Crystal structures and properties of two modifications of the molecular charge-transfer salt (BEDT-TTF)2Ag(CN)2[BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene]

Abstract

Crystal structures are reported of two modifications of the molecular charge-transfer salt (BEDT-TTF)₂Ag(CN)₂[BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene]. Both structures contain alternating layers of BEDT-TTF and [Ag(CN)₂]^–. Structure I is orthorhombic (P2₁2₁2), a= 9.519(2)Å, b= 4.952(1)Å, c= 33.927(7)Å, Z= 2, R= 0.054, and structure II is monoclinic (P2₁/n), a= 10.996(1)Å, b= 4.281(1)Å, c= 34.093(5)Å, β= 93.13(2)°, Z= 2, R= 0.084. The modes of packing of the BEDT-TTF within layers are similar; the BEDT-TTF are inclined to the stacking axis and there are no significant S⋯S contacts within stacks. In adjacent stacks in I the molecules are inclined in the opposite sense, with dihedral angles of 77.3° and 81.5° for the two crystallographically independent layers, giving a number of short S⋯S distances. In II the dihedral angle between adjacent stacks is smaller (55.3°). Phase I is metallic from 300 to 1 K, while for II electron paramagnetic resonance (EPR) gives evidence for a metal–insulator transition near 100 K. The EPR linewidths are 25–30G (I) and 17–27 G (II). Analysis of the reflectivity data of I gives plasma frequencies of 9200 cm^–1(‖a) and 7000 cm^–1(‖b). The band structures of both materials are broadly similar, with a Fermi surface consisting of a nearly symmetric hole pocket centred at the zone centre of ca. 50% of the Brillouin zone. In I the two layers are found to have slightly different Fermi surfaces.