CAREER: Decoupling Structure and Composition with Zintl Polymorphs

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PART 1: NON-TECHNICAL SUMMARY One of the most fundamental challenges in the design of new functional materials is decoupling the role played by the crystal structure from that of chemical composition. Polymorphism – the ability of a compound to form two or more crystal structure types – provides an attractive solution to this dilemma by enabling experiments in which the crystal structure is the only variable. However, examples of polymorphs are still relatively rare. This project, supported by the Solid State and Materials Chemistry Program in the Division of Materials Research, employs a systematic, theory-guided approach to expand the current library of polymorphs among a family of semiconductors known as Zintl compounds. Zintl compounds are best known for their excellent thermoelectric performance (i.e., their ability to convert thermal energy into electrical energy). This research explores several different routes to discovering new Zintl polymorphs, including the application of high pressure to control bond length and the use of high-throughput synthesis to rapidly explore a wide phase space. PI Zevalkink posits that characterization of the elastic properties of newly discovered compounds helps to shed light on the relationship between crystal structure and bond stiffness, which in turn may pave the way for targeted design of improved thermoelectric materials. This research also provides training and mentorship for graduate and undergraduate students who are involved in the synthesis and characterization of new materials. This project leverages modern digital resources at MSU including the Planetarium, Science on a Sphere at the MSU Museum, and the 360º room at the central library to provide intuitive platforms for student presentations, to develop introductory crystallography curriculum, and to create an interactive outreach exhibit on thermoelectric materials. By involving students in the development of new curriculum and in K-12 and community outreach, the educational plan provides opportunities for college students to develop their teaching skills using new technologies. PART 2: TECHNICAL SUMMARY The primary goal of the proposed research is to develop a systematic, theory-guided approach to polymorph discovery, with an emphasis on Zintl intermetallics, a class of thermoelectric materials known for their complex, multi-component structure types. The synthetic approach employs a suite of experimental strategies to achieve polymorphism, including i) the application of high pressure to increase coordination environment, ii) alloying to encourage superstructure formation, and iii) high-throughput synthesis to explore the phase boundaries between competing structures. The secondary goal of the proposed work is to exploit the newly discovered polymorphs to develop structure-property relationships in the realm of electronic and thermal transport. This project, which is supported by the Solid State and Materials Chemistry Program in the Division of Materials Research, emphasizes characterization of elasticity. Elasticity plays a central role in material properties mediated by or limited by phonons, including thermal conductivity (phonon velocities, phonon-phonon coupling strength), electrical conductivity (electron-phonon coupling) and ionic conductivity. The elastic moduli of solids are extremely sensitive to crystal structure and the nature of chemical bonding. By leveraging newly-discovered Zintl polymorphs and recently-developed orbitally-decoupled elastic tensor calculations developed by collaborators, this project aims to unravel the role that structural features such as polyanion connectivity, cation coordination and order vs. disorder play in determining elastic properties. The proposed research aims to bridge the gap between theory and experiment to provide a new perspective on the origin of the divergent functional behaviors found in different Zintl structure types.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.