Johnson, D

Interdiffusion and Nucleation at the Interface of Reacting Solids
Dave Johnson, Chemistry
By using elementally modulated reactants in Johnsonwhich composition can be controlled on an angstrom length scale, we have been able to use slow solid state diffusion rates as a synthetic advantage. Controlled crystallization of elementally modulated reactants results in the rational synthesis of targeted compounds. There are several projects for REU students. These include preparing new thermoelectric materials and measuring their properties, preparing new extended cluster compounds, and preparing crystalline superlattices containing interwoven layers of different compounds. All of these projects involve preparing thermodynamically unstable compounds which cannot be prepared using conventional synthesis techniques. Although the compounds are thermodynamically unstable, they are kinetically stable. The structure of the superlattice compounds is controlled by the structure of the initial modulated elemental reactant. The rational synthesis of superlattice compounds permits the tailoring of physical properties as a function of compositional layer thicknesses and native properties of the parent compounds. The structure of the resulting products is characterized using transmission electron microscopy and x-ray diffraction. The student will measure the properties of the compounds he/she prepares, using electrical conductivity, Seebeck coefficients, Hall measurements and thermal conductivity measurements to correlate properties with structure and composition. We have had an excellent track record (in publications alone) in involving both undergraduate physics and chemistry majors in projects like these and we anticipate that this will continue in the future.