Experimental equilibrium structures of solids and gases

Abstract

In the past sixty years, X-ray, neutron and electron dffraction have emerged as the structural techniques of choice in the solid state. However, despite many advances in theory and instrumentation, these diffraction methods are still reliant on a number of assumptions. Chief amongst these is that the atoms in the crystal vibrate in a harmonic fashion. This thesis is concerned with understanding the effects of anharmonic motion on crystal structure determination and developing new ways of moving beyond the harmonic approximation used in crystallography. A method has been developed, using molecular dynamics simulations, to correct experimental structures to equilibrium structures. This has been applied to the crystal structures of phase-I deutero-ammonia, deutero-nitromethane and benzophenone. Path-integral molecular dynamics simulations have been used to obtain meaningful comparison with experimental data collected at low temperatures. The simulations also offer information on the probability density functions that describe thermal motion in solids. Using data from simulations of nitromethane and other compounds it has been demonstrated that the molecular dynamics-derived data can be used to assess and develop new functions for modelling thermal motion in crystal structure refinements. Finally, similar molecular dynamics techniques have been applied to determine the equilibrium structures of some polyhedral oligomeric silsesquioxanes in the gas phase. Some members of this class of compounds feature such strong anharmonic motion that refinement of the structures using gas electron diffraction is impossible without taking into account the effects of the anharmonicity.