Non-equilibrium phases during the early growth of crystals - an ultrafast in-situ SAXS, -WAXS, and -PDF analysis

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Two central questions shall be answered within this project:1) The evolution of the structural properties during nucleation and growth of small crystals in precipitation reactions shall be made visible in-situ already 10 μs after mixing of the reactants by SAXS (morphology of the particles), WAXS (coherence volumina, crystal structure and phase transitions), and Total Scattering (non-periodic arrangements, defects and phase transitions). This approach will opened up a so far unexplored time regime in the field of nucleation and early structure formation for scattering methods. The information content of the three methods is in part overlapping. This assures a reliable interpretation of the experimental data and it provides further a solid foundation for model calculations.2) What is the minimum size a crystal must have in order to adopt his adult structure? Small agglomerates or crystallites may have amorphous or non-periodic arrangements e. g. with a high density of stacking faults which are usually referred to structural defects. We are concerned with the question if these observed non-crystalline orderings represent non-equilibrium phases or if they should be considered as an intrinsic property of small particles. The calculations by D. Zahn (structural evolution of ion clusters from dimers to aggregates with hundreds of ions including structural ripening) and by R. Weihrich (very high quality ab initio calculations of crystal nuclei regarding their structure, dynamics, and stability) are of fundamental importance for this discussion.Based on our experience we intend start out with investigations on sulfides synthesized from aqueous solutions (CdS, PbS, ZnS, Ag2S). However, our novel experimental approach will be also made available to other sample systems within the project and beyond.The construction of a mixing cell is an essential part of the experimental program. An early data point after the beginning of the crystallization shall become accessible (10 μs) and a high resolution for the crystallization time must be assured (15 % for short crystallization times, better 1 % for crystallization times longer 1 ms). This is achieved by a rapid mixer cell with a free jet for the product. The free jet further reduces background scattering significantly thus assuring a high data quality.