Mechanisms of Nucleation and Crystal Growth of Metal Halide Networks

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This project will use isothermal time and temperature resolved two-dimensional X-ray diffraction (2-D t/TRXRD) techniques to independently measure the rates of intrinsic, and extrinsic nucleation, and crystal growth, as well as to perform simultaneous pair distribution function (PDF) analysis of the crystalline and amorphous fractions. This will be accomplished using high energy X-rays, area detectors with up to 30 Hz time resolution, development of melt-quenching techniques with rates greater than 500/min, and data processing methods to accommodate collection and analysis of massive amounts of data. The studies will be conducted with a series of halide materials that exhibit melting points between room temperature and 350 C, which are temperatures that are readily amenable to the proposed experimental methods. Specifically, the crystallization of clathrated networks (our halozeotypes), probing the crystallization of parent binary condensed phases, and considering step-wise structural ordering through the formation of molecular plastic crystalline phases will be studied. This project will provide invaluable training of students in fundamental scientific discoveries with implications for the atmospheric and geologic sciences as well as for the design and function of advanced materials. %%%Fast time-resolved diffraction techniques are being developed, and facilities at Argonne and Brookhaven National Laboratories are being used to independently measure the rates of crystal nucleation and growth. This work is transforming classical understandings of the mechanisms by which crystals are formed from the molten state. Contrary to the assumptions of classical theory, this project will demonstrate that it is the organization of atomic and molecular level structure in the molten state, rather than surface energy that controls the rate of nucleation and crystal growth. New instrumentation and data processing tools will be developed, and series mechanistic studies performed to develop and refine a new structural-order-driven-crystallization (SODC) theory. Answers from these fundamental mechanistic studies of crystal growth will help decipher mysteries such as ice-crystal growth in clouds which dramatically impact global climates, and will be applicable to the design and function of advanced technologies such as phase change materials utilized for data storage for which understanding of the rapid amorphous-to-crystal transitions will lead to superior performance.