Solid-State NMR Characterization of the Local Structure of Lead-Free Relaxor Ferroelectrics

• Applicant: Professor Dr. Gerd Buntkowsky
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 397608312

Sodium-bismuth titanate and its solid-solutions with barium (100-x)(Na1/2,Bi1/2)TiO3 (x)BaTiO3 (NBT xBT) stand out as a promising and environmentally friendly alternative to lead-based piezoelectric ceramics. These intrinsically inhomogeneous materials exhibit structural distortions with small magnitude and short coherence length, which pose a challenge to their characterization with conventional methods that are sensitive only to the average structure. As a consequence, key aspects of the structure-property relations in these materials are still unclear, as for example the role of chemical modifications, the nature of phase transitions (either temperature or electric field-induced) and the occurrence of a relaxor ferroelectric state. In this project, we employ solid-state nuclear magnetic resonance (ssNMR) spectroscopy in order to face the challenges posed by this class of materials and to learn about their structure at the local scale. This will be achieved by the analysis of static, one-dimensional MAS and in particular two-dimensional 3QMAS NMR spectra, which enable a description of the local structure in terms of the nuclear interactions present for 23Na and other NMR-active nuclei available. The investigation of the temperature dependence of relevant NMR parameters should shed light on the changes occurring to the local structure of the NBT-xBT compositions, both unpoled and poled, throughout the temperature range from below the depolarization temperature (Td) over the ferroelectric-to-relaxor transition temperature (TF R) and the dielectric permittivity’s maximum temperature (Tm) and above. Dynamical aspects of the local structure will be investigated both in the ferroelectric and relaxor states by the investigation of NMR relaxation times, aiming at the identification of the nature of local structural fluctuations present in these materials. The focus of the first application period was the characterization of the dependence of the local structure and electric polarization of (Na1/2,Bi1/2)TiO3 solid-solutions with barium titanate (NBT xBT, 0≤x≤15) with solid-state NMR. In the second application period we first want to (a) finish our studies on the nature of the apparently cubic phase in the NBT xBT system. For this we will (a) solve the problem of the missing side-band intensity in the MAS-NMR spectra and (b) complete the field dependent 23Na-T1-relaxation curves in order to be able to have enough data to model the T1- and T2-dispersion and reveal information about the dynamical nature of the apparent cubic phase. In parallel, we want (b) to employ these techniques to a detailed analysis of solid solutions of BaTiO3-CaTiO3, (barium calcium titanate, BCT), its poling/depoling behavior, the precipitation hardening process and its effect on the poling/depoling process inside these materials. Our interest in this type of materials is triggered by their favorable electric loss properties.

• DFG Programme: Research Grants