D. Jung

Strain index, lattice softness and superconductivity of organic donor-molecule salts: Crystal and electronic structures of three isostructural salts k-(BEDT- TTF)2Cu[N(CN)2]X (X=Cl, Br, I)

Abstract The recently discovered k -phase salts k -(BEDT-TTF) 2 ]Cu[N(CN) 2 ]X (X=Cl,Br, I) are isostructural but differ in their superconducting properties: the Br-salt is an ambient-pressure superconductor with T c =11.6K, the Cl-salt becomes superconducting ( T c =12.8K) under a slight applied pressure of 0.3 kbar, and the I-salt does not exhibit superconductivity under an applied-pressure of up to 5 kbar. We examine the structural and electronic properties of these three salts on the basis of their crystal structures determined at 127 K by single crystal X-ray diffraction measurements. The band electronic structure of the three salts are virtually the same and do not account for the differences in the conduction properties. The differences in the three salts with regard to superconductivity are examined from the viewpoint of lattice softness by calculating strain indices for short intermolecular contacts. The random potential arising from ethylene group conformational disorder prevents superconductivity in the iodide salt.

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.

Charge-Transfer Salts Derived from the New Electron-Donor Molecule BEDO-TTF: ESR, Superconductivity and Electrical Properties, and Crystal and Band Electronic Structure

Abstract Charge transfer salts of bis(ethylenedioxy)tetrathiafulvalene, BEDO-TTF, namely (BEDO-TTF)2AuBr2, and charge transfer salts (BEDO-TTF)mXn where X− is I3, AuI2 −, Au(CN)2−, ClO4 −, BrO4 −, BF4 −, PF6 −, AsF6 −, NO3 −, C(CN)3 −, and HgBr3− have been synthesized. The AuBr2− salt is the first BEDO-TTF salt to be structurally characterized. Crystallographic investigations of several of the other (BEDO-TTF)mXn salts have shown that (with the exception of the I3− salt) these new synmetals are structurally very similar to (BEDO- TTF)2AuBr2. Therefore, a 2:1 stoichometry is expected for BEDO-TTF salts. The observation of similar ESR linewidths for all the BEDO-TTF salts reinforces this conclusion. While (BEDO-TTF)2PF6 is semiconducting and the AuBr2−, Au(CN)2−, and ClO4− salts are metallic only near room temperature and semiconducting at low temperatures, (BEDO-TTF)2AuI2 shows metallic conductivity to low temperatures. RF penetration depth measurements on crystals derived from BEDO-TTF/KSCN/CuSCN show the...

Importance of C-H⋯Donor and C-H⋯Anion Contact Interactions for the Crystal Packing, the Lattice Softness and the Superconducting Transition Temperatures of Organic Conducting Salts

The organic donor molecule BEDT-TTF and its analogs 2--4 have yielded a number of ambient-pressure superconducting salts. What structural and electronic factors govern the magnitudes of their superconducting transition temperature {Tc} has been a topic of intense studies. Examination of the band electronic structures of closely related superconducting salts shows, that the magnitudes of their {Tc}s are primarily determined by the softness of their crystal lattices. The crystal packing and the lattice softness of organic donor salts are strongly influenced by the donor{hor ellipsis}donor and donor{hor ellipsis}anion contact interactions involving the donor-molecule C-H bonds. In the present work, we briefly review the electronic structures of some representative organic salt superconductors and discuss the softness of their crytsal lattices on the basis of the interaction energies calculated for the C-H{hor ellipsis}donor and C-H{hor ellipsis}anion contact interactions. 34 refs., 14 figs., 8 tabs.

Structural and Electronic Properties of K-Phase Organic Donor Salts: κ-(DMET)2AuBr2 and κ-(BEDT-TTF)4Hg3Cl8

Abstract Electronic structures of κ-(BEDT-TTF)4Hg3Cl8 and κ-(DMET)2AuBr2 were examined by performing tight-binding band calculations, and the crystal and electronic properties of the two salts were compared with those of other super conducting and non super conducting κ-phase salts. A possible reason for why κ-(BEDT-TTF)4Hg3Cl8 requires an applied pressure to achieve superconductivity was discussed.

Recent Progress in the Development of Structure-Property Correlations for κ-Phase Organic Superconductors

Abstract An examination of the molecular packing arrangements of the electron-donor molecules in κ-phase organic conductors and superconductors [κ-(BEDT-TTF)2X (X ˭ I3−, Cu(NCS)2−), κ-(MDT-TTF)2AuI2, and κ-(BMDT-TTF)2Au(CN)2] reveal that the main predictors of superconductivity appear to be (i) bond-over-ring intradimer molecular packing and (ii) relatively short (∼ 3.35A) intradimer separations. These structural features in newly discovered κ-phase organic conductors may be requirements for the occurrence of ambient pressure superconductivity.

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.