Materials World Network: Gradient-Enabled Ferroic Phenomena: Tunable Metastable States, Roto-Flexo, and Transport Properties

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TECHNICAL SUMMARY: The functional properties of ferroelectrics and ferroelastics, materials with built-in polarization and elastic distortion states in their crystal structure, respectively, are typically reliant on their response under uniform spatial elastic and electric fields, e.g., switching, piezoelectric, and electro-optic responses. There is also a rich range of ferroic phenomena arising under gradient fields, which receive much less attention. This project is based on two new discoveries/ideas initiated by the US/Ukraine team: New highly tunable metastable states and roto-flexo phenomena. Strong gradient fields created at and in the proximity of domain walls can result in local phase transitions that lead to new bulk polar phases not normally expected in classic textbook ferroelectrics, and even in non-polar ferroelastics. In all oxide interfaces with oxygen octahedral tilts, the creation of a polarization (up to 1-10 microC/cm2) is predicted through a rotostriction-flexoelectric product effect that can significantly impact the interface charge transport. Using optical second harmonic generation microscopy, Raman microscopy, scanning probe microscopy, nanoscale X-ray diffraction imaging, z-contrast scanning transmission electron microscopy, analytical theory, phase-field modeling, and first principles theory, this collaborative team explores these new phenomena. Broadly speaking, the US team (Pennsylvania State University, Oak Ridge National Labs, Argonne National Labs) conducts experimental research and performs phase-field simulations, and the Ukrainian team (National Academy of Sciences, Ukraine) focuses on developing the theoretical framework. NON-TECHNICAL SUMMARY: This project can potentially lead to new highly tunable, large piezoelectric response lead-free materials useful for precision motion and sensors. Gradient couplings at interfaces can lead to two-dimensional electron gas systems of great current interest for next generation high-speed transistors. This project also develops cutting-edge quantitative microscopy tools and advances theoretical modeling by simulating flexoelectric and other gradient effects. The NSF award provides funds to energize and sustain an international research team, which started in 2007. It funds undergraduate and graduate students to work and collaborate in a global context, supports extended visits across the Atlantic by PIs and students, furthers interactions between a university (Penn State), national labs (Oak Ridge and Argonne) and international collaborators (NAS-Ukraine), supports outreach activities through K-12, and provides research opportunities for women and underrepresented groups.This project is supported by the Electronic and Photonic Materials program and Office of Special Programs, Division of Materials Research.