NMR Studies of structural features

Intermetallic compounds are investigated by solid-state Nuclear Magnetic Resonance (NMR). NMR as a local probe is able to gain access to the information about chemical bonding and disorder phenomena, and is thus complementary to diffraction methods, which usually observe an average of the crystal structure. A comparison between quantum mechanical calculations and the measured NMR parameters is a good test for the validity of the models.

The research of the electrically non-conducting and non-magnetic materials focuses mainly on the chemical shift and the chemical shift anisotropy, which depend on chemical bonding, local symmetry, and possible disorder. Through dipole interaction, the distances between the atoms can be evaluated.

The main effect of the conduction electrons in the electrically conductive materials is the Knight shift of the NMR signal. In case of the non-cubic crystal structures, the investigations of the powdered samples, oriented in the magnetic field, are possible, in addition to the usual powder and single crystal measurements.

Most of the observed isotopes have their nuclear spins higher than ½ and thus possess a nuclear quadrupole moment. In non-cubic local environments, this results in an additional broadening of the signals due to the interaction of the quadrupole moment with the local electric field gradient (EFG), which can thus be measured.

The temperature dependent measurements of the shifts, widths and relaxation constants of the signals are a window into the physics of the solid state systems. Thus the dynamics, phase transitions, and physical properties of the investigated materials are indirectly observed.

Our research is currently focused on the structural features of the intermetallic clathrates. These are cage compounds, whose frameworks are mainly built by the atoms of the 13th and 14th group, while the cages are occupied by the alkali or alkaline-earth metals.

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