location and sufficient signal above noise for quantification. Modern aberration-corrected optics provide intense electron probes allowing for the highest spatial resolution and beam current density possible. On the other hand, non-destructive analysis requires low irradiation doses, so that there is a limit to the achievable signal-to-noise ratio. Here, we employ the StripeSTEM method that sacrifices the resolution in one spatial dimension in return for decreased radiation damage to the sample. Using this technique, radiation damage effects and achievable quantification accuracy are examined on the example of bulk SrTiO₃ and a one unit cell thick layer of LaAlO₃ in SrTiO₃. The results show that valency artefacts are expected for conventional recording conditions where the electron dose is concentrated to a few atomic columns. Likewise a high accuracy for measuring the oxygen defect chemistry without radiation damage requires spreading out the irradiation dose.     

Synthesis of heteroleptic anthryl-substituted beta-ketoenolates of rhodium(III) and iridium(III): photophysical,electrochemical, and EPR study of the fluorophore-metal interaction

The anthryl-substituted rhodium(III) and iridium(III) heteroleptic beta-ketoenolato derivatives of general formula [M(acac)(2)(anCOacac)] [acac = pentane-2,4-dionate; anCOacac = 3-(9-anthroyl)pentane-2,4-dionate], 3 (M = Rh) and 4 (M = Ir), and [M(acac)(2)(anCH(2)acac)] [anCH(2)acac = 3-(9-anthrylmethyl)pentane-2,4-dionate], 5 (M = Rh) and 6 (M = Ir), were prepared by reacting the corresponding tris(pentane-2,4-dionate)metal complexes, [M(acac)(3)], with 9-anthroyl chloride and 9-chloromethylanthracene, respectively, under Friedel-Crafts conditions. 3-6 were characterized by elemental analysis, ion spray mass spectrometry (IS-MS), (1)H NMR, and UV-vis spectroscopy. The structure of 3 was also elucidated by single-crystal X-ray analysis. When excited at 365 nm, 3-6 result to be poorly luminescent compounds; while the free diketone, i.e., 3-(9-anthrylmethyl)pentane-2,4-dione 1, whose structure was established also by single-crystal X-ray analysis, results to be a strongly light emitting molecule. The study of the electrochemical behavior of 3-6 as well as of the corresponding tris-acetylacetonates of rhodium(III) and iridium(III) allows a satisfactory interpretation of their electrode process mechanism, and gives information about the location of the redox sites along with the thermodynamic and kinetic characterization of the corresponding redox processes. All data are in agreement with the hypothesis that the quenching of the anthracene fluorescence, observed for compounds 3-6, can be due to an intramolecular electron transfer process between the anthryl moiety and the metal-beta-ketoenolato component. Moreover, a study was carried out of the redox behavior of the dyads 3-6 under chemical activation. The one-electron oxidation of compounds 3-6 by thallium(III) trifluoroacetate leads to the formation of the corresponding cation radicals, 3(+)-6(+), whose highly resolved X-band EPR spectra were fully interpreted by computer simulation as well as by semiempirical and DFT calculations of spin density distribution.     

Reaktion von Diazoniumsalz mit Tyrosin-Resten in Polypeptide

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Local ordering and magnetism in Ga0.9Fe3.1N

Prior investigations of the ternary nitride series Ga_1-xFe_3+xN (0≤x≤1) have indicated a transition from ferromagnetic γ′-Fe₄N to antiferromagnetic “GaFe₃N”. The ternary nitride “GaFe₃N” has been magnetically and spectroscopically reinvestigated in order to explore the weakening of the ferromagnetic interactions through the gradual incorporation of gallium into γ′-Fe₄N. A hysteretic loop at RT reveals the presence of a minority phase of only 0.1-0.2 at%, in accord with the sound two-step synthesis. The composition of the gallium-richest phase “GaFe₃N” was clarified by Prompt Gamma-ray Activation Analysis and leads to the berthollide formula Ga_0.91(1)Fe_3.09(10)N_1.05(7). Magnetic measurements indicate a transition around 8 K, further supported by Mössbauer spectral data. The weakening of the ferromagnetic coupling through an increasing gallium concentration is explained by a simple Stoner argument. In Ga_0.9Fe_3.1N, the presence of iron on the gallium site affects the magnetism by the formation of 13-atom iron clusters.