CAREER: A roadmap to atomically ordered complex materials via control of entropic mixing

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Non-Technical AbstractThe ability to atomically order crystalline materials is central to advancing technology. In the 1980s, 99% atomic ordering in two-element materials was developed, wherein two elements alternate nearly perfectly in their atomic site occupations. This enabled high-frequency transistors, which led to the cell phone revolution and high-efficiency solar panel technologies. Theoretical predictions tout revolutionary new material properties in complex (3+ elements) materials that will make possible new devices with broad application in information technology, solar cells, lighting, microwave communications, thermoelectrics, and power electronics. However, achieving the 99% atomic ordering required to realize those properties has remained elusive. The goal of this project is to systematically gain an understanding of the fundamental ordering mechanisms in complex materials. This research integrates computational theory and experimental results to create a set of criteria that can be used to design materials of sufficiently high atomic ordering (99%) to realize their intrinsic properties. This research directly integrates educational activities to impact underrepresented minorities, women, and underserved rural communities in STEM fields, and ensure that undergraduate education includes research experience. This mentor-based strategy focuses on elevating science, technology, engineering and mathematics (STEM) educators in underrepresented communities in rural Alabama and Mississippi. K-12 educators gain access to university faculty and specialists at the Alabama Science in Motion program to plan classes and laboratory sessions, and through the Alabama Math, Science and Technology Summer Institute receive training to qualify their rural school district for program/equipment funding. Undergraduate summer researchers are recruited from local Historically Black Colleges and Universities, minority-serving institutions and the American Physical Society’s Conferences for Undergraduate Women in Physics. Undergraduate students will also work as research assistants during each school year.Technical AbstractImperfect atomic ordering in complex materials is a pervasive issue in the condensed matter and materials communities: A model that accounts for entropic mixing disorder is required before complex materials can be applied widely. First principles calculations can accurately predict the intrinsic (structural, electronics, magnetic/magnetodynamic) material properties of an ordered system, but near-perfect (99%) atomic ordering is needed to manifest those properties. At present, it is a complex and computationally expensive task to determine if the system can form with the required atomic ordering in the presence of thermal and growth energies. The central hypothesis of this research is that high atomic ordering can be predicted by incorporating existing metallurgical metrics that indicate the level of entropic mixing. To test the hypothesis, computational predictions of ordering and properties are produced for a range of three-element L21-ordered Heusler alloys, chosen specifically due to decades-long frustration in realizing predictions of high spin polarization due to atomic disorder. Although applications often prefer highly ordered systems, materials with a range of ordering levels are selected to refine a robust quantitative model and provide a roadmap for material design. Thin films of each material system are grown with low energetics by the Sputter Beam Epitaxy method invented by the principal investigator, such that atomic ordering and material properties of each system can be compared to predictions with minimal extrinsic contribution. The results form a feedback loop between theory and experiment to establish and refine a quantitative model of atomic ordering when three or more elements are used. A quantitative predictive model for atomic ordering in complex alloys broadly translates to the many other fields whose material systems are plagued by entropic mixing.This project is jointly funded by the Electronic and Photonic Materials Program and the Established Program to Stimulate Competitive Research.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.