Superstructures of Magnetic Materials

This dissertation reports on the results of studies on magnetic materials as the ternary carbide SmNiC2 and the perovskite-related layered compounds La5(Ti0.80Fe0.20)5O17 and La6(Ti0.67Fe0.33)6O20. Studies were performed with single-crystal X-ray diffraction and subsequently refinement and analysis of the crystal structures. Measurements of SmNiC2 were performed within the temperature range of 8 - 300 K. Refection peaks were analyzed for a possible peak-splitting as a consequence of a lowering of the symmetry of the crystal lattice in the charge-density wave state as well as in the ferromagnetically ordered state. It was found, that no lattice distortion is associated with the phase transitions. In both phases the crystal lattice has orthorhombic symmetry. A structure model of the incommensurately modulated structure in the charge-density wave state describes the structure with layers of Sm and Ni atoms, stacked along the c axis. Between the layers C atoms are arranged as C2-pairs. Ni atoms form dimerized chains along the a axis and show the largest displacement modulation of all atoms as well as the largest variations of the bond distances to neighboring atoms. Thus, the Ni atoms carry the valence band, which is responsible for the charge-density wave. For a single layer of Ni atoms a commensurate charge-density wave would result. A frustration is caused by the interchange of the modulation with the lattice centering of the structure, which renders the modulation incommensurate in all layers. The two n = 5 and 6 members of a homologous series of perovskite-related layered compounds show completely different magnetic properties. Indeed both compounds remain paramagnetic in the complete analyzed temperature range of 4{400 K. But especially the n = 6 compound shows a behavior clearly different from the common paramagnetic characteristic with a ferromagnetic interaction at higher temperatures. At about room temperature a crossover to an anti-ferromagnetic interaction appears. Whereas the n = 5 compound does not show a crossover nor a clear (anti-) ferromagnetic interaction can be found. Structures of both compounds were successfully described on the basis of a unique structure type within (3+1)-dimensional superspace with modulation functions and modulation wave vectors depending in a systematic way on n. Selected structural parameters, as for example the B{O{B bond angle, do not show any noticeable change at temperatures of about 290 K. Instead the maximum concentration of magnetic Fe atoms realized in the central layers of each slab is suggested as the mechanism for the different magnetic properties. In the case of the n = 5 compound the maximum concentration is too low for the formation of magnetic interacting clusters. In the case of n = 6 clusters of 52 iron ions in average are formed. At temperature above room temperature the magnetic interaction within the clusters dominates, while at lower temperatures the anti-ferromagnetic interaction between clusters in neighboring layers is dominant. The thermal expansion of both compounds anisotropic with the strongest expansion along the a axis, parallel to the layers. This is different to the expansion in many other layered structures. Thereby the slabs show a clear expansion with rising temperature, while the interslab distances are almost temperature-independent. Valences of Ti and Fe ions were calculated with the Bond-Valence method. The results show, that calculated valences depend on the site occupancy by Ti and Fe ions. The best agreement between formal and calculated valences are found for sites, which are almost exclusively occupied by Ti ions. In these cases the oxygen environment and the valences are that of Ti4+. At sites occupied by both ions the refined oxygen positions represent the average of oxygen positions in TiO6 and FeO6 octahedra, weighted to the occupancies of Ti and Fe.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2013

Author dc:creator

• Wölfel, Alexander

Contributors dc:contributor

• van Smaalen, Sander

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/106/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:106