Presenter: Amanda A. Josey
Advisor(s): James D. Martin
Author(s): Amanda A. Josey, Jacob C. W. Folmer, Stephen J. Goettler and James D. Martin
Graduate Program: Chemistry

Title: Probing the Mechanism of Nucleation and Crystal Growth in Inorganic Networks

Abstract: Understanding mechanism(s) of crystallization is critical for application to atmospheric science and advanced technologies. Ice crystal formation in cirrus clouds is critical for understanding the earth's radiative balance, and single crystalline materials are the foundation of many advanced optical and electronic technologies. Classic nucleation theory, developed to describe crystallization from dilute systems, does not effectively describe crystallization of condensed matter, such as the examples above. Using zinc chloride, a material that forms a crystalline network of intermediate strength between water and silicates, we have been able to directly measure mechanistic details of crystal nucleation and growth. These data provide the basis for a new model of condensed phase crystal nucleation and growth based on the structural order of the materialsí liquid and crystalline states.

Specifically, the crystallization of three representative materials is discussed: zinc chloride, ZnCl2, a low melting analog of SiO2; CZX-1, a copper-zinc chloride halozeotype, analogous to the aluminosilicate sodalite; and CuMCl4 (M = Al or Ga), a halide analog to aluminophosphates. These corner-shared tetrahedral networks all exhibit melting points below 325 oC, and several can be quenched into a glassy state. Synchrotron and Neutron diffraction studies demonstrate that significant features of crystalline structures are retained into the liquid state. Time-resolved X-ray diffraction, coupled with differential scanning calorimetry, is used to probe crystallization kinetics. Crystallization reactions run above the glass transition temperature, Tg, but well below the melting point, Tm, result in polycrystalline products. However, isothermal crystallization near Tm, where nucleation is slow with respect to growth, yields single crystal growth. These experiments further allow measurement of crystal growth rates independently from nucleation. These mechanistic data demonstrate that the rate of crystallization is strongly dependent upon the structure of the liquid (or glass) from which the crystals are grown.