COMPARISON OF THE ELECTRONIC-STRUCTURES OF ISOSTRUCTURAL (BEDT-TTF)(3)(HSO4)(2) AND [NI(DDDT)(2)](3)(HSO4)(2) MOLECULAR-METALS

The isostructural charge transfer salts (BEDT-TTF)(3)(HSO4)(2) and [Ni (dddt)(2)](3)(HSO4)(2) are metallic at room temperature and exhibit me tal to insulator transitions at 130 K and 25 K, respectively. X-ray di ffuse scattering measurements for (BEDT-TTF)(3)(HSO4)(2) and tight-bin ding band structure calculations for both salts suggest that these tra nsitions are most likely due to slight structural modifications, proba bly donor displacements. which suppress the semi-metallic overlap in t hese 3:2 salts. Although the Fermi surface of (BEDT-TTF)(3)(HSO4)(2) i s made of closed loops. our study suggests that this salt has remarkab le pseudo-1D properties. We show that even when they are isostructural , the BEDT-TTF and Ni(dddt)(2) salts can be electronically quite diffe rent, because the LUMO of the latter donor is not too far in energy fr om the HOMO. Consequently, the LUMO of Ni(dddt)(2) can mix into the pa rtially filled bands of their charge transfer salts, which are based o n the HOMO of the donor. This feature can change noticeably the topolo gy of the band structure and the Fermi surface. We show that this actu ally happens in [Ni(dddt)(2)](3)(HSO4)(2) and that it can be the clue to understand the differences in physical properties of the (BEDT-TTF) (3)(HSO4)(2) and [Ni(dddt)(2)](3)(HSO4)(2) salts.