Gießelmann, Elias

Structural and spectroscopic investigations of (oxidized) intermetallic aluminum compounds of the early transition metals

In the context of the PhD thesis, binary and ternary intermetallic aluminum compounds of the early transition metals are synthesized and investigated. Besides the classical investigation methods (X-ray diffraction on powder and single crystal), the synthesized compounds can be investigated by ²⁷Al-MAS solid state nuclear magnetic resonance spectroscopy (NMR), as well as Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), due to their dia- / Pauli paramagnetic properties.

Beside the validation of the crystal structure (number of crystallographic layers correlates with the number of signals), NMR spectroscopic investigations enable one to make statements about structural features (local distortions, formation of solid solutions). In addition, the interpretation of the chemical shift allows an assertion about the electron transfer in these materials. Raman spectroscopy provides insights to force constants of lattice vibrations, which can be useful for quantum chemical calculations. Finally, XPS measurements allow conclusions about the formal oxidation state as well as the charge transfer in these compounds. The analysis of the electronic structure and the bonding situation is performed by band structure calculations on DFT level. With the help of computer-aided theory, Bader charge analyses can be performed in addition to band structures and the interpretation of interatomic interactions with respect to their bonding character. These also allow an estimation of electron transfer in these compounds. Real space analyses (ELF) can also be used to illustrate the bonding situation. Furthermore, density functional theory allows the calculation of NMR parameters (quadrupole interactions, asymmetry parameters), which support the interpretation of the corresponding spectra.

Since phase pure samples are required for the planned spectroscopic and physical investigations, the synthesis of these compounds represents a central aspect. Due to the high melting and boiling points of the elements involved, the compounds can be synthesized by conventional arc melting. The exact synthesis strategy, the appropriate temperature profiles for reaction control as well as the corresponding annealing sequences have to be optimized for the respective systems in order to finally obtain radiographically clean samples for property investigations and single crystals for structural analysis.

Another central aspect of the research is the targeted oxidation of the intermetallic compounds synthesized. This may offer the possibility of obtaining novel oxides that are not accessible by direct reaction of the oxides. The previously mentioned investigation methods can also be applied to these compounds. In addition, there is the possibility of studying the reaction events in-situ by means of temperature- and atmosphere-dependent X-ray powder diffraction. Thus, also with regard to the other characterization methods, there is not only a comparative interpretation but also, if necessary, the possibility to directly observe the phase formation as well as to quantify the obtained phase fractions.