Anion chain structure controlled behavior of phase transition in quasi-two-dimensional organic metal (EDT-TTF) ₄[Hg ₃i ₈] _1-x

The correlation between the structure of anionic chains in crystals of quasi-two-dimensional organic metal (EDTTTF) ₄[Hg ₃I ₈] _1-x and the type of phase transition (metal-superconductor or metal-insulator) was found. Crystal structures of four single crystals of ethylenedithiotetrathiafulvalene (EDT-TTF) octaiodomercurate, which differed in the resistivity behavior, were determined at 90 K. The crystals 1-3 underwent a superconducting transition: Crystal 1 became a superconductor at ambient pressure, and crystals 2 and 3 became superconductors at 0.3 kbar. Crystal 4 underwent a metal-insulator transition. It was shown that 1-3 have a nonstoichiometric composition, (EDT-TTF) ₄[Hg ₃I ₈] _1-x, while 4 has a stoichiometric composition (EDT-TTF) ₄Hg ₃i ₈. In all of the crystals, the structures are formed by conducting β-like layers of EDT-TTF and insulating layers built of {[Hg ₃I ₈] ^2-} _∞ chains, in which the [Hg ₂I ₆] ² anions alternate with the HgI ₂ molecules. In 4, the chains are fully ordered. In 1-3, the [Hg ₃I ₈] ^2- units are disordered in two positions related by the approximate inversion center, the total occupancy of both positions being less than 1.0. It implies that in superconducting crystals iodomercurate chains involve defect sections with the inverse alternation of the Hgl2 molecules and the [Hg ₂I ₆] ² anions. In the anion chains of 1, the defects occur more often than in 2 and 3.