Frustrated and Allowed Structural Transitions: Towards a Predictive Framework for the Structural Chemistry of Intermetallic Phases

非技术总结：新技术的发展与新型固态材料的发现和优化密切相关。金属间相——金属在一起合金化形成的化合物——代表着这类材料的丰富来源，因为元素周期表几乎提供了金属元素无限可能的组合和比例。然而，基于金属间相的有用应用的实现受到无法在原子水平上控制它们通常复杂和不可预测的几何排列的限制。该项目由美国国家科学基金会材料研究部固态和材料化学项目支持，通过创建化学原理来理解这些材料的晶体结构，并最终指导它们以增强所需性能的方式来促进科学进步。理论用于确定原子的不同方面（例如它们的大小和键合方式）可以合作以稳定结构并促进结构之间的转换的方式。这些结论随后指导新化合物的实验发现，它们的结构和行为为理论提供反馈。此外，该项目继续开发在线资源，使固态和材料化学对公众开放，包括生活教科书“互动固态化学”（作为化学教育数字图书馆的一部分）和“科学通过漫画”。该项目还为本科生研究人员提供教育经验，并每年夏天为一名来自科学领域代表性不足的人口群体的高中生提供实习机会。技术概述：金属间相的结构——源自金属和类金属元素的固态化合物——被认为是由与分子相同的化学性质支配的：电子数、原子尺寸要求和电负性。然而，与分子化学相比，目前还没有广泛适用的预测方案来指导金属间结构化学通过固态合成，这是一个限制因素，以充分利用这些化合物作为应用材料的潜力。该项目由美国国家科学基金会材料研究部固态和材料化学项目资助，利用最近开发的理论工具来可视化金属间化合物的原子包装应变和局部电子配置，以建立一个预测这些化合物结构现象的概念框架：受挫和允许结构转变（FAST）模型。在这里，母体结构的潜在重排被分为那些被阻止或允许的重排，这取决于驱动转变的主要因素（原子包装或电子）是否受到各自的第二因素的支持或阻碍，被允许的转变预测很容易在实验中实现。通过理论和实验的结合，主要研究者的研究小组通过预测和发现新的结构进展（包括无扩散相变）以及通过元素取代对二元系统中浅亚稳结构的选择性稳定来测试和改进该模型。因此，他们通过FAST方法研究了大量的金属间结构，以寻找电子和包装因素（分别违反电子计数规则和强烈的化学压力特征）的响应性标志。那些先前未观察到的结构化学被这个模型认为是允许的相被优先用于实验研究。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。

NON-TECHNICAL SUMMARY: The development of new technologies is closely connected to the discovery and optimization of new solid state materials. Intermetallic phases-compounds formed upon alloying metals together-represent a rich source of such materials, as the periodic table offers virtually infinite possible combinations and ratios of metallic elements. However, the realization of useful applications based on intermetallic phases is limited by the inability to control their often complex and unpredictable geometrical arrangements at the atomic level. This project, supported by the Solid State and Materials Chemistry program in the Division of Materials Research at NSF, is promoting the progress of science through the creation of chemical principles for understanding the crystal structures of these materials and eventually guiding them in ways that enhance a desired property. Theory is used to identify ways that the different aspects of atoms (such as their sizes and ways of bonding) can cooperate to stabilize structures and facilitate transformations between structures. These conclusions then guide the experimental discovery of new compounds, with their structures and behavior providing feedback for theory. In addition, this project continues the development of online resources that make solid state and materials chemistry accessible for the public, including the living textbook "Interactive Solid State Chemistry" (available as part of the Chemical Education Digital Library), and "Science through Comics". The project also provides educational experiences for undergraduate researchers as well as an internship for a high-school student from a demographic group underrepresented in the sciences each summer. TECHNICAL SUMMARY: The structures of intermetallic phases - solid state compounds derived from metallic and metalloid elements - are known to be governed by the same chemical properties as molecules: electron counts, atomic size requirements, and electronegativities. However, in contrast to molecular chemistry, no broadly applicable predictive schemes for guiding intermetallic structural chemistry through solid state synthesis has been realized yet, which is a limiting factor in utilizing the full potential of these compounds as applied materials. This project, funded by the Solid State and Materials Chemistry Program in the Division of Materials Research at NSF, draws on recently developed theoretical tools for visualizing strains in atomic packing and local electron configurations in intermetallics to build a conceptual framework for predicting structural phenomena in these compounds: the Frustrated and Allowed Structural Transitions (FAST) model. Here, potential rearrangements of a parent structure are divided into those that are frustrated or allowed, depending on whether the primary factor driving the transition (either atomic packing or electronics) is supported or impeded by the respective second factor, with allowed transformations predicted to be readily realizable experimentally. Using a combination of theory and experiment, the principle investigator's research group tests and refines this model through the anticipation and discovery of new structural progressions (including diffusionless phase transitions) and the selective stabilization of shallow metastable structures in binary systems through elemental substitution. Therefore they examine a vast range of intermetallic structures through the FAST approach in search of hallmarks of responsiveness in the electronic and packing factors (the violation of electron counting rules and intense chemical pressure features, respectively). Those phases for which previously unobserved structural chemistry is deemed allowed by this model are prioritized for experimental investigation.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.