Structural and electronic properties of the molecular conductors (EDTTTF)x[Pd(dmit)2]y(x:y=2:3 and 2:2)

Two new charge transfer salts combining the EDTTTF (ethylenedithiotetrathiafulvalene) donor and the Pd(dmit)2 acceptor (dmit2-=4, 5-dimercapto-1, 3-dithiole-2-thione) have been prepared and characterized. The molecular structure of the (EDTTTF)2(Pd(dmit)2)3 salt is quite unusual since the Pd(dmit)2 entities form true trimers through two strong interactions between the Pd atoms. These trimers of acceptors form slabs which alternate with slabs of (EDTTTF)2 pairs. Band structure calculations indicate a charge transfer of one electron per EDTTTF molecule and we expect this 2:3 salt to be a semiconductor. In the (EDTTTF)2[Pd(dmit)2]2 salt, the EDTTTF units form centrosymmetric pairs which pile up along (100) while the [Pd(dmit)2]2 dimers pile up along (101). Stacks of donors and acceptors form segregated slabs which alternate along (010). (EDTTTF)2[Pd(dmit)2]2 is metallic, and our studies suggest the occurrence of a charge transfer of 3/4 electrons per molecule. This salt exhibits a resistivity anomaly, a drop of spin susceptibility and a structural phase transition at 50 K. These data combined with electronic structure calculations show that this transition must be associated with a charge density wave (CDW) instabilty of the EDTTTF stacks. Finally we suggest that the resistivity anomaly of the related alpha -(EDTTTF)[Ni(dmit)2] salt is also due to a CDW instability of the donor slabs.