The first polymorph, κ″-(ET)2Cu[N(CN)2]Cl, in the family of κ-(ET)2Cu[N(CN)2]X (X=Cl, Br, I) radical cation salts

The first polymorph, κ″-(ET)2Cu[N(CN)2]Cl, of the well known Mott insulator κ-(ET)2Cu[N(CN)2]Cl has been prepared and its crystal and electronic structures have been examined. The polymorph has monoclinic symmetry in contrast with the orthorhombic one of the isostructural κ-salts of the family (ET)2Cu[N(CN)2]X (X=Cl, Br, I). The monoclinic phase exhibits a layered structure in which the conducting layers are packed in a κ-type arrangement and the anion sheets consist of polymeric zigzag chains formed by Cu[N(CN)2]Cl units. The main structural differences with the orthorhombic κ-salts are that the anion sheets are disordered and the ET molecules are less planar. The new polymorph shows metallic type resistivity down to 4.2 K.