STRUCTURAL AND ELECTRONIC-PROPERTIES OF THE ONE-DIMENSIONAL ORGANIC METAL BIS(THIODIMETHYLENE)-TETRATHIAFULVALENE TETRACYANOQUINODIMETHANE

The synthesis and structural characterization at room temperature and 130 K of the organic charge-transfer salt bis(thiodimethylene)-tetrathiafulvalene tetracyanoquinodimethane (BTDMTTF-TNCQ) is reported. X-ray diffuse scattering as well as ir and Raman measurements show that the charge transfer in this salt is similar to 0.5. BTDMTTF-TCNQ remains metallic down to very low temperatures but exhibits an anomaly at 175 K in the conductivity and thermopower which could be related to some unusual structural modification of the unit cell. Band-structure calculations show that this salt is a narrow-band one-dimensional metal. The high value of the spin susceptibility, the low value of the conductivity, and the observation of a 4k(F) charge-density-wave (CDW) instability around room temperature prove that it is a highly correlated metal. In contrast with other salts of the TTF-TCNQ family, BTDMTTF-TCNQ does not exhibit critical 2k(F) structural fluctuations. It is proposed that the absence of the 2k(F) CDW transition is due to the stronger interlocking of the donor and acceptor molecules due to the presence of the sulfur atoms in the central position of the outer five-membered rings of the BTDMTTF donor.