Résumé
The isostructural charge transfer salts (BEDT-TTF)(3)(HSO4)(2) and [Ni(dddt)(2)](3)(HSO4)(2) are metallic at room temperature and exhibit metal to insulator transitions at 130 K and 25 K, respectively. X-ray diffuse scattering measurements for (BEDT-TTF)(3)(HSO4)(2) and tight-binding band structure calculations for both salts suggest that these transitions are most likely due to slight structural modifications, probably donor displacements. which suppress the semi-metallic overlap in these 3:2 salts. Although the Fermi surface of (BEDT-TTF)(3)(HSO4)(2) is made of closed loops. our study suggests that this salt has remarkable pseudo-1D properties. We show that even when they are isostructural, the BEDT-TTF and Ni(dddt)(2) salts can be electronically quite different, because the LUMO of the latter donor is not too far in energy from the HOMO. Consequently, the LUMO of Ni(dddt)(2) can mix into the partially filled bands of their charge transfer salts, which are based on the HOMO of the donor. This feature can change noticeably the topology of the band structure and the Fermi surface. We show that this actually 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.