CAREER: Decoupling Structure and Composition with Zintl Polymorphs

第一部分： 设计新功能材料的最基本挑战之一是将晶体结构所起的作用与化学组成的作用解耦。 多晶型现象--化合物形成两种或多种晶体结构类型的能力--通过使晶体结构成为唯一变量的实验成为可能，为这一困境提供了一个有吸引力的解决方案。 然而，多晶型的例子仍然相对罕见。 该项目由材料研究部的固态和材料化学计划支持，采用系统的理论指导方法来扩展被称为Zintl化合物的半导体家族中的多晶型物的当前库。Zintl化合物以其优异的热电性能（即，它们将热能转换成电能的能力）。 这项研究探索了几种不同的途径来发现新的Zintl多晶型物，包括应用高压来控制键长，以及使用高通量合成来快速探索宽的相空间。 PI Zevalkink认为，对新发现的化合物的弹性特性进行表征有助于揭示晶体结构和键刚度之间的关系，这反过来可能为改进热电材料的目标设计铺平道路。 这项研究还为参与新材料合成和表征的研究生和本科生提供培训和指导。 该项目利用MSU的现代数字资源，包括天文馆，MSU博物馆的球体科学和中央图书馆的360º室，为学生演示提供直观的平台，开发介绍性晶体学课程，并创建热电材料的互动外展展览。 通过让学生参与新课程的制定以及K-12和社区外联，教育计划为大学生提供了利用新技术发展教学技能的机会。 第二部分： 所提出的研究的主要目标是开发一种系统的、理论指导的多晶型物发现方法，重点是Zintl金属间化合物，这是一类以其复杂的多组分结构类型而闻名的热电材料。 该合成方法采用一套实验策略来实现多晶型，包括i）施加高压以增加配位环境，ii）合金化以促进超结构形成，以及iii）高通量合成以探索竞争结构之间的相边界。拟议工作的第二个目标是利用新发现的多晶型物来开发电子和热传输领域中的结构-性质关系。 该项目由材料研究部的固态和材料化学计划支持，强调弹性的表征。弹性在由声子介导或限制的材料性质中起着核心作用，包括热导率（声子速度，声子-声子耦合强度），电导率（电子-声子耦合）和离子电导率。 固体的弹性模量对晶体结构和化学键的性质极其敏感。 通过利用新发现的Zintl多晶型物和合作者最近开发的轨道解耦弹性张量计算，该项目旨在揭示结构特征（如聚阴离子连接性，阳离子配位和有序与无序）在确定弹性特性中的作用。 该研究旨在弥合理论和实验之间的差距，为不同Zintl结构类型中发现的不同功能行为的起源提供新的视角。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.