Electronic Structure of the?-(BEDT-TTF)₂MHg(XCN)₄ (M?=?Tl, K, NH₄; X?=?S, Se) and Related Phases. Synthesis and Crystal Structure of the New Stable Organic Metal ?-(BEDT-TTF)₂TlHg(Se_1-xS_xCN)₄ (x?=?0.125)

The synthesis, crystal structure, conductivity and electronic band structure of the new molecular metal alpha-(BEDT-TTF)(2)TlHg(Se1-xSxCN)(4) (x = 0.125) are reported. alpha-(BEDT-TTF)(2)TlHg(Se1-xSxCN)(4) (x = 0.125) is isostructural with all other members of the family of alpha-(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 alpha-(BEDT-TTF)(2)MHg(XCN)(4) salts as well as alpha-(BEDT-TTF)(2)I-3, alpha-(BEDT-TTF)(2)IBr2 and alpha-(BEDT-TTF)(4)PtBr6 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 alpha-(BEDT-TTF)(2)MHg(XCN)(4) salts (i.e., 1D electrons but 2D holes),the metal to insulator transition in alpha-(BEDT-TTF)(2)I-3 and the semiconducting behavior of alpha-(BEDT-TTF)(2)IBr2 and alpha-(BEDT-TTF)(4)PtBr6 are also discussed.