Band structures and physical properties of new organic salts based on unsymmetrical donors and square planar dianions

Abstract

Several organic salts based on the organic radical cations, MDTTTF (methylenedithion-tetrathifulvalene), EDTTTF (ethylenedithio-tetrathifulvalene) and EDOEDTTTF (ethylenedioxo-ethylenedithio-tetrathiafulvalene), have been prepared with the square planar dianions, M(CN)4 (M=Pt, Pd). We have determined their crystal structures and their crystal structures and their physical properties (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, (BEDTTTF)4Pt(CN)4H2O. It is important to note that the (MDTTTF)4Pt(CN)4 salts are the first κ-phase observed in such materials. The salts based on the EDTTTF molecule belong to the β-phase. The transport and magnetic properties of these compounds are contrasted. (MDTTTF)4Pt(CN)4(CH2Cl2)2 exhibits a quasi metallic behavior down to 170K where a broad metal-semiconductor regime change is detected. The temperature dependence of the ESR line4ewidth of (EDTTTF)4Pt(CN)4(H2O)2 does not ehibit any phase transition down to 4K, while a metal to insulator transition occurs in (EDTTTF)4Pd(CN)4-(H2O)2 at 100K. The Pt and Pd salts of EDOEDTTTF are very similar and the poor quality of the crystals prevents any analysis for T<20K. Finally, the transfer integrals and band structures have been calculated in order to analyse these different behaviors.