Martin, Lee; (1999) Structures and Properties of Magnetic Molecular Charge-Transfer Salts. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The superconducting state may be destroyed by magnetic fields which unpair the electrons of a Cooper pair of superconducting charge carriers. This occurs whether the field is applied externally or arises internally from magnetic moments within the material under investigation. However, there is evidence to suggest that S = 1 Cooper pairs can exist in strong magnetic fields. Both magnetic and conducting properties within a lattice may be combined through the synthesis of charge transfer salts which contain organic donor molecules and magnetic counter-ions. With this molecular approach, structural complexities can be invoked which would not be achievable via conventional solid state synthesis. With the above criteria in mind, this thesis reports a series of molecular charge-transfer salts of the organic donor molecule BEDT-TTF, bis(ethylenedithio)tetrathiafulvalene, with anionic tris(oxalato)-mtetallate(III) and (IV) complexes. The structures of these compounds consist of segregated layers containing BEDT-TTF interleaved with layers containing the oxalato complex. Narrow conduction bands are formed by frontier orbital overlap in the BEDT-TTF layers, while localised spins may be introduced in the anionic layers. Several structures in the series (BEDT-TTF)x[AM(C2O4)3].C 6H5CN, where A = H3O+, NH 4+ or K+ and M = Cr3+, Fe3+ Co3+, (x = 4) or Ge4+ (x = 2) have been characterised by X-ray diffraction. The tris(oxalato)-metallate anion can exist in two enantiomeric forms. This thesis shows how the packing of these two enantiomers determines the arrangement of the BEDT-TTF molecules in the organic layer, and thus the conducting properties. It details the synthesis, structure and physical properties of this series of charge-transfer salts. Synthesis is by conventional electrocrystallisation techniques and physical characterisation includes measurement of transport, magnetic susceptibility, polarised infrared reflectivity, circular dichroism, Raman, epr, and ultraviolet spectroscopies. Electronic band structure calculations have also been performed on selected materials.

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