Authors: Sifkovits, Mark
Title: Elektronische Struktur und physikalische Eigenschaften der Eisennitride und einiger Nitride des Si, Ge und Sn
Language (ISO): de
Abstract: In this thesis, the physical properties of and the chemical bonding in two classesof nitrides are studied in the framework of density functional theory(DFT). The phase diagram of the iron nitrides is very complex. Over a broad range of nitrogen concentrations they are ferromagnetic metals. The spin density is concentrated around the iron atoms and the value of the magnetic moment of the iron atom is characteristic of the number of nearest neighbour nitrogen atoms. A careful analysis of the results of a series of DFT calculations shows, that the strong covalent iron-nitrogen bond causes the characteristic dependence of the magnetic moment on the nitrogen environment. On the basis of this analysis a simple model is proposed, that correctly describes the decrease of the magnetic moment of the unit cell with increasing nitrogen concentration, that is found in the DFT calculations. However, close to the composition Fe_2N the calculations (and the model as well) give a ferromagnetic ground state. In contrast to this, the experimental results rule out a ferromagnetic ground state. It seems, that current exchange and correlation functionals do not describe the electronic structure of the iron nitrides with the composition Fe_2N. The strong iron-nitrogen bonding affects the nitrogen diffusionas well. The activation energy and the transition rate between adjacent interstitial sites were calculated for typical diffusion paths in the ferromagnetic phase of gamma-Fe_4N and epsilon-Fe_3N. In the paramagnetic phase the activation energies are significantly smaller, so that a higher transition rate can be expected. Possible links to the observed disorder of nitrogen in paramagnetic samples are discussed. The second class of nitrides studied are those of the composition A_3N_4, where A in {Si, Ge, Sn}. The ground state structure of these insulators was determined by calculating the energy vs. volume curves for the compounds in the phenakite and the spinell structure. Whereas Si_3N_4 and Ge_3N_4 cristallize in the phenakite structure, the ground state structure of Sn_3N_4 is the spinell structure. It is shown, that the chemical bond between the group IV element and nitrogenis of a mixed covalent-ionic type. The heavier the group IV element, the larger is the ionic contribution to the bond. Thus, for Sn_3N_4 the close-packed spinell structure is energetically more favourable than the phenakit structure.
Subject Headings: Dichtefunktionaltheorie
itineranter Ferromagnetismus
density functional theory
iron nitrides
Issue Date: 2000-05-08
Publisher: Universität Dortmund
Appears in Collections:Theoretische Physik II

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