K. D. Carlson

The role of anions on the crystal structures and electrical properties of the organic metals and superconductors, (BEDT-TTF)2X (X = trihalide anions)

Abstract A systematic study of the crystal structures of (BEDT-TTF) 2 X salts [BEDT-TTF is bis-(ethylenedithio) tetrathiafulvalene, C 10 S 8 H 8 , and X is a linear trihalide anion] reveal that the different packing motifs for the BEDT-TTF molecules are directly related to the electrical properties of these materials. The packing of the organic molecules is influenced by the cation-anion interactions of the -CH 2 …X − type and is observed to change systematically with anion size.

States determined by photoelectron spectroscopy in the perchlorate and perrhenate of TMTSF

The photoelectron spectroscopic binding energies of Se 3d5/2, O 1s, and Cl 2p3/2 or Re 4f7/2 have been measured in tetralmethyltetraselenafulvalenium (TMTSF) perchlorate and perrhenate, tetrabutylammonium perchlorate and perrhenate, and potassium perchlorate and perrhenate. It has been found that the binding energies of Se 3d5/2 are the same in the perchlorate and the perrhenate even though the spectrum is more asymmetrically broadened in the perrhenate. The binding energy of O 1s depends on the cation and on the central atom in the anion. In the organic perrhenates it is at 530.96 eV, whereas in the perchlorates it is at 532.08 eV. In the potassium salts the O 1s binding energies differ by 1.89 eV in the perchlorate and the perrhenate. The binding energies of Cl 2p3/2 and Re 4f7/2 differ by 0.84 and zero respectively from those in the potassium salts. The energetic differences among the values for O 1s indicate the extent to which the electronic structure of the TMTSF salts are determined by the electronic structure of the anions. The difference in binding energies between the TMTSF perchlorate and perrhenate is such that the application of pressure to the perrhenate should transfer electrons to the cation and presumably to the conduction band.

Synthesis, Structure and Electrical Conductivity of (BEDT-TTF)X(BrO4)Y Organic Metals

Abstract The electrochemical oxidation of BEDT-TTF (bis(ethylenedithio)-tetrathiafulvalene) in 1,1,2-trichloroethane solution in the presence of (n-Bu4N)Br04 as supporting electrolyte produces three distinct morphologies: needles, thick plates, and thin plates. These crystal habits have been identified with different crystallographic phases: needles as (BEDT-TTF) 2Br04, thick plates as (BEDT-TTF)2(Br04)(TCE)0.5, and thin plates as (BEDT-TTF)3(Br04)2. The structural characterization and conductivity for these materials is presented.

Organic superconductors: structure-property relations and new materials design

Most known organic materials are electrical insulators having extremely low electrical conductivities of δ < 10-10 Ω-1 cm-1. A small number of organic materials are semiconductors having, for classification purposes, conductivities of δ ≈ 10-10-1 Ω-1 cm-1. A very small, but growing, number of organic substances are metallic in nature, i.e. having conductivities that rise with decreasing tem perature (δ ≈ 1−1010 Ω-1 cm-1). The latter systems comprise a class of intensely studied materials known as ‘organic metals’ of which fewer than ten can display the complete absence of electrical resistance at low temperatures, i.e. superconductivity (δ ≈ infinity). The known organic superconductors are novel, being derived from radical-cation donors and monovalent anions, X. The donors are derived from two kinds of molecules, neither of which contain any metallic elements. These are TMTSF (tetramethyltetraselenafulvalene) and BED T-TTF (bis-ethylenedithiotetrathiafulvalene, or ‘ET’ in abbreviated form). Most of the (TMTSF)2X and (ET)2X conducting materials require applied pressure to induce superconductivity that is thus far observed at very low temperatures (Te ≈ 1–2 K). However, two materials, (TMTSF)2ClO4 and (ET)2I3 are ambient pressure organic superconductors (Te = 1.2 and 1.4 K, respectively). Within each class the crystal structures have many similarities, the most important being a complex ‘infinite sheet networkߣ of short Se-Se interactions in (TMTSF)2X and a ‘corrugated sheet network’ of short S-S interactions in ET 2X. In this paper we discuss structure-property relations of the (TMTSF)2X salts, and of the (ET)2X salts as far as is known. In addition, we attempt to provide insight and guidelines for the synthesis of new highly conducting anionic derivatives of TMTSF and ET. It appears that while highly conducting (TMTSF)2X materials can be designed before synthesis, the onset of superconductivity depends heavily on the presence of anion order in the crystal, which is a parameter not easily controlled. For the (ET)2X systems the structural disorder apparent at 298 and 125 K may persist to very low tem perature, making it difficult to correlate structural order with superconductivity as is the case for (TMTSF)2X systems.

Asymmetric Broadening of SE(3d5/2) XPS Spectra of (TMTSF)2ClO4 and (TMTSF)2ReO4

Abstract A comparison of the x-ray photoelectron spectra in TMTSF, (TMTSF)2ClO4, and (TMTSF)2ReO4, shows that the spectra of Se(3d5/2) for the first is nearly symmetrical, whereas those for the other two are broadened asymmetrically. The broadening is associated with polarization of the selenium by the anion.