Studying orientational disorder with neutron total scattering.

PhD ThesesThe orientations of chemical bonds and molecular groups are an important characteristic of the local structure of materials, and may determine the macroscopic performance of materials. Because orientational disorder happens over small length scales and fast time, it may not be accessible to many common characterisation techniques. The properties of neutron allow them to be an excellent tool for the study of orientational disorder in materials. In this thesis, I will demonstrate how orientational disorder can be studied using neutron total scattering and its Fourier transform, namely the pair distribution function. Combining neutron total scattering, the reverse Monte Carlo (RMC) and molecular dynamics (MD) simulations has shown how the phase transitions of KCN and BaCO3 are associated with the orientational disorder of cyanides and carbonate molecular anions respectively, and has given quantitative information on the degree of disorder through analysis of the orientational distribution function. The study of BaCO3 has been supported by molecular dynamics simulations, giving us a new picture of the phase transitions in both BaCO3 and the related mineral calcite. The porous aromatic framework (PAF) has been studied by neutron total scattering and small angle scattering combined with molecular dynamics simulations. It has shown that the amorphous phase of PAF has the structure of a continuous random network similar to that of amorhous silica for example, but with orientational disorder of the biphenyl molecular groups about their long axes. Finally,the orientational disorder of ammonium and sulfate tetrahedra in (NH4)2SO4 was studied by both neutron total scattering and RMC. It was found the orientational disorder plays an important role in the explanation of the giant entropy change during the ferroelectric phase transition