Structural Plasticity in Intermetallics: Shaping the Crystal Structures of Metals and Alloys with Chemical Pressure

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TECHNICAL SUMMARYIntermetallic phases combine a diverse structural chemistry with a range of valuable physical and chemical properties, including superconductivity, thermoelectric, hydrogen storage, catalysis, and a variety of magnetic phenomena. A limiting factor in the development of materials from these compounds is the difficulty encountered when attempting to synthetically guide or control their crystal structures. The focus of this project supported by the Solid State and Materials Chemistry (SSMC) program is the development of a new conceptual framework for understanding and harnessing the chemical driving forces underlying intermetallic crystal structures: Structural Plasticity. The structures of many complex intermetallic phases can be viewed as being built from fragments of simpler structure types, which are separated by interfaces. Such atomic arrangements recall the dislocations that are induced by mechanical stress in a malleable metal and mediate a metal's response to such stress. The structural plasticity model hypothesizes that the connection between intermetallics and malleable metals goes beyond geometry: interfaces inserted into a simple structure to create a more complex one alleviate internal stresses that would be present if that original structure were to be adopted in an unmodified form. This project uses a combination of theoretical calculations and experimental work to explore this hypothesis. This includes the development and application of the Density Functional Theory-Chemical Pressure (DFT-CP) analysis, which extracts information about the local pressures, or chemical pressures, acting on individual atoms within a crystal structure from DFT results. The results of chemical pressure analysis and empirical reasoning are used in the synthesis and structure determination of new intermetallic structures that emerge from chemical pressure-induced Structural Plasticity. Three mechanisms by which structures cope with chemical pressure will be explored through this joint theoretical and experimental approach: (1) the insertion of defect planes into a simple structure type, (2) the hosting of one structure type by another, and (3) the formation of quasicrystalline order. NON-TECHNICAL SUMMARYMetals and alloys exhibit a host of useful properties that are expected to play key roles in energy applications, such as storage of hydrogen as a fuel source, the catalysis of chemical reactions at fuel cell electrodes, and the extraction of electrical energy from temperature gradients. Optimizing these properties for applications is severely complicated by the difficulty of controlling the packing geometries of atoms in these materials. The central goal of this research project supported by the Solid State and Materials Chemistry (SSMC) program is to develop an understanding how the chemical bonding determines these atomic arrangements, and strategies based on these insights for tailoring atomic packing to enhance the desired materials properties. This objective will be attained through combining theoretical and computational modeling with the synthesis and characterization of new materials. The project also includes the creation and expansion of online resources for facilitating the incorporation of materials chemistry into high school and undergraduate courses. Included among these resources are the Solid State Chemistry Resource Library and the online textbook Interactive Solid State Chemistry. Together, these will help systematize the wealth of on-line content available as teaching aides for solid state and materials chemistry, and will fill gaps in the coverage of these subjects left by the existing content. Both are to be incorporated in the Chemical Education Digital Library, a pathway in the National Science Digital Library.