Publikationsserver der RWTH Aachen University
Preparation, structural characterization and properties of Oxo-vanadates-IV with unusual magnetic states
Abstract
dc:descriptionWithin the scope of this work temperature dependences of both novel and known low-dimensional S=1/2 vanadium oxides have been studied at low temperatures. The basic structural unit of all investigated compounds is the V4+O5 pyramid arranged in layers. Besides structural characterisation alternative synthetic routes were explored. The work focuses on detailed single crystal and powder X-ray diffraction studies, for selected compounds magnetometric measurements were performed. For low-temperature measurements a split coldhead closed cycle He refrigerator was adapted to the image plate diffractometer IPDS I. The most important results are summed in the following:CaV4O9: The tetragonal compound consists of V4O9 layers that are characterised by interacting quadripartite V units (metaplaquettes). It is the first known two-dimensional spin-gap compound. Crystals were grown by flux synthesis at moderate temperatures for the first time. The material shows no structural phase transition in the temperature range 20 K to 320 K. It reaches a minimum in the a lattice parameter at 150 K and a change in the slope of the c lattice parameter is observed at the same temperature. Magnetometric measurements confirm the presence of a broad susceptibility maximum at ~110 K and the presence of a gap in the magnetic excitation spectrum at low temperatures.A2V4O9•LiCl (A= Rb, K): These two isostructural compounds, which form a solid solution series were grown from the flux. They are structurally related to CaV4O9. The temperature dependence is different from that observed for the former; no structural phase transition was observed at low temperatures. The magnetometric measurements show a broad transition at T=60K for the Rb compound, possibly a transition from the paramagnetic to the antiferromagnetic state. This needs to be checked by neutron scattering.A2V4O9 (A= Rb, Cs): Investigated crystals of Rb2V4O9 and beta-Cs2V4O9 have a structure which deviates from the aforementioned plaquette compounds. The edge sharing VO5 pyramids point to the same side of the V4O9 layer and form V2O8 dimer units. The structure of Rb2V4O9 was determined in space group P-4n2 contrary to previously reported results. Between RT and 100 K no structural phase transition was observed. At T= 190 K and T= 200 K a locking in of the dimers at a minimum V-V distance was seen. Magnetic properties in these substances are dominated by the interaction between the dimers within the V4O9 layers.AV3O7 (A= Ca, Sr): It was found that these two compounds are not isostructural contrary to the assumption made in previous works. Single crystal structure determination shows that SrV3O7 crystallizes in space group Pmmn. No structural phase transition was observed between RT and 100 K. For both compounds the in-plane lattice parameter perpendicular to the infinite chains of edge sharing VO5 pyramids within the V3O7 layer shows a negative linear thermal expansion coefficient which results from a competition of increasing buckling and thermal expansion of the layer. Given the fact that the two compounds are not isostructural the differently oriented magnetic spin structures reported for the two compounds should be readdressed.Rb2V3O8: The structure of this mixed valence compound shows a negative linear thermal expansion coefficient within the V3O8 layers between RT and 14 K. This is a result of a combination of a stretching of the VO4 tetrahedra in the a1a2 layer and a translation of the VO5 pyramids along c.Li2(VO)SiO4: Single crystal and powder diffraction indicates very weak temperature dependence of the structure. A maximum of the magnetic susceptibility was observed at ~8 K, confirming the results of previous works. Crystals of the two noncentrosymmetric V4+ and Cu2+ bearing compounds Rb2(VO)2[Si8O19] and Cs2Cl2CuV2O6 that display potentially interesting magnetic properties have been grown.Rb2(VO)2[Si8O19]: This anhydrous diphyllosilicate crystallizes in space group Pmc21. It consists of alternating silicate double layers and V2O8 dimer layers. First results show that a phase transition without symmetry change, the nature of which is not yet clear, takes place at 250 K.Cs2Cl2CuV2O6: This novel oxy chloride crystallizes in space group P41212. The structure consists of chains of corner sharing VO4 tetrahedra which are separated by Cs in the a1a2 plane and by CuCl2 layers along c.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2008
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Prinz, Sebastian
- Contributors dc:contributor
-
- Roth, Georg
Subjects
dc:subject × 9Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng