NMR-Spektroskopie an intermetallischen Phasen in den Systemen EA-Al-Ga mit EA = Ca, Sr und Ba : experimentelle und theoretische Untersuchungen der chemischen Bindung

Abstract

Intermetallic phases are widely used in industrial and technical applications. Until this day, the modification of properties is still based on empiric approaches. The lack of a precise knowledge about the chemical bonding in intermetallic phases limits the target oriented synthesis of materials with designated properties. Achieving detailed information about the structure-bonding-property relationships in intermetallic phases is a necessary step to open new opportunities for applications and to increase efficiency of existing materials. Since substitutional disorder is of pronounced role concerning the properties of intermetallic phases, the impact on the local bonding situation in the systems Sr1-xBaxGa2, Sr(Al1-xGax)2, M(Al1-xGax)4 with M = Sr, Ba and Ca1-xGa2+3x as well as Ca3-xGa8+3x was investigated, respectively. The combined application of diffraction (XRD), NMR spectroscopy and quantum mechanical (QM) calculations is well suited to gain information on atomic interactions by an analysis of the electric field gradient (EFG). Within the scope of this work, a specially designed probe system for orientation dependent NMR experiments on magnetically aligned powder samples was commissioned to establish the XRD-NMR-QM approach on intermetallic phases being strongly influenced by substitutional disorder. To ensure single phase material for NMR spectroscopy, a detailed characterization of the samples was performed using X-ray diffraction, thermoanalytical and metallographic investigations. This resulted in a re-investigation of the Ga-rich part of the phase diagram Ca-Ga focussing on the intermetallic phases Ca1-xGa2+3x and Ca3-xGa8+3x. Furthermore, conclusions about the structural chemistry and miscibility of the solid solutions Sr1-xBaxGa2, Sr(Al1-xGax)2 as well as M(Al1-xGax)4 with M = Sr, Ba were achieved. Orientation dependent 27Al and 69;71Ga NMR experiments on aligned powder samples were used to analyse the local Al and Ga environments. The influence of alkaline earth metal substitution on the Ga atoms was investigated in the solid solution Sr1-xBaxGa2. NMR spectroscopy revealed two different species Ga and Ga' with similar values of the EFG. A local symmetry reduction for Ga' was proven. In the solid solution Ba(Al1-xGax)4 a significant influence of group 13 elements substitution on the bonding situation with preferred occupancy of four (4b) and five bonded (5b) Al and Ga positions was studied, respectively. In Ca1-xGa2+3x triangular Ga3 units partially substitute the Ca atoms. Different three (3b) and four bonded (4b) Ga environments could be resolved and analysed by NMR experiments. The local atomic environments could partially be modelled in superstructures enabling QM calculations of NMR coupling parameters. From that, a verification of experimental results was possible. Future studies will have to focus on an analysis of the electronic contributions dominating the EFG