A. N. Bloch

Resistive and Magnetic Susceptibility Transitions in Superconducting (TMTSF)2C104

Abstract We report measurements of the ac magnetic susceptibility and dc resistive superconducting transitions in the organic superconductor (TMTSF)2C104. Inductive measurements show complete diamagnetic shielding below a broad transition and initial flux penetration at very low fields [Hc1(0) < 1 Oe]. The resistive transition is also broad, but occurs at a significantly higher temperature than the inductive transition, Tc = 1.0 K and 0.65 K respectively. Resistance measurements also show evidence of a phase transition in the vicinity of 24 K. Magnetic field induced transitions, measured both inductively and resistively, show marked anistropy both in magnitude and in breadth of the transition. Results suggest that (TMTSF)2C104 is a quasi ID or 2D superconductor at high temperatures and high magnetic fields and an anisotropic bulk superconductor at low temperatures and fields. Associated thermoelectric power measurements suggest that spin density waves coexist with the superconducting state.

Structure of an organic charge‐transfer salt derived from dibenzotetrathiafulvalene and tetrafluorotetracyanoquinodimethane (DBTTF–TCNQF4). Observation of a high‐temperature phase transition

The crystal structure of the organic charge‐transfer salt DBTTF–TCNQF4 is reported. The structure has been determined by single‐crystal x‐ray diffraction techniques and has been shown to consist of segregated stacks of donor and acceptor molecules. Within both the donor and acceptor stacks, significant dimerization is observed at room temperature. From the geometries of the DBTTF and TCNQF4 molecules, it is concluded that the charge transfer is complete. The electrostatic contribution to the crystal cohesion for DBTTF–TCNQF4 has been calculated and compares well to those for similar salts of unit charge transfer. It is suggested that the room‐temperature structure may well be representative of the low‐temperature phase of a system affected by a Peierls instability. Based on diffraction data, a phase transition near 390 K is reported. The driving force for the transition is likely a spin–phonon instability. Crystal data for DBTTF–TCNQF4 are triclinic, space group P1; a = 13.159(3) A, b = 13.703(4) A, c = ...

Charge Transfer Salts of Highly Oriented Fibers of Discotic Liquid Crystal HET-n

Abstract The charge transfer salt of highly oriented fibers of discotic liquid crystals of 2,3,6,7,10,11-hexa-alkoxytriphenylene (HET-n) and 2,3,6,7,10,11-hexa-alkanoyloxytriphenylene (HAT-n) can be made by a strand technique followed by a doping process. As a result, the bromine doped HET-5 fiber shows a conductivity of three orders of magnitude higher than that of the bromine doped HET-5 powder.

Synthesis and Study of Electrochemically Grown Salts of Organic II-Donors

Abstract The effects of purity and crystal perfection on the properties of (TMTSF)2ClO4 are examined via SEMs and transition temperatures. Careful electrochemical techniques using gradient sublimed TMTSF starting material and short term oxidation result in high quality crystals. Preliminary work on HMTSF salts of PF − 6 and AsF− 6 is also reported.

Synthesis and study of multiple cationic salts of hexamethoxytriphenylene (HMTP)

Abstract 2,3,6,7,10,11-Hexamethoxytriphenylene (HMTP) is known to exhibit multiple reversible oxidation states. We report on the synthesis and the physical properties of stable monocation, dication, and alloy salts between HMTP and ClO 4 − . The monocation salt, HMTP-ClO 4 , is a half-filled band, quasi one-dimensional semiconductor with a possible spin-Peierls transition at 240K. The dication is also found to be a semiconductor.

Synthesis of organic solids with a high spin density as an approach to organic ferromagnets

Abstract A synthetic approach modified from the McConnell-Breslows model for the preparation of organic ferromagnetism is described. Our study has been to manipulate an array of ground state high spin radicals into an ordered matrix such that they are molecularly separated by a second set of radicals with a different spin state. It was accomplished by utilizing a chemical doping technique at the solid state of organic charge transfer complexes to oxidize donor molecules to dications in triplet state while the structure of the studied complexes is preserved. Specifically, we demonstrate this synthetic approach to prepare high spin density (1.6 spins 1 2 per formula unit) organic solids of doped (HMT)2-TCNQF4 with anomalously small interspin coupling. The resulting system stabilizes the ground high spin state of HMT in a wide range of temperature (5K to room temperature) in a solid matrix.

On the Crystal Structure of the Organic Charge-Transfer Salt Derived from Hexamethylenetetraselenafulvalene (HMTSF) and Tetrafluoro-7,7,8,8- Tetracyano-p-Quinodimethane (TCNQF4), HMTSF-TCNQF4

Abstract The charge-transfer salt HMTSF-TCNQF4 crystallizes in the monoclinic system, space group C2/m, with the following crystal data: a = 21.906(5) A, b = 12.918(3) A, c = 4.018(1) A, β = 91.66(1)°, V = 1136.5 A3 Z = 2, D cale, = 2.186 g cm−3 Dcale,= 2.17(2) g cm−3. A structural model was readily deduced from standard Patterson-Fourier methods and has been refined (based on 1175 counter-collected X-ray data) to an R value of 0.048. The crystalline motif is dominated by uniform, segregated stacks of donors and acceptors. Within each stack, the molecular overlap pattem is of the ring-over-bond type and the mean interplanar separations are 3.68 A (donor column) and 3.24 A (acceptor column). Principal intercolumn contacts are of two types: Se(donor) … F(acceptor) at 3.19 A and Se(donor) … N(acceptor) at 3.36 A. Comparisons are made between the crystal structure and physical properties of HMTSF-TCNQF4 and HMTSF-TCNQ (to which it is isostructural) and to those of HMTTF-TCNQ and HMTTF-TCNQF4 (to which it is c...

Charge and Spin Density Waves in Charge-Transfer Solids

Abstract It is shown that the strengths and periodicities of charge and spin density waves in the organic charge-transfer solids depend very strongly on the degree of charge transfer ρ. We thereby explain both the appearance of the 4kF-, charge density wave near ρ = 0.5 and its nonexistence for ρ ≳ 0.6, while for ρ slightly larger (but not smaller) than 0.5 we predict a new kind of Coulomb-induced defect.

(TMTSF)2(2,5-Tcnqbr2): Structure and Physical Properties

Abstract The 2:1 charge-transfer salt (TMTSF)2(2,5-TCNQBr2) has been prepared and its physical properties investigated. Its crystal structure consists of segregated stacks of TMTSF donors (ring-over-bond overlap pattern; mean interplanar spacing of 3.6A) and chains of edge-on and disordered 2,5-TCNQBr2 acceptors. Infrared data are suggestive of unit charge on the 2,5-TCNQBr2 molecule and, therefore, half charge on the TMTSF donor. Resistivity data are successfully interpreted on the basis of a percolation construction. Magnetic data are also presented.

Synthesis of Substituted Tetraselenafulvalenes from Dimethylphosgene Iminium Chloride

Abstract Tetramethyltetraselenafulvalene (TMTSF) and hexamethylenetetraselenafulvalene (HMTSF) can be conveniently synthesized using dimethyl phosgene iminium chloride as a starting material. The method and detailed experimental conditions are described.