The magnetic susceptibility and EPR of the organic conductors α'-(BEDT-TTF)2X, X=AuBr2, CuCl2and Ag(CN)2

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Abstract
The authors have investigated the magnetic properties of some linear chain anion salts of BEDT-TTF which adopt the alpha '-structure. These are (BEDT-TTF)2X, where X=AuBr2-, CuCl2- and Ag(CN)2-. In the alpha '-morphology the donor stacks are formed from twisted BEDT-TTF dimers, with the BEDT-TTF molecules within the dimeric unit rotated with respect to each other about the stacking axis by about 32 degrees . Contact between the dimer pairs along the stacking axis is poor, and this has a strong influence on the electronic properties. They find that this series of compounds are highly localised, low-dimensional spin systems with S=1/2 per BEDT-TTF dimer. They describe this behaviour as that of a Mott-Hubbard insulator, and consider that it is due here to the narrow band width along the stacking direction. The behaviour of the susceptibility at low temperatures indicates weak antiferromagnetic coupling, with an exchange energy of about 50 K. The Ag(CN)2- salts show a sharp fall in susceptibility at 7 K, and they consider that this is probably associated with a spin-Peierls transition.