Piezoelectric materials couple polarization Pα and mechanical strain εβδ [1]. The technologically most used piezoelectric material system is the ferroelectric PbZr1− xTixO3 (PZT) [2,3]. Due to the toxicity of lead oxide, and to obtain properties beyond the range of PZT [2], such as a higher temperature of depolarization, larger Young’s modulus or higher cohesive stress, current research on ferroelectric materials devotes large resources to the identification of new lead-free materials [3].
The aim of this thesis is to improve the understanding of lead-free ferroelectric perovskite materials and eventually to guide the search for new ferroelectric materials. This is done by analyzing the structure and thermodynamic stability of different ferroelectric perovskite materials, focusing on PZT as reference and (Na1/2Bi1/2TiO3)1− x-(BaTiO3)x (BNT-BT). Ferroelectricity is an intrinsic material property that occurs only in materials with certain crystal structures [1]. Therefore, atomistic simulations are an appropriate tool to analyze ferroelectric materials. In this thesis the structure and thermodynamic stability of different ferroelectric perovskites are analyzed based on density functional theory (DFT) calculations.
As a first step chemical ordering and its influence on relaxation is analyzed. Chapter 3 shows that although chemical ordering is preferred in thermodynamic equilibrium for PZT, the driving force is too small to overcome diffusion barriers in bulk materials. In Chapter 4 a combination of DFT calculation and high resolution transmission electron microscopy (HRTEM) is used to analyze the cation distribution in BNT-BT. Finally, the solid solution BNT-BT is modeled according to the atomic distribution found in Chapter 4, and cation displacement is used as a measure of ferroelectricity. It is found, that the instabilities of the cation sites are a bilinear function of lattice parameter and composition. Also traits of the region showing improved ferroelectric properties are identified
