New organic superconductors β-(BDA-TTP)2X [BDA-TTP=2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene; X- = SbF6-, AsF6-, and PF6-]

The synthesis, electrochemical properties, and molecular structure of a new pi -electron donor, 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene (BDA-TTP), is described. In contrast to the hitherto-known tetrachalcogenafulvalene pi -donors providing organic superconductors, this donor contains only the bis-fused I,3-dithiole-2-ylidene unit as a pi -electron system, yet produces a series of ambient-pressure superconductors beta-(BDA-TTP)(2)X [X = SbF6 (magnetic T-c = 6.9 K, resistive T-c = 7.5 K), AsF6 (magnetic T-c = 5.9 K, resistive T-c = 5.8 K), and PF6 (magnetic T-c = 5.9 K)], which are isostructural. The values of the intermolecular overlap integrals calculated on the donor layers of these superconductors suggest a two-dimensional (2D) electronic structure with loose donor packing. Tight-binding band calculations also indicate that these superconductors have the 2D band dispersion relations and closed Fermi surfaces.