Ferroelectricity is a phenomenon which scientists have investigated since its discovery – for its own sake, as well as the practical applications which have been later revealed. Although most research has historically focused on inorganic ferroelectrics such as the perovskites lead zirconate titanate and barium titanate, organic ferroelectrics often possess different characteristics which may inspire novel applications in the future.
An important aspect of a ferroelectric’s behaviour is the mechanism by which its polarisation switches in an external field. This thesis describes the development of an effective Hamiltonian to simulate polarisation switching in the organic, proton transfer ferroelectric, croconic acid (CRCA). CRCA is a single-component, room temperature ferroelectric with a large spontaneous polarisation (30 μCcm⁻²). Density functional theory (DFT) was used to map out the potential energy surface (PES) of CRCA from first principles, and to generate the data which was used to train the effective Hamiltonian.
The effective Hamiltonian was developed to satisfy translational invariance in a way which allows the molecular degrees of freedom to be coarse-grained. This improves the efficiency with which the effective Hamiltonian can be trained and used relative to previous versions of the method from which our model was adapted. The accuracy of the effective Hamiltonian was verified by calculating the minimum energy path (MEP) of a single unit cell between its two domain states and comparing it to the results of DFT calculations of the MEP. Polarisation switching in the non-activated regime of CRCA was then simulated using the kinetic Monte Carlo method with the effective Hamiltonian. These simulations suggest that the domain wall velocity and the electric field strength follow a linear relationship with a negative intercept. The similarities and differences between this result and the results of previous theoretical studies
are considered and discussed.