Modelling local order in organic and metal-organic ferroic materials using the reverse Monte Carlo method and total neutron scattering

Abstract

The ordering processes of ferroelectric and multiferroic materials were investigated via neutron total scattering and the reverse Monte Carlo method. The results presented in this thesis are from three materials where ferroelectric behaviour is a result of ordering of molecular groups rather than individual atoms. Two of the materials are metal-organic frameworks, three dimensional cage-like structures with guest ions inside the pores; the third material, is a room temperature ferroelectric. In the high-temperature phase of dimethylammonium manganese formate, the framework distorts around the disordered cation, and the cations form shorter hydrogen bonds with the formate framework than the average structure suggests. Framework deformations became increasingly unfavourable as the material cooled. The cations continue to order as the material was cooled below Tc. Analysis of the high-temperature phase atomistic configurations showed that in addition to the three known orientations about the threefold axis, a significant minority of the cations lie mid-way between these positions, a feature which could not have been observed via standard crystallographic techniques. The mechanisms for thermal expansion of potassium imidazolium hexacyanoferrate change between the intermediate-temperature phase and the high-temperature phase. In the hightemperature phase the framework distorts around the disordered guest, but in the intermediatetemperature phase the framework stiffens. I propose that the temperature of the dielectric transition is dependent of the volume inside the framework, but that the temperature range of the intermediate-temperature phase is dependent on the rate of contraction of the framework around the guest cation.. For triglycine sulfate no correlation was observed between the orientation of the polar molecules and the motion of the intermediate deuterium. Furthermore, in the high temperature phase the atomistic configurations produced models with macroscopic polarisation. I propose that this material forms domains of aligned polar molecules above Tc and that these domains are larger than the atomistic configurations.