W0221

Effects of Impurity Poisoning During Crystal Growth. Terry A. Land, Ruth Hawley-Fedder, Randy Floyd, Alan Burnham, James J. De Yoreo, Martin DeHaven and Mark Evans, Dept. of Chemistry and Materials Science, Lawrence Livermore National Laboratory, Livermore, CA 94550.

Growth and dissolution of crystal surfaces is central to processes as diverse as pharmaceutical manufacturing, corrosion, single crystal production and mineralization in geochemical and biological environments. Impurities are either unavoidable features of these processes, or are intentionally introduced to modify the products. Those that act as inhibiting agents induce what is referred to as the “dead zone”, a regime of low supersaturation where growth ceases to occur. This talk will focus primarily on KDP (KH₂PO₄) but the effects of impurities are very relevant to other small molecule and macromolecular systems and this will be mentioned.

So why KDP? KDP crystals will serve as the optical components in the National Ignition facility (NIF) which is designed to permit the study of fusion of atomic nuclei. Critical to NIF is the use of large 42 cm plates of single crystal KDP as switching crystals and frequency conversion elements. The challenge is to suppress secondary nucleation and control growth condition to yield large (~700lb), nearly perfect, single crystals. A recently developed technology for producing these crystals involves a rapid-growth method which is extremely sensitive to certain impurities. Specifically, trivalent cations of Fe, Al and Cr are known to adversely affect the stability of elementary molecular steps on the surface of the crystals. It is imperative to eliminate the defects and growth instabilities that are generated by these impurities. Results from multi-scale experiments detailing the fundamental physics of crystal growth on the nanometer scale and its adaptability to large (700 lb+) crystal growth technology will be presented. AFM is used to follow the behavior of elementary steps on the surface in clean solution and in the presence of Fe³⁺ impurities as a function of supersaturation. The effects of impurity poisoning are observed as the surface emerges from the dead zone. We show that traditional models do not predict the behavior of this system and why. These experiments are aimed at developing a fundamental understanding of how impurities interact with steps during crystal growth.