The mechanisms of enhanced electromechanical coupling in uniaxial ferroelectrics

• Applicant: Dr. Carsten Richter
• Subject Area: Synthesis and Properties of Functional Materials ; Experimental Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 521034088

Piezoelectricity and electromechanical coupling have enormous technological relevance and underpin the functionalities of indispensable electronic devices. Despite the strong interest, only a handful of technology-relevant piezoelectric materials have been found and implemented by industry. Examples are single-crystalline quartz or polycrystalline lead-containing PbZr1-xTixO3. A lot of scientific efforts have been invested in the last decade towards lead-free alternatives with comparable piezoelectric properties. However, the research is mostly based on empirical approaches, such as high-throughput synthesis and characterization of complex solid solutions. It is known that the combination of piezoelectricity with ferroic ordering (e.g. ferroelectricity and ferroelasticity) can enhance the piezoelectric coefficients by two-three orders of magnitude. However, the coexistence of the multiple effects makes it hard to understand the reasons of this enhancement. This is why there is still a lack of fundamental understanding of structural mechanisms of domain dynamics and piezoelectricity in ferroelectrics. Here we propose a profound study of electric field induced strain in uniaxial ferroelectrics – materials where ferroelasticity is forbidden by symmetry. Focusing on such materials should reveal the structural relationship between piezoelectricity and ferroelectricity in the isolated form. We intend to employ advanced synchrotron X-ray diffraction techniques for operando structure analysis of uniaxial ferroelectric single crystals during electric field induced polarization reversal. The results will allow us to answer a) if atomic displacements in the unit cell are the dominating effect behind electromechanical coupling in uniaxial ferroelectrics; and b) if the presence of 180° inversion domains can enhance piezoelectricity. To better understand the role of symmetry for electromechanical coupling, we will study materials with stepwise increasing complexity: starting from centrosymmetric SrTiO3 via its polar phase transition towards uniaxial ferroelectrics BaMgF4 (BMF) and Li(Nb,Ta)O3 (LNO, LTO). The formulated goals pose several methodological challenges and require further development in the relevant branches of operando X-ray crystallography. These are e.g. time-dependent structure analysis in the presence of inversion domains. In order to pave the way towards future microscopic operando studies of dynamics of ferroelectric domains under external bias, we also plan to develop imaging of ferroelectric domains using X-ray diffraction microscopy at the ESRF synchrotron (Grenoble, France). The recently upgraded ESRF source and existing setups for X-ray diffraction microscopy provide ideal groundwork for this goal. On success, this development will be the basis for investigations of the microstructural effects close to domain walls that are expected to drive the enhanced electromechanical coupling in uniaxial ferroelectrics.

• DFG Programme: Research Grants
• International Connection: Israel
• International Co-Applicant: Dr. Semen Gorfman