Résumé
The BEDT-TTF4[M(CN)4] (M = Ni, Pt) and BEDT-TTF4[M(C2O4)2] (M = Pt, Cu) salts are metallic at room temperature but exhibit metal-to-semiconductor transitions at lower temperatures. Their electronic structures have been studied by performing tight-binding band structure calculations on their cationic sublattices. All of these salts possess electron and hole Fermi surfaces, in agreement with their metallic character at room temperature. Although the calculated Fermi surfaces for the two series of salts are not very different, the analysis of their crystal structures suggests that the BEDT-TTF4[M(C2O4)4] (M = Pt, Cu) salts should have a more anisotropic character than the BEDT-TTF4[M(CN)4] (M = Ni, Pt) ones. The analogy between the crystal and electronic structures of the BEDT-TTF4[M(C2O4)2] (M = Pt, Cu) and the BEDT-TTF2ReO4 salts, and the fact that the shape of the Fermi surface of BEDT-TTF4[Pt(CN)4] does not change appreciably when the temperature is lowered, suggest that the metal-to-semiconductor transition is due to a Peierls type mechanism for BEDT-TTF4[M(C2O4)2] (M = Pt, Cu ) but not for BEDT-TTF4[M(CN)4] (M = Ni, Pt). The occurrence of a Peierls transition in the BEDT-TTF4[M(C2O4)2] (M = Pt, Cu) Salts is explained in terms of hidden Fermi surface nesting.