High-pressure transport measurements of α'-BEDT-TTF salts

Abstract

The authors report on transport and optical measurements of some alpha '-phase (BEDT-TTF)2X salts with X=AuBr2-, Ag(CN)2- and CuCl2-. These salts are shown to be Mott-Hubbard semiconductors. Transport measurements were made under hydrostatic pressures up to 20 kbar and temperatures down to 4 K. At room temperature all three systems have low conductivities, approximately=0.05 S cm-1 and small activation energies, approximately=300 meV. On cooling, the AuBr2- and Ag(CN)2- salts show a transition between two semiconducting phases with slightly different activation energies, and for both salts the transition temperature increases under pressure. For all three salts the activation energy for conductivity decreases under pressure. In the AuBr2- system the transition changes abruptly from second to first order at 8.5 kbar. This is not seen for Ag(CN)2-. The thermopower has been measured for all three salts at ambient pressure. There are differences, with the AuBr2- system showing hole conduction and the other showing electron conduction. The authors have measured the optical absorption spectrum of the Ag(CN)2- salt, and this shows a peak at 1.3 eV in the solid state which is missing in a dilute solution of BEDT-TTF. They attribute this to a Hubbard correlation. Finally, they suggest an explanation for the similarities of the differences in behaviour between these three systems and discuss the general properties in terms of a large-U model.

Crystal structures and physical properties of bis(ethylenedithio)-tetrathiafulvalene charge-transfer salts with FeX4–(X = Cl or Br) anions

“…The last few years have seen the synthesis of numerous chargetransfer salts of the S-heterocyclic molecule bis(ethy1enedithio)tetrathiafulvalene (bettf), following the discovery of superconductivity, first under pressure in [bettfl2[ReO4] (T, 2 K, P x 4.5 kbar)' and later at ambient pressure in P-[bettQ2 [13]-(T, 1.4K).2 These efforts culminated recently in the preparation of ~-[bettfl,[Cu(NCS)~] with the highest superconducting critical temperatures yet found in a molecular solid (10.4K). 3 One aspect of the potential flexibility of structures in this class of compound that has not received much attention so far4 is the incorporation of anions carrying a magnetic moment, although the interplay of magnetism and superconductivity is a recurring theme in the theory of classical superconductors.…”

New Family of Six Stable Metals with a Nearly Isotropic Triangular Lattice of Organic Radical Cations and Diluted Paramagnetic System of Anions: κ(κ⊥)-(BDH-TTP)4MX4·Solv, where M = CoII, MnII; X = Cl, Br, and Solv = (H2O)5, (CH2X2)

“…Δ E can be approximated as a twice the intradimer transfer integral t D : Δ E = 2· t D . The experimental U 0 value for ET is 1.3 eV and for BDH is essentially the same as the differences between the first and the second electrochemical oxidation potentials are the same: E 2 (ET) – E 1 (ET) = E 2 (BDH) – E 1 (BDH) = −0.26 V . Then, an inequality U 0 ≫ 2·Δ E takes place and the formula gives approximately U D ≈ Δ E ≈ 2· t D .…”

Influence of the cooling rate on low-temperature Raman and infrared-reflection spectra of partially deuterated κ-(BEDT-TTF)2Cu(N(CN)2)Br

“…ra −U/2 ≈0.15 eV (taking U ≈1 eV, t ra ≈0.2 eV). In comparison with U ef f ≈0.6 eV, it is a considerable value but the estimate is very rough because the value for U is not accurately known 18,19,20 . In addition, U is affected strongly by the screening effect.…”