W. K. Kwok

New κ-phase materials, κ-(ET)2Cu[N(CN)2]X.X=Cl, Br and I. The synthesis, structure and superconductivity above 11 K in the Cl (Tc = 12.8 K, 0.3 kbar) and Br(Tc = 11.6 K) salts

Abstract The syntheses, structures, selected physical properties, and band electronic structures of three copper (I) dicyanamide halide salts of bis(ethylenedithio)tetrathiafulvalene [κ-(ET) 2 Cu[N(CN) 2 ]X, where X = Cl, Br, and I] are discussed. X-ray crystallographic studies demonstrate that the three derivatives are isostructural. The bromide salt is an ambient pressure superconductor with an inductive onset at 11.6 K and a resistive onset at 12.5 K. κ-(ET) 2 Cu[N(CN) 2 ]Cl exhibits the highest reported superconducting transition temperature (T c = 12.8 K. 0.3 kbar) for an organic superconductor, once a semiconductor-semiconductor transition (42 K) is suppressed. The application of GE varnish or Apiezon N grease to crystals of κ-(ET) 2 Cu[N(CN) 2 ]Cl provides sufficient stress to induce superconductivity at “ambient pressure”. Crystals of the iodide remain metallic to ∼150 K, where they become weakly semiconductive. No sign of superconductivity was detected at pressures (hydrostatic and shearing) up to 5.2 kbar and at temperatures as low as 1.1 K. The band electronic structures of the three salts are essentially identical. The differences in superconducting properties are explained in terms of differences in lattice softness, which are strongly influenced by short C-H…donor and C-H…anion contacts.

The synthesis and characterization of radical cation salts of bis(ethylenedithio)tetraselenafulvalene

Abstract Four 2:1 radical cation salts (SbF 6 , CF 3 SO 3 , GaCl 4 , Cu[N(CN) 2 ]Br) of bis(ethylenedithio)-tetraselenafulvalene (BETS) have been prepared and scrutinized by X-ray crystallography, DC resistivity measurements (300-10K), rf penetration depth experiments (0.5K), pressure studies (0.5–5kbar, 4K), and tight-binding band calculations. The salts are κ-phase, and three are metallic to low temperatures. κ-(BETS) 2 Cu[N(CN) 2 ]Br is isostructural with κ-(ET) 2 Cu[N(CN) 2 ]Br.

Thermal conversion of α-(BEDT-TTF)2IBr2 to superconducting β-(BEDT-TTF)2IBr2

Abstract Crystals of semiconducting α-(BEDT-TTF) 2 IBr 2 , derived from the sulfur-based organic donor bis(ethylenedithio)tetrahiafulvalene (BEDT-TTF), transform at ambient pressure to the superconducting phase β-(BEDT-TTF) 2 IBr 2 on heating to temperatures near 416 K. This solid-state transformation occurs well below the melting temperature and can be followed in detail by ESR measurements. Inductive studies of the superconducting β-phase product (T c ∼ 2.7 K) indicate an essentially total conversion of the α-phase crystals.

Structure-Property Relationships for β- and κ-Phase BEDT-TTF Salts and their use in the Synthesis of κ-(BEDT-TTF)2Cu[N(CN)2]Br: A Salt Having the Highest-TC (Inductive Onset = 11.6 K, Resistive Onset = 12.5 K) Yet Observed in an Organic Superconductor

The electron-donor molecule BEDT-TTF (or ET, 1) and its salts have yielded the highest number of ambient pressure organic superconductors and also the highest 7 c ’s reported to date compared to any other electron-donor molecule.