Research of Prof. Hans-Beat Bürgi

Research in chemical crystallography: much work has been done on the correlation of structures with structures and with chemical properties. The aim of this work is to trace reaction pathways (nucleophilic addition, nucleophilic substitution) and interpret kinetic processes structurally. More recent work deals with spin crossover compounds in the solid state. These solids may be considered prototypes of chemical switches as they show complex phase transitions which may be induced by temperature and pressure changes or by light irradiation. 

Research on motion in crystals: atoms and molecules in crystals are quantum objects and show motion down to the absolute zero of temperature. In an X-ray experiment atomic and molecular motions are visible as atomic mean square displacements, which provide detailed information on low-frequency, large-amplitude atomic and molecular vibrations. A normal mode analysis has been developed to extract this information from the temperature dependence of mean-square displacement amplitudes.

Research on disorder and diffuse scattering: crystals are never as nicely ordered as the text books suggest. Their order is disrupted by many kinds of faults. For many materials these faults are essential as they are responsible for whatever interesting property the materials possess (e.g. alloys, high-Tc superconductors, relaxor ferroelectrics, up-conversion materials). If the concentration of faults and thus the degree of disorder becomes sufficiently high, diffuse scattering will accompany the usual Bragg scattering. Diffuse scattering is measured and interpreted in terms of atomistic models of the underlying disorder. This kind of structure analysis is far more complex than conventional structure determination of ordered crystals from Bragg scattering. The necessary software makes use of genetic algorithms and Monte Carlo calculations. It is computationally very demanding and is being developed in a worldwide cooperation. This work is undertaken in collaboration with the group of Prof. Dr. Tony Linden