Search, synthesis and structural investigations of organic molecular ferroelectrics

This thesis is inspired by the symmetry relationships frequently observed between paraelectric and ferroelectric phases. It aims to search for new molecular ferroelectrics, identify their structural phase transitions and understand how the behaviours of these materials are associated with phase transitions. Chapter 1 is a literature review on two aspects of organic materials in the solid state which are closely related to structural phase transitions: ferroelectricity and unusual thermal expansion. It gives a brief introduction to ferroelectricity and uses a number of ferroelectric examples to illustrate the different phase transition types that can lead to ferroelectricity. An overview of unusual (negative) thermal expansion observed in inorganic, organic-inorganic and pure organic compounds is also included. Chapter 2 introduces diffraction methods (single crystal and powder X-ray diffraction and neutron diffraction), complementary structural techniques (solid state nuclear magnetic resonance, thermal analysis, elemental analysis and second harmonic generation) and the theory behind them. It also introduces the concepts underlying the molecular symmetry-adapted distortion mode analysis, which are used in Chapter 3 and 4. Chapter 3 describes a comprehensive study of chloranilic acid pyrazine (CA-Pyz) which undergoes a reversible phase transition at ~300 K and shows remarkable thermal expansion properties associated with the phase transition. We show that CA-Pyz has uniaxial negative thermal expansion over a wide temperature range with a linear contraction coefficient as negative as −1500 ×10-6 K-1 at 250 K. The concepts of symmetry-adapted rotational modes and strain modes are used to successfully understand the structural changes and the unusual thermal expansion behaviour of CAPyz. Chapter 4 demonstrates the use of systematic and exhaustive symmetry-adapted distortion mode subgroup search methods to solve the high temperature structure of 5,6dichloro-methylbenzimidazole (DC-MBI) successfully from powder diffraction data. It describes how different models can be automatically tested against powder diffraction data and how the best structural description can be chosen. This is the first example that uses exhaustive symmetry-adapted distortion mode subgroup search method to solve a molecular structure. DC-MBI has the unusual property of its high temperature structure being of lower symmetry than its low temperature structure. Chapter 5 reports two new polymorphic forms of the 2:1 cocrystal of benzoic acid and ethylenediamine (2BA-ETD). Variable temperature PXRD measurements revealed two new forms of 2BA-ETDA (the β- and γ-forms). The structure of the β-form is solved successfully from powder diffraction data by using the symmetry descent subgroup search method. The structure of the γ-form has also been solved but is less clear cut. Three possible structure models for the γ-form are obtained from powder diffraction data using simulated annealing. Structural optimisation and 13C solid state NMR study provide a result that is inconsistent with an SHG study. As a result, we present a structural model for the γ-form that we believe is most likely based on the data available. Chapter 6 describes attempts to grow cocrystals of chloranilic acid (CA) with pyrazole (PYR) and benzimidazole (BEN) as potential hydrogen-bonded ferroelectric materials. It reports two previously-unknown structures of the monohydrate CA-PYR and CA-2BEN. The structure of monohydrated CA-PYR is solved using single crystal X-ray diffraction. The structural model helps rationalise why a dehydrated sample cannot be formed by gradually heating. The structure of CA-2BEN is solved from both powder and single crystal diffraction data. The structure solution from powder diffraction data is similar to that from single crystal diffraction data. This gives a confidence test on the methods we have used in other chapters