Graham, Anthony William; (1996) Synthesis and properties of charge transfer salts of BEDT-TTF. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This research project has investigated the interplay of superconductivity and magnetism in organic systems, with the ultimate aim of observing their co-existence. In order to pursue this study, a series of charge transfer salts of BEDT-TTF (BEDT- TTF = bis ethylenedithio tetrathiafulvalene) have been successfully prepared with M(C2O4)33- anions (M = Fe, Cr and Co). The anion has been varied to systematically monitor the effect of localised moments on the conducting properties. Crystals with the general formula (BEDT -TTF)4AFe(C2O4)3.C6H5CN (A = K, NH4 or H2O) have been electrocrystallised and identified by X - ray crystallography. In the salts where A = K or NH4 a novel packing arrangement of the BEDT-TTF moiety is observed which involves the formation of monopositive BEDT-TTF units dimerising. These dimers are surrounded by six neutral BEDT-TTF molecules. Both of these materials are semiconductors with an activation energy of 0.14eV. Susceptibility measurements indicate that the dimers are spin paired and therefore the magnetic properties are dominated by the high spin Fe3+ complex. These results are reported in Chapter 3. The A = H2O salt is reported in Chapter 4. It is the first molecular superconductor containing localised magnetic moments, which has a transition temperature of 8.5K to the superconducting state. The BEDT-TTF molecules pack in the [beta]" fashion, which facilitates considerable overlap of the Sulfur orbitals. Susceptibility measurements at low field verify the superconducting transition, but as the field is increased, the paramagnetic signal from the Fe3+ becomes more dominant. At fields greater than 1 Tesla, the superconductivity is destroyed and the material obeys the Curie - Weiss law to 2K. A contribution from the conduction electrons is also observed. Band structure calculations suggest two dimensional metallic behaviour. Chapter 5 collates other compounds which have been prepared from solutions of BEDT-TTF and M(C2O4)33- (M = Fe, Cr, Co) in Benzonitrile. This has resulted in several new phases being isolated but due to multiple twinning, their structures have not yet been determined. Crystals grown from the solution where M = Cr are semimetallic to ∼ 150K at which temperature it undergoes a transition to the insulating state. The solvent has also been modified to include 5% Methanol. This has resulted in a superconducting phase whose lattice parameters have been determined. Efforts are currently being made to determine the full crystal structure. SQUID measurements indicate a Tc of 7.5K.

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