Gindele, OT; (2017) Atomistic Simulations of Ferroelectric Lead Zirconate Titanate. Doctoral thesis , UCL (University College London).

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Abstract

Ferroelectric materials are of technological importance for many applications and a thorough understanding of the origins of their high piezoelectric and dielectric properties is needed to optimise materials performance. One of the most widely used ferroelectrics is Pb(Zr1 xTix)O3 (PZT), which shows excellent piezoelectric response near its morphotropic phase boundary (MPB). To study PZT in large scale molecular dynamics (MD) simulations, I have developed a shell model force field that reproduces the details of the phase diagram of the PZT solid solution. The developed force field supports the temperature induced phase transitions from cubic to low symmetry phases over the whole composition range and additionally reproduces the composition driven phase transitions. This force field was subsequently used to study a variety of different effects in PZT. First, the polarisation switching in near morphotropic PZT at different temperatures was investigated, where I report a reduction of the saturation polarisation, accompanied by an exponential increase of the coercive field, as the temperature decreased. The simulations further demonstrated that the switching mechanism is fundamentally different at low temperatures, where it occurs via polarisation rotation, than at high temperatures, where domain nucleation dominates. Second, a set of simulations on near morphotropic PZT showed that the response of the local structure to electric fields highly depends on the local B-cation environment. Furthermore, I find that B-cation clustering largely influences the piezoelectric properties of the material. In a third MD study the electrocaloric effect (ECE) in the PZT compound was calculated, finding that PbTiO3 outperforms other compositions with its giant temperature change of 16 K. Lastly, density functional theory (DFT) calculations were performed to study defects and domain walls in the PZT end member PbTiO3, where we show that the presence of a couple donor-acceptor defect pair stabilises charged head-to-head and tail-to-tail 90 domain walls in PbTiO3.

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