Accurate Atomic Structure and Symmetry Determination of New Hybrid Improper Ferroelectric Phases

Non-Technical Abstract. Ferroelectric materials comprise a $7 billion market owing to their unique properties (a spontaneous electric polarization), and find use in many applications such as medical imaging for disease diagnosis, large-scale storage of information such as in data centers, or in high-density energy storage for solar or wind farms for times when these forms of energy are not available. The most common ferroelectric materials utilize elements that are difficult to obtain in nature and/or are toxic. Fortunately, the number of ferroelectric systems has been expanded significantly with the discovery of hybrid improper ferroelectrics, which can be composed of readily available, cheap elements and have low toxicity. The goal of this work is to determine what the atomic structures of these materials are under high pressure, as new unique ones can be created which will have high technological applicability for information storage or energy storage. Graduate and undergraduate students are involved in all levels of this work, including sample preparation, laboratory and synchrotron-based measurements, modeling, and data analysis. Under the direction of the research team and graduate students, Newark-area high school students are being trained in a seven-week summer research and teaching program on material preparation and advanced materials characterization. It includes a one-week workshop for high school teachers to enable them to implement components of the program into their laboratory experiments. The education and research are a collaboration between the New Jersey Institute of Technology, Rutgers University, the University of Michigan, Brookhaven National Laboratory, Argonne National Laboratory, Lawrence Berkeley National Laboratory, and the SEED program for high school students (American Chemical Society). Technical Abstract. Ferroelectric materials are essential to high-density data storage and are used in solid-state drives, but basic physics requirements for the known materials have limited the chemical and structural space available for the development of new ones. Recently, this space has been significantly expanded with the discovery of hybrid improper ferroelectrics, which have multiple nonpolar distortions that stabilize a polar ferroelectric state. External conditions can stabilize new phases. To fully exploit this new class of ferroelectrics, their full pressure- and temperature-dependent phase diagrams are needed. To develop a detailed understanding of structural changes in the general class of transition metal oxide-based hybrid improper ferroelectrics, single-crystal diffraction measurements as a function of pressure and temperature are being carried out. The derived detail structure is being used to inform density functional theory (DFT) calculations by constraining the crystal structures investigated computationally. Newly discovered phases are being prepared as metastable forms and integrated into ferroelectric-based devices for improved performance. The societal impact of the proposal will come from broadening the range of oxides available for applications in data storage devices and enabling the use of nontoxic earth-abundant materials systems. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.