CAREER: Ligand Engineering of Structure and Electronic Function in Complex Metal Oxyfluorides

Northwestern University, Evanston IL

Investigators

Abstract

NON-TECHNICAL SUMMARY The CAREER project supports computational materials science research and education aimed at understanding and designing electronic properties of materials, including electrical resistivity and optical behavior, by control over material structure at the level of atoms. The specific compounds of interest include oxides with transition metal cations, and focus on the effect that fluorine, which can substitute for oxygen in the materials, has on the electronic functionality. Previous research on transition metal oxides has established the importance of atomic structure engineering of electronic responses in many crystal families. Most conventional property-by-design routes rely, however, on changing the transition metal cation in oxides. This project seeks to apply another route - tuning the interactions in the crystal through anion (fluorine and oxygen) substitution. Changes in the geometry, arrangement and composition of the anion atoms will be explored through a confluence of materials theory techniques based on symmetry analyses, materials informatics (machine learning), and quantum mechanical calculations. The PI has ongoing collaborations with leading experts in synthesis and characterization of such materials composed of transition metals, oxygen, and fluorine; understanding derived here will stimulate experimental methods and vice versa. Continued investigation of known materials, while valuable, is inadequate to formulate strategies for deterministic property control. Knowledge obtained here will facilitate the selection and design of materials with tunable electronic states. It will benefit society by advancing the repertoire of structure-based design strategies to control electronic structure, which could lead to the discovery of new functional materials, e.g. for better performing energy storage and conversion systems, materials for transparent electronics, and optical technologies relying on laser generated light. This project will further the educational opportunities of students at Northwestern University and other academic institutions, precollege students, and contribute to the professional development of high school teachers. The PI will implement an educational plan to foster awareness, understanding, and appreciation of advanced technology materials and data-driven scientific methods through two main tasks. First, he will create Engineering Design Modules capable of engaging students in grades 9 through 12 by fostering model building skills to analyze and communicate concepts taught in secondary chemistry and physics courses that underpin many modern technologies, and support teachers preparing for the recently adopted Next Generation Science Standards. Second, he will create a Materials Informatics Curriculum to engage university students in modern informatics-based science problem-solving methods. All contextual learning activities will build knowledge, promote scientific and engineering literacy, and provide greater insight into the societal needs for engineering solutions, fostering cognitive skills through an emphasis on cause/effect relationships that are axiomatic to the research objectives and vital to the next-generation workforce. Assessment of the proposed educational activities and broad dissemination through multiple platforms will determine the efficacy of the educational activities, improve their implementation, and maximize impact. TECHNICAL SUMMARY This CAREER award supports synergistic research, education, and outreach activities which focus on the design of functional electronic behavior in transition metal oxyfluorides using control over the ligand sublattice by oxygen/fluorine substitution and ordering. Conventional routes to direct the responses in transition metal oxides primarily rely on cation substitution and interfacial effects in thin films and superlattices, which offer limited control owing to a single (oxygen) anion - this makes materials discovery challenging. Remarkably, ligand (anion) engineering with mixed anion polyhedral building blocks remains to be fully exploited for property control and design, especially in these materials which already find use in energy generation and storage, phosphors, and catalysis. Combinations of applied group theory, informatics, and density functional theory calculations will be applied to achieve the main research objectives, which include (1) Advancing new theoretical methods to establish structure-function axioms for how anion order can be used to direct crystal structure and properties; (2) Formulating a quantitative theory of structure stability based on understanding the ligand sublattice symmetry and local bonding interactions; and (3) Understanding the consequences of mixed-anion polyhedral topologies on electronic properties. Structure-property axioms will be extracted by studying the consequences of anion substitution using oxyfluoride building blocks on physical properties in cryolite and elpasolite structures. With that knowledge, quantitative guidelines will be constructed to tailor electronic structure and properties in new oxyfluorides: metal-insulator transitions, electronic band gaps, and large non-linear optical responses. Success in this research will produce new knowledge underlying crystal stability, chemical bonding, and electronic behavior. It will articulate predictive rules for selecting new oxyfluorides, accelerate discovery, and enable an unprecedented expansion of compounds with varying electronic functions. Ultimately, interactions with experimental groups will lead to the discovery of functional properties in structurally and chemically more complex (hybrid) organic and inorganic materials than those proposed. Such collaborations will ensure that the virtual predictions translate into realistic models and new materials, which may transform the discovery process for materials deployed in glasses, phosphors, fuel-conversion, and Li-ion batteries technologies. This project will further the educational opportunities of students at Northwestern University and other academic institutions, precollege students, and contribute to the professional development of high school teachers. The PI will implement an educational plan to foster awareness, understanding, and appreciation of advanced technology materials and data-driven scientific methods through two main tasks. First, he will create Engineering Design Modules capable of engaging students in grades 9 through 12 by fostering model building skills to analyze and communicate concepts taught in secondary chemistry and physics courses that underpin many modern technologies, and support teachers preparing for the recently adopted Next Generation Science Standards. Second, he will create a Materials Informatics Curriculum to engage university students in modern informatics-based science problem-solving methods. All contextual learning activities will build knowledge, promote scientific and engineering literacy, and provide greater insight into the societal needs for engineering solutions, fostering cognitive skills through an emphasis on cause/effect relationships that are axiomatic to the research objectives and vital to the next-generation workforce. Assessment of the proposed educational activities and broad dissemination through multiple platforms will determine the efficacy of the educational activities, improve their implementation, and maximize impact.