CAREER: Decoupling Structure and Composition with Zintl Polymorphs

新功能材料设计中最基本的挑战之一是将晶体结构与化学成分的作用解耦。多态性——一种化合物形成两种或两种以上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，多态的例子仍然相对罕见。该项目由材料研究部固态和材料化学项目支持，采用系统的、理论指导的方法来扩展被称为Zintl化合物的半导体家族中的多晶化合物的当前库。Zintl化合物以其优异的热电性能（即将热能转化为电能的能力）而闻名。本研究探索了几种不同的途径来发现新的Zintl多晶，包括应用高压来控制键长和使用高通量合成来快速探索宽相空间。PI Zevalkink认为，新发现化合物的弹性特性的表征有助于阐明晶体结构和键刚度之间的关系，这反过来可能为有针对性地设计改进的热电材料铺平道路。本研究也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。该项目利用了密歇根州立大学的现代数字资源，包括天文馆、密歇根州立大学博物馆的球体科学和中央图书馆的360º房间，为学生展示提供直观的平台，开发入门晶体学课程，并创建热电材料的互动推广展览。通过让学生参与新课程的开发，K-12和社区外展，该教育计划为大学生提供了使用新技术发展教学技能的机会。本研究的主要目标是开发一种系统的、理论指导的方法来发现多晶型，重点是锌金属间化合物，一类以其复杂、多组分结构类型而闻名的热电材料。合成方法采用了一套实验策略来实现多态性，包括i)应用高压来增加配位环境，ii)合金化来促进上层结构的形成，以及iii)高通量合成来探索竞争结构之间的相边界。提出的工作的第二个目标是利用新发现的多晶来发展电子和热输运领域的结构-性质关系。本项目由材料研究部固态与材料化学项目支持，重点研究弹性的表征。弹性在由声子介导或受声子限制的材料特性中起着核心作用，包括导热性（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为敏感。通过利用新发现的Zintl多晶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（如聚阴离子连通性、阳离子配位和有序与无序）在决定弹性特性中的作用。本研究旨在弥合理论与实验之间的差距，为不同Zintl结构类型中不同功能行为的起源提供新的视角。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识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.