Materials World Network: Computer-based discovery of novel low-energy structures in intermetallic compounds

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Intermetallic compounds can exist in a variety of crystal structures. Roughly, 2N structures can be made from a binary A1-xBx system with N lattice sites. The structure determines properties such as ductility, hardness, and conductivity. Yet the structure of many intermetallics is unknown experimentally. This proposal offers a theoretical methodology for predicting the stable crystal structures for a given compound from first principles. In particular, we are concerned with the study of different intermetallic compounds at non-ideal stoichiometries and the development of additional techniques to study such systems. We will use the Mixed-Basis Cluster Expansion in conjunction with modern statistical mechanics ground state search techniques and predict the stable structures for intermetallics that have so far eluded experimental determination. This will include: (a) one- and two-dimensional structures of CuPd and CuMn-these are mysterious structures that exhibit one- and two-dimensional periodicities, (b) ordering of vacancies in early transition metal nitrides and carbides. In these systems, the vacancies form peculiar spatial arrangements, (c) determination of the fundamental phase behaviour of Mo-Ta and Nb-W. The study of these material systems also requires development of new methodologies: 1. Development of new kinetic-Monte Carlo codes to study evolution of microstructures. 2. Development of a "genetic algorithm" approach to determining ground states in alloy systems. 3. Development of mixed-basis cluster expansion codes that handle BCC-based alloys The broader impacts of the project include: 1. Systems of study are archetypes in broad classes of materials and the knowledge gained for these systems will aid in our understanding of a broad range of materials. 2. Development of tools for studying alloys that will be freely available to other reseachers. 3. Provide undergraduate students, especially those from demographic groups underrepresented in physics and materials science, with research experience and one-to-one faculty mentoring. G. Hart is currently mentoring three undergraduate students, including two women, one a native American. 4. Train students in UNIX operating system, executing computional codes, scripting & programming, and other general computer skills that will be useful to students. 5. Long term involvement of the students, from project inception to final publication. The proposed collobaration combines the strength of three groups: (1) the NREL group of A. Zunger (a no-cost PI to NSF) where the mixed-basis cluster expansion was originally developed and has been applied to many systems. This group has been a world leader in the theory of alloys for 20 years. (2) The new research group of S. Müller in Erlangen Germany, particularly noted for its interest in low-dimensional aspects of materials. (3) The new research group of G. Hart, a new faculty member at Northern Arizona University (NAU). This group, noted for its experience in studying vacancy ordering compounds, is providing undergraduates with research experience working on significant projects, particularly native Americans, Hispanics, and women.