Elektrische Quadrupolwechselwirkung in defektreichen und deformierten MAX-Phasen

Abstract

In this work, it is shown how hyperfine interaction methods can be used to in-situ detect defects and deformations even in crystallographically complex materials such as the MAX-Phases, a class of ternary carbides and nitrides. The sensitive quantity is given by the electric field gradient (EFG), a measure of the asymmetry of the charge distribution around a particular probe atom's nucleus. Two mechanisms of interplay between defect and EFG are considered: the long-ranged effects of elastic strain and the direct influence of a defect on its local electronic surrounding. The EFG's elastic response is determined by means of ab-initio methods in the framework of density functional theory. This approach permits to analyze the reason behind the strain dependence of the EFG and allows insights into any other strain dependence of the EFG such as volume and structure dependencies. Distributions of the EFG for certain defect arrangements are calculated by means of Monte-Carlo simulations based on the aforementioned elastic response. The hereby predicted EFG distributions are experimentally verified in uniaxially deformed, polycrystalline MAX-Phase by means of the perturbed angular correlation (PAC) method which in turn allows to in-situ determine defect densities. The local influence of defects is systematically studied within MAX-Phase solid solutions. In this regard, the synthesis of a new solid solution, Ti$_2$(Al$_{0,5}$,In$_{0,5}$)C, is reported. The corresponding lattice parameters are determined by a Rietveld refinement of XRD-data.