Assembly of Molecular Units for the Crystallization of Electronic and Magnetic Materials
Dr. John Schlueter (Materials Science Division, Argonne, (USA))

Competition for intermolecular hydrogen or halogen bonds in molecule-based materials frequently leads to the stabilization of polymorphic structures with similar energies. In many cases, these directional interactions also enable stimuli-responsive behavior, including pressure-induced phase transitions. This talk provides an overview of the assembly of molecular components in conductive cation radical salts and magnetic coordination polymers, including the selective crystallization of various polymorphs. Among magnetic coordination polymers, the selective crystallization of the [Cu(HF2) (pyrazine)2]SbF6 and [Cu2F(HF)(HF2) (pyrazine)4](SbF6)2 polymorphs can be achieved through control of pressure and temperature during the crystallization process. Through this manner, orbital order and magnetic dimensionality (2D vs. 1D) can be controlled. For cation radical salts, five polymorphic phases in the (BEDT-TTF)2Ag(CF3)4(1,1,2-trichloroethane) system have been crystallized, four of which have superconducting ground states. In this case, phase selectivity is enabled by the current density during the electrocrystallization process. Among the superconducting salts, the ones with intercalated charge ordered layers exhibit a five-fold enhancement of Tc. The application of external pressure provides a means to control electronic and magnetic coupling. For example, pressure-induced sequential reorientation of the Jahn-Teller axis in CuF2(H2O)2(pyrazine) results in a change in magnetic dimensionality with higher pressure leading to a chemical transformation. In the related CuF2(H2O)2(3-chloropyridine) system, pressure also enables a reorientation of the magnetic structure.