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

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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.