Melanie Gröting

Reconciling Local Structure Disorder and the Relaxor State in (Bi1/2Na1/2)TiO3-BaTiO3

Lead-based relaxor ferroelectrics are key functional materials indispensable for the production of multilayer ceramic capacitors and piezoelectric transducers. Currently there are strong efforts to develop novel environmentally benign lead-free relaxor materials. The structural origins of the relaxor state and the role of composition modifications in these lead-free materials are still not well understood. In the present contribution, the solid-solution (100-x)(Bi1/2Na1/2)TiO3-xBaTiO3 (BNT-xBT), a prototypic lead-free relaxor is studied by the combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, dielectric measurements and ab-initio density functional theory (DFT). For the first time it is shown that the peculiar composition dependence of the EFG distribution width (ΔQISwidth) correlates strongly to the dispersion in dielectric permittivity, a fingerprint of the relaxor state. Significant disorder is found in the local structure of BNT-xBT, as indicated by the analysis of the electric field gradient (EFG) in 23Na 3QMAS NMR spectra. Aided by DFT calculations, this disorder is attributed to a continuous unimodal distribution of octahedral tilting. These results contrast strongly to the previously proposed coexistence of two octahedral tilt systems in BNT-xBT. Based on these results, we propose that considerable octahedral tilt disorder may be a general feature of these oxides and essential for their relaxor properties.

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.

Structure, Electronic Structure and Defect Formation Energies of LixCo1-yNiyO2 as a Function of x (0<x<1) and y (y = 0, 0.5, 1)

Abstract LixCoO2 and LixNiO2 (0.5<x<1) are prototype cathode materials in lithium ion batteries.Both systems show degradation and fatigue during electrochemical cycling. We have performed band structure calculations based on density-functional theory for a series of compounds Lix(Co,Ni)O2 (0<x<1). The distribution of the transition metals (TM) cobalt and nickel on TM sites as well as the electronic structure of these compounds is investigated with focus on the change of oxidation states of cobalt, nickel and oxygen during lithium de-intercalation. We also study the total energy as a function of the lithium content x, including the vibrational energy Ev and the formation energy of lithium vacancies E(VLi). It is found that Ev is small compared to E(VLi) and that E(VLi) is increasing with increasing x for all systems.

Octahedral tilt transitions in the relaxor ferroelectric Na{sub 1/2}Bi{sub 1/2}TiO{sub 3}

The kinetics of octahedral tilt transitions in the lead-free relaxor material sodium bismuth titanate Na1/2Bi1/2TiO3 (NBT) is investigated by electronic structure calculations within density functional theory. Energy barriers for transitions between tetragonal, rhombohedral and orthorhombic tilts in cation configurations with [001]- and [111]-order on the A-sites are determined by nudged elastic band calculations. By tilting entire layers of octahedra simultaneously we find that the activation energy is lower for structures with 001-order compared to such with 111-order. The energetic coupling between differently tilted layers is, however, negligibly small. By introducing a single octahedral defect we create local tilt disorder and find that the deformation energy of the neighboring octahedra is less in a rhombohedral than in a tetragonal structure. By successively increasing the size of clusters of orthorhombic defects in a rhombohedral matrix with 001-order, we determine a critical cluster size of about 40 A . Thus groups of about ten octahedra can be considered as nuclei for polar nanoregions, which are the cause of the experimentally observed relaxor behavior of NBT.