Research at the Laboratory of Crystallography
Low-dimensional magnetic crystals
In one-dimensional (1D) or two-dimensional (2D) magnetic crystals the interactions between localised magnetic moments are highly anisotropic. Although the magnetic atoms are evenly distributed throughout the structure, strong interactions between neighbors exist in only one direction (1D system) or in only two directions (2D system). Examples of compounds with these properties are CuGeO3 and SrCuO2 .
The interest in low-dimensional crystals derives from the fact that the physics in 1D or 2D is qualitatively different from the physics of 3D systems. Specific properties of magnetic chain compounds include the spin-Peierls transition between a paramagnetic state and a state of magnetic order coupled to a structural distortion. In 2D systems the magnetic ordering transition might be coupled with charge ordering and structural distortions. Crystals containing 2 or 3 coupled chains ("spin ladders") exhibit a characteristic dynamical behaviour.
The structural distortions related to the phase transitions in low-dimensional magnetic crystals can be studied by x-ray scattering. Scattering experiments in dependence on temperature allow the study of the critical behaviour near the phase transition. They allow the study of the superstructures at low temperature. And they allow the study of the evolution of the structure with temperature. The resulting information can be used to compute the electronic band structure and the magnetic exchange constants in dependence on temperature. They provide essential information for the understanding of the properties of such materials .
At the Laboratory of Crystallography we study the low-dimensional magnetic crystals like TiOCl;TiOBr, NaV2O5, Sr14Cu24O41 and (VO)2P2O7. The experimental work includes:
Determination of the low-temperature superstructures using synchrotron radiation x-ray diffraction .
Studies of the accurate electron densities employing the Maximum Entropy Method (MEM).
Search for superstructure reflections and the measurement of their temperature dependence and critical exponents.
 S. van Smaalen : structural aspects of spin-chain and spin-ladder oxides. Z. Kristallogr. 214, 786-802 (1999).
 J. Ludecke, A. Jobst, S. van Smaalen, E. Morre, C. Geibel and H.G. Krane (1999): Acentric low-temperature superstructure of NaV0O0. Phys. Rev. Lett. 82, 3633 (1999).
 S. van Smaalen and J. Ludecke (1999): The valence states of vanadium in the low-temperature superstructure of NaV0O0. Europhys.Lett., (2000).
 S. van Smaalen, A.Schoenleber and L.Palatinus: Incommensurate interactions and non-conventional spin-Peirls transition in TiOBr, Phys.Rev. B 72, 020105(R) (2005).