First-principles calculations on the structural and thermodynamic stability of (Na1/2Bi1/2,Ba)TiO3 and Pb(Zr,Ti)O3

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

Chemical order and local structure of the lead-free relaxor ferroelectric Na1/2Bi1/2TiO3

The A-site mixed perovskite sodium bismuth titanate (Na1/2Bi1/2)TiO3 (NBT) is investigated by means of first-principles calculations based on density functional theory. By studying different geometries with varying occupations of the A-site, the influence of chemical order on the thermodynamic stability and local structure is explored. We find that the hybridization of Bi 6sp with O 2p-states leads to stereochemically active Bi3+ lone pairs and increases the stability of structures with high Bi concentrations in {001}-planes. This goes along with displacive disorder on the oxygen sublattice, which up to now has been neglected in experimental studies. The calculated ordering energies are, however, small as compared to the thermal energy and therefore only short-range chemical order can be expected in experiments. Thus, it is conceivable that chemically ordered local areas can act as nucleation sites for polar nano-regions, which would explain the experimentally observed relaxor behavior of NBT

Chemical order and local structure of the lead-free relaxor ferroelectric (Na1/2Bi1/2)TiO3

The A-site mixed perovskite sodium bismuth titanate (Na1/2Bi1/2)TiO3 (NBT) is investigated by means of first-principles calculations based on density functional theory. By studying different geometries with varying occupations of the A-site, the influence of chemical order on the thermodynamic stability and local structure is explored. We find that the hybridization of Bi 6sp with O 2p-states leads to stereochemically active Bi3+ lone pairs and increases the stability of structures with high Bi concentrations in {001}-planes. This goes along with displacive disorder on the oxygen sublattice, which up to now has been neglected in experimental studies. The calculated ordering energies are, however, small as compared to the thermal energy and therefore only short-range chemical order can be expected in experiments. Thus, it is conceivable that chemically ordered local areas can act as nucleation sites for polar nano-regions, which would explain the experimentally observed relaxor behavior of NBT

A-site occupancy in the lead-free (Bi1/2Na1/2TiO3)0.94-(BaTiO3)0.06 piezoceramic : combining first-principles study and TEM

The crystal structure of the lead-free piezoelectric ceramic(Bi1/2Na1/2TiO3)0.94–(BaTiO3)0.06 was investigated by first-principles calculations and high-resolution transmission electron microscopy(HRTEM) imaging. Structures with different A-site occupation were relaxed by total energy calculations within density functional theory and then used for simulating the corresponding HRTEM images. Simulated and experimental HRTEM images were compared and the closest match selected for structure interpretation. By combining these techniques, we have identified the Bi(Ba)/Na distribution on the A-site to be homogeneous. We exclude the possibility that regions visible in HRTEM images within one grain can be attributed to different ordering but to a slight tilting of the structure with respect to the electron beam

A-site occupancy in the lead-free (Bi1/2Na1/2TiO3)0.94–(BaTiO3)0.06 piezoceramic: Combining first-principles study and TEM

The crystal structure of the lead-free piezoelectric ceramic (Bi1/2Na1/2TiO3)0.94–(BaTiO3)0.06 was investigated by first-principles calculations and high-resolution transmission electron microscopy (HRTEM) imaging. Structures with different A-site occupation were relaxed by total energy calculations within density functional theory and then used for simulating the corresponding HRTEM images. Simulated and experimental HRTEM images were compared and the closest match selected for structure interpretation. By combining these techniques, we have identified the Bi(Ba)/Na distribution on the A-site to be homogeneous. We exclude the possibility that regions visible in HRTEM images within one grain can be attributed to different ordering but to a slight tilting of the structure with respect to the electron beam