Studying orientational disorder with neutron total scattering.
Abstract
The 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.
Authors
Cai, GuanqunCollections
- Theses [4225]