BAND STRUCTURES AND PHYSICAL-PROPERTIES OF NEW ORGANIC SALTS BASED ONUNSYMMETRICAL DONORS AND SQUARE-PLANAR DIANIONS

Several organic salts based on the organic radical cations, MDTTTF (me thylenedithio-tetrathiafulvalene), EDTTTF (ethylenedithio-tetrathiaful valene) and EDOEDTTTF (ethylenedioxo-ethylenedithio-tetrathiafulvalene ), have been prepared with the square planar dianions, M(CN)4 (M=Pt, P d). We have determined their crystal structures and their physical pro perties (conductivity and ESR). The inclusion of solvent molecules in the crystal favors one-dimensional polymers built on the M(CN)4 anions and the solvent. This situation, which is found in the MDTTTF salts, is reminiscent of the crystal packing in the superconducting salt, (BE DTTTF)4Pt(CN)4H2O. It is important to note that the (MDTTTF)4Pt(CN)4 s alts are the first kappa-phase observed in such materials. The salts b ased on the EDTTTF molecule belong to the beta-phase. The transport an d magnetic properties of these compounds are contrasted. (MDTTTF)4Pt(C N)4(CH2Cl2)2 exhibits a quasi metallic behavior down to 170K where a b road metal-semiconductor regime change is detected. The temperature de pendence of the ESR linewidth of (EDTTTF)4Pt(CN)4(H2O)2 does not ehibi t any phase transition down to 4K, while a metal to insulator transiti on occurs in (EDTTTF)4Pd(CN)4(H2O)2 at 100K. The Pt and Pd salts of ED OEDTTTF are very similar and the poor quality of the crystals prevents any analysis for T<20K. Finally, the transfer integrals and band stru ctures have been calculated in order to analyse these different behavi ors.