Lawrence K. Montgomery

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.

ESR studies of the 10.4 K ambient-pressure organic superconductor κ-(BEDT-TTF)2Cu(NCS)2

Abstract The novel ESR behavior of single crystals of the ambient-pressure organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 is studied from 440 to 4.2 K. The ESR peak-to-peak linewidth increases with decreasing temperature and there is a well-defined inflection temperature which suggests a structural phase transition. The linewidth behavior is opposite to that of the well-known β- (ET) 2 X( X − = I − 3 , IBr − 2 , AuI − 2 ) organic superconductors. No signal attributable to Cu(II) species is observed. The relative ESR spin susceptibility is approximately constant from 440 K to the inflection temperature.

How well do we understand the synthesis of (ET) 2I3 by electrocrystallization? ESR and X-ray identification of (ET) 2I3 crystals which are mixtures of phases and observation of high-Tc states of (ET) 2I3, ranging from 2.5–6.9 K

Abstract A previously unreported, perhaps widely occurring, phenomenon has been observed in the ESR examination of α- and s-(ET) 2I3 crystals prepared by electrocrystallization. Crystals that are in themselves α-s-mixtures have been identified. The unusual mixed-phase crystals were first discovered in attempts to alloy (ET) 2I3 and triiodides of closely related donors but have subsequently been detected in simple (ET) 2I3 electrocrystallizations. In all cases where mixed-phase systems have been isolated, crystals have been noted growing on the bottom of the anodic portion of the H-cell. It is not known at present whether the α/s-crystals were formed on the bottom, on the anode, or at both sites, although experiments are in progress to clarify this point. A second novel observation that has been made in investigating the mixed-phase systems is that crystals of (ET) 2I3 have been found that have high-Tc states, ranging from 2.5 to 6.9 K (rf penetration depth). It appears that α-, and s-, and s∗ (or αt)-(ET) 2I3 phases may all occur in one “single” crystal, thereby producing a range of Tcs from 1.5–8 K. In the minimum, these findings suggest that crystals of (ET) 2I3 prepared by electrocrystallization should be carefully screened. In all likelihood, the results have significant ramifications for electrocrystallization preparations in general. An ESR spectroscopic procedure has been developed for identifying and quantifying the composition of s-(ET) 2I3 crystals that are mixtures of phases.

New electron donors related to BEDT-TTF

Abstract Recent results on the synthetic aspects of new electron donors related to BEDT-TTF are presented.

Crystal structure and electrical properties of (BEDT-TTF)2Cp(CN)5·(solvent)x

Abstract The crystal structure and electrical properties of (BEDT-TTF) 2 Cp(CN) 5 · (Solvent) x , where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene (ET), Cp(CN) 5 is pentacyanodicyclopentadienide (PCCP) and the solvent is TCE or THF, have been determined. The crystals are orthorhombic, space group Pncm , with unit cell dimensions a = 6.774(2), b = 14.657(5) and c = 41.05(1)A. The ET molecular network is similar to that of the α′ phase of (ET) 2 AuBr 2 with interstack S…S interactions between almost coplanar donor molecules ranging from 3.532(5) A to 3.566(5) A and intrastack interactions greater than 3.7 A. The TCE and THF solvates have room-temperature conductivities of 0.32 μ −1 cm −1 and 0.45 μ −1 cm −1 , respectively, and are semiconductors with activation energies of about 0.12 eV. The ESR studies indicate an antiferromagnetic ordering below 90 K, leading to an insulator ground state.

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.

Magnetic relaxation of the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2Br

Abstract Magnetic relaxation was measured on a κ-(ET) 2 Cu[N(CN) 2 ]Br (ET=BEDT-TTF) single crystal for H/b (perpendicular to the conducting layers) at 6 K. The relaxation rate, dM/dlnt was found to have a maximum at 30 G. This maximum in the relaxation rate is thought to be the field of full penetration. H ∗ , and agrees with the peak observed in the magnetization measurements for H//b at 6 K. The field dependence of the relaxation rate can be described by a thermally activated flux creep model where U o ≈8.3 meV.

Characterization of the electrocrystallization products of the mixed-donor system ET:MET (1:1) / I3 / TCE: How to get crystals with the ordered ß∗-(ET) 2I3 structure and a Tc of 4.6 K without applied pressure

Crystals that result from the electrocrystallization of a 1:1 mixture of the organic donor molecules ET and MET with the I3- anion are superconducting at ambient pressure, Tc = 4.6 K. Single crystal X-ray diffraction investigations (298 K) show that the crystals are s-(ET) 2I3 and little or no MET is incorporated. However, at 125 K no modulated structure is observed, instead the ethylene groups of the ET molecule are ordered. The conformation of the ET molecule is the same as that observed in s∗-(ET) 2I3, the high pressure form of s-(ET) 2I3, with Tc ∼ 8 K.

Electrical conductivity, crystal structure, and variable temperature ESR investigation of (BEDSe-TTF)2IBr2

Abstract The electrocrystallization of 3,4;3′,4′-bis(ethylenediseleno)-2,2′,5,5′-tetrathiafulvalene in the presence of [NBu 4 ] [IBr 2 ] in THF yields a 2:1 charge transfer complex that crystallizes with space group PĪ and lattice parameters a = 6.863(2) A , b = 10.065(3) A , c = 13.0183 (3) A , α = 87.94(2)°, β = 100.49(2)°, γ = 98.84(2)°, Z = 1, V = 884.8(4) A 3 . This product is semiconducting with a room temperature resistivity of 285 Ωcm and is isostructural with the 2:1 s-phase BEDT-TTF salts containing the ICI 2 − and BrICI − anions. ESR data obtained over the temperature interval 10–300 K show a monotonic decrease in peak-to-peak linewidth and the spin susceptibility data show that a sharp decrease in the sample magnetic susceptibility occurs with an onset temperature of approximately 12 K.