MRSEC: Center for Nanoscale Science

Work at Northwestern University will use first-principles density functional theory (DFT) meth-ods, supplemented with phenomenological models, group theory (symmetry), materials informat-ics, and chemistry principles to: - Explore routes to design ferroelectricity in cation ordered perovskites from non-polar perovskite building blocks, thereby developing materials selection criteria to expand the number of compounds with sizeable electric polarizations and low switching barriers. - Evaluate the energetic competition between ferro- and anti-ferroelectricity in layered perovskites with respect to cation order and periodicity. - Formulate models of unconventional ferroelectric mechanisms based on layered dimen-sionality, n¸ in Ruddlesden-Popper and Aurivillius oxides using a novel inverse structural search based on group theoretical methods and a crucial orbital radii descriptor. The planned investigations by Rondinelli form a key component of the proposed project. The technical proposal, therefore, is narrated in a manner to clearly delineate the specific objectives and computational methods that his group will use to achieve the target deliverables—a class of novel ferroics based on tuning electronic and steric interactions through real-space control of structure topology, chemical ordering, and polyhedral geometric connectivity. For that reason, we generally only describe here the planned work; complete investigation details are available in the Technical Narrative. Calculations will be performed by the budgeted personnel (see below) in concert with the PI whom will be actively involved in supervising the entire project, analyzing results and mentoring research personnel, and exchanging information with IRG team members. During project periods 1-6, a graduate student supervised by Rondinelli will investigate novel routes to induce hybrid improper ferroelectricity in layered oxides characterized by octahedral units. He or she will perform DFT calculations and analyze the effect of cation order ar-rangements, dimensionality, and mechanical strain on the tendency to polar and anti-polar dis-placements. Lattice normal modes and energy landscapes with respect to mode distortions will be computed. The ground state atomic structures will also be computed for a variety of chemistries; this information will be fed into genomic models and combined with Bayesian analysis to make probabilistic recommendations on compositions for superior ferroics. For these promising candidate compounds, we will compute the electric polarizations, formation energies, and elec-tronic structure, to understand the microscopic mechanisms producing the ferroic behavior. These results will be used to suggest chemistries for experimental synthesis and characterization by the IRG team members. It is anticipated that a shared website and data server, bi-monthly virtual meetings, and visits to Penn State will facilitate the transmission of new data resulting directly from this project.