Florian Schwarz «  Udo Schwingenschlögl »  Armin Seibert
Udo Schwingenschlögl
The Interplay of Structural and Electronic Properties in Transition Metal Oxides
Supervisor: Priv.-Doz. Dr. Volker Eyert [Theoretical physics II]
Date of oral examination: 02/23/2004
177 pages, english , ISBN 3-8325-0530-X
Density functional theory has been established as a very powerful approach to the ab initio description of condensed matter for several decades. In combination with the local density approximation it allows for an accurate calculation of a large variety of ground state properties. Starting just with the atomic arrangement it yields, for instance, the electronic, magnetic, or optical properties. Density functional theory paves the way for materials tailoring and is thus an important tool in materials science. However, more fundamental issues are likewise covered by the theory.

Aiming at an understanding of the metal-insulator transitions of the prototypical vanadium oxides VO2 and V2O3 an identification of the relevant electronic states and an investigation of their response to the changes of the crystal structure is necessary. This issue is addressed in the present thesis by analyzing the relations between the structural and electronic properties in the broader class of the vanadium Magnéli phases, which form a homologous series and reveal crystal structures comprising typical dioxide and sesquioxide-like regions. As they are intermediate between the structures of their end members VO2 and V2O3, these materials allow for insight into the crossover between the latter oxides. Analogous with the vanadium compounds, the titanium oxides likewise give rise to a Magnéli series. Due to structural similarities knowledge of the phase transitions in the vanadium oxides can be transferred to the titanium case.

Despite its striking simplicity the perovskite structure allows for many crystallographic variations giving room for a wide class of compounds. For optimal tailoring of material properties uch research on perovskite-related compounds concentrates on the interrelations of structural distortions and resulting physical properties. In this context octahedral tiltings as realized in the ruthenates ACu3Ru4O12 are the most important class of distortions. The relationship between the tiltings and the electronic features of these materials is investigated, giving rise to a universal picture of octahedral tilting in perovskites.

Low-dimensional systems are known for their fascinating physical properties. It has recently been possible to synthesize a new structural family of (quasi) one-dimensional materials characterized by extraordinary magnetic properties, which lack microscopic understanding. Motivated by electronic structure calculations for Ca3Co2O6 the related compounds Ca3CoRhO6 and Ca3FeRhO6 are studied, allowing for an interpretation of the magnetic couplings on a common basis.