Engel, Stefan

Studies concerning the electronic structure and the electron transfer in electron poor intermetallics

The research of intermetallic compounds provides a field of materials with highly interesting properties, e. g. the hard magnet Nd₂Fe₁₄B, CeCu₂Si₂ which possesses the super conductivity below 0.6 K, shape memory alloys such as NiTi, or thermoelectric compounds. But also intermetallic aluminum compounds like Ti₃Al, which is used for high temperature alloy and Ni₃Al a super alloy thanks to its acid resistance are known and widely used. Especially the research on new intermetallic aluminum compounds in combination with elements with a low electron negativity, like alkali earth elements, is highly interesting since these compounds exhibit not purely metallic, ionic or covalent binding character, but rather a mixture of all of these. Within these structures the understanding of binding situation of Al in compounds is a crucial point of the research.

Within the framework of the PhD thesis, the electron transfer in intermetallic aluminum compounds is examined from both an explorative and a systematic preparative point of view. The investigation of selected ternary systems M-T-Al, where M is an alkali metal, alkaline earth metal or europium and ytterbium, which have been little studied so far, is conducted. The transition metals of the cobalt, nickel and copper groups will be investigated as possible transition metal components T. Phase pure compounds known to literature will be prepared for spectroscopic and physical investigations and the phase diversity of these systems will be explored via X-ray diffraction and electron microscopy. Due to the low melting and boiling points of the alkali, alkaline earth and rare earth elements involved (Eu and Yb), intermetallic compounds with these constituents cannot be synthesized by arc melting. Therefore, synthesis must be carried out in inert metal ampoules (Ta or Nb). Precursor compounds can also be used to specifically eliminate some problems in order to gain access to additional new compounds. Alternatively, the metal ampoules can be fused into evacuated quartz ampoules and heated in conventional tube or resistance furnaces. The exact crucible selection, 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 X-ray pure samples for property studies and single crystals for structural analysis. Compounds that do not contain low-boiling elements can be synthesized by classical arc melting and, if necessary, post-annealed in metal or glass ampoules as described above. The growth of single crystals is easily possible in these systems.

The samples obtained are first examined by means of X-ray powder diffraction. This is followed, if required, by optimization of the synthesis conditions, in which case powder and single crystal X-ray diffraction are coupled with metallography and scanning electron microscopy / EDX. Of the compounds obtained, the crystal structures are characterized with the aim of structural systematization and determination of the exact composition. Based on these, the synthesis of the X-ray pure sample can be performed, if necessary. In particular, the different structural influences between the electron-deficient and electron-rich compounds are to be investigated. After optimization of the synthesis conditions, X-ray pure samples can be further investigated for their physical properties. In addition to magnetic behavior, electrical conductivity and specific heats can also be measured. Furthermore, ¹⁵¹Eu Mößbauer spectroscopic investigations will provide deeper insights into the magnetic ground states of the europium compounds. The analysis of the electronic structure and the bonding situation will be performed by band structure calculations on the DFT level. Using computational theory, Bader charge analyses can be performed in addition to band structures and interpretation of interatomic interactions with respect to their bonding character. These also allow estimation of electron transfer in these compounds. Real space analyses (ELF) will be used to illustrate the bonding situation. Furthermore, the theory allows the calculation of NMR parameters (quadrupole interactions, asymmetry parameters), which support the interpretation of the corresponding spectra. Dia- or Pauli-paramagnetic samples can be studied by solid-state NMR spectroscopy. Here, ²⁷Al represents an excellent probe. Using ²⁷Al solid-state NMR spectroscopy, information about the local coordination as well as the electronic environment of the Al nuclei can be obtained. This information can be obtained from the experimental spectra by evaluating the quadrupole coupling (C_Q) and the asymmetry parameter (η_Q).