Electronic Structure of the $\mathsf{\alpha}$-(BEDT-TTF)$\mathsf{_2}$MHg(XCN)$\mathsf{_4}$ (M = Tl, K, NH$\mathsf{_4}$; X = S, Se) and Related Phases. Synthesis and Crystal Structure of the New Stable Organic Metal $\alpha$-(BEDT-TTF)$\mathsf{_2}$TlHg(Se$\mathsf{_{\rm 1-x}}$S$\mathsf{_{\rm x}}$CN)$\mathsf{_4}$ (x = 0.125)

The synthesis, crystal structure, conductivity and electronic band structure of the new molecular metal α-(BEDT-TTF)2TlHg(se 1 -xSxCN) 4 (x = 0.125) are reported. α-(BEDT-TTF) 2 TlHg(Se 1-x S x CN) 4 (x = 0.125) is isostructural with all other members of the family of α-(BEDT-TTF) 2 MHg(XCN) 4 salts and is a stable metal down to very low temperatures. A general study of the electronic structure of all structurally characterized α-(BEDT-TTF) 2 M Hg(XCN) 4 salts as well as α-(BEDT-TTF) 2 I 3 , α-(BEDT-TTF) 2 IBr 2 and α-(BEDT-TTF) 4 PtBr 6 is also reported. Different aspects related to the electronic structure of these salts like the stability of the metallic state and the nature of the Fermi surface are discussed and correlated with the details of their crystal structures. The origin of the different dimensionality of the carriers for the α-(BEDT-TTF) 2 MHg(XCN) 4 salts (i.e., 1D electrons but 2D holes), the metal to insulator transition in α-(BEDT-TTF) 2 I 3 and the semiconducting behavior of α-(BEDT-TTF) 2 IBr 2 and α-(BEDT-TTF) 4 PtBr 6 are also discussed.