BEDT-TTF is the short form of bisethylenedithio-tetrathiafulvalene commonly abbreviated with ET. These molecules form planes which are separated by anions. The pattern of the molecules in the planes is not unique but there are several different phases growing, depending on the anion and the growth conditions. Important phases concerning superconductivity are the α- and θ- phase with the molecules ordering in a fishbone structure and the β- and especially κ-phase which order in a checkerboard structure with molecules being dimerized in the κ-phase. This dimerization makes the κ-phases special as they are not quarter- but half-filled systems, driving them into superconductivity at higher temperatures compared to the other phases.
The amount of possible anions separating two sheets of ET-molecules is nearly infinite. There are simple anions such as triiodide (), polymeric ones such as the very famous Cu[N(CN)]Br and anions containing solvents for example Ag(CF)·112DCBE. The electronic properties of the ET-based crystals are determined by its growing phase, its anion and by the external pressure applied. The external pressure needed to drive an ET-salt with insulating ground state to a superconducting one is much less than those needed for Bechgaard salts. For example, κ-(ET)Cu[N(CN)]Cl needs only a pressure of about 300 bar to become superconducting, which can be achieved by placing a crystal in grease frozen below  and then providing sufficient stress to induce the superconducting transition. The crystals are very sensitive, which can be observed impressively in α-(ET)I lying several hours in the sun (or more controlled in an oven at ). After this treatment one gets α-(ET)I which is superconducting.
In contrast to the Fabre or Bechgaard salts universal phase diagrams for all the ET-based salts have only been proposed yet. Such a phase diagram would depend not only on temperature and pressure (i.e. bandwidth), but also on electronic correlations. In addition to the superconducting ground state these materials show charge-order, antiferromagnetism or remain metallic down to lowest temperatures. One compound is even predicted to be a spin liquid.
The highest transition temperatures at ambient pressure and with external pressure are both found in κ-phases with very similar anions. κ-(ET)Cu[N(CN)]Br becomes superconducting at T = 11.8 K at ambient pressure, and a pressure of 300 bar drives deuterated κ-(ET)Cu[N(CN)]Cl from an antiferromagnetic to a superconducting ground state with a transition temperature of T = 13.1 K. The following table shows only a few exemplary superconductors of this class. For more superconductors, see Lebed (2008) in the references.
Even more superconductors can be found by changing the ET-molecules slightly either by replacing the sulfur atoms by selenium (BEDT-TSF, BETS) or by oxygen (BEDO-TTF, BEDO).
Some two-dimensional organic superconductors of the κ-(ET)X and λ(BETS)X families are candidates for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase when superconductivity is suppressed by an external magnetic field.