An Integrated Approach to Molecular Ferroelectric Discovery

This thesis was motivated by the search for new molecular ferroelectric materials. A new method for identifying molecular ferroelectrics has been developed, and materials suggested by it have been synthesised and their properties and phase transitions have been investigated. Chapter 1 provides a background to the field of molecular ferroelectrics. A short history of ferroelectrics and their applications is followed by descriptions of some of the key molecular ferroelectric discoveries from the literature. Chapter 2 reviews the theoretical background of X-ray diffraction and outlines the other experimental techniques used in this thesis. Chapter 3 introduces FERROSCOPE, a database searching program for finding potential molecular ferroelectric materials in the Cambridge Structural Database. Chapter 4 elucidates the complex chain arrangement of piperaziniums in piperazinium tetrafluoroborate. These hydrogen-bonded chains are present in all observed phases, including the highly disordered high-temperature phase. The four polymorphs of piperazinium tetrafluoroborate, four polymorphs of piperazinium perchlorate from the literature and two polymorphs of newly-discovered piperazinium perrhenate have been structurally related on a group-subgroup tree. Chapter 5 argues that fast molecular dynamics can allow chiral molecules to crystallise in non-Sohncke space groups. By synthesising chiral versions of (Rac-3-chloro-2- hydroxypropyltrimethylammonium)2MCl4, three new potentially ferroelectric structures were discovered. Along with the other chiral and racemic members of this family, these transition to a shared centrosymmetric high-temperature cubic phase upon warming. This suggests the potentially ferroelectric structures may be multiaxial. Density-functional theory calculations on the polar structures show they may exhibit potentially useful values of spontaneous polarisation. Chapter 6 details the structural analysis of molecular ferroelectric 18-crown-6 oxonium tetrachloro-gallium(III). The structure of the high-temperature paraelectric phase was solved from PXRD data. An intermediate phase was identified, isolated, and revealed to be structurally distinct to the ferroelectric and paraelectric phases but related to a newly discovered low-temperature phase. This relationship was described using symmetry inspired rotational-mode analysis using the ISODISTORT methodology. Chapter 7 assesses the ferroelectricity of anilinium tetrafluoroborate, a potential ferroelectric suggested by FERROSCOPE. Three new phases were discovered and the structures solved by diffraction methods. Room temperature polarisation-electric field measurements showed good dielectric behaviour at the temperatures and switching frequencies measured. Chapter 8 reanalyses the structure of dibetaine tetrafluoroborate. Incorrect symmetry assignment in the Cambridge Structural Database meant the expected ferroelectricity is impossible. Two new structures were discovered and analysed including a complex super structure at low temperatures solved using SXRD. Chapter 9 describes the structure solution of two new phases of aminoacetonitrile hydrochloride. A potentially antiferroelectric orthorhombic phase and a transient tetragonal plastic phase were solved directly from powder X-ray diffraction data. Chapter 10 reports a high-temperature phase transition in tetraethylammonium tetrachloroferrate(III). The room-temperature hexagonal phase transitions to a highly disorderd phase that adopts a caesium chloride-like structure. Chapter 11 summarises the findings and developments made during this thesis.