Ilsabe Heinen

Superconductivity at 10 K and ambient pressure in the organic metal (BEDT-TTF)2Cu(SCN)2

We confirm the observation of superconductivity at ambient pressure above 10 K in the organic metal (BEDT-TTF)2Cu(SCN)2 as reported recently by Urayama et al [12]. In addition we have measured ESR, ac-susceptibility and thermopower in crystals of (BEDT-TTF)2Cu(SCN)2 and have shown that in contrast to other organic superconductors here a relatively sharp superconducting transition even in the ac-susceptibility can be observed which saturates already around 8 K. The thermopower measurements indicate a clear metal-metal phase transition at 100 K and a possible second phase transition at around 50 K, while from the temperature dependence of the resistivity and susceptibility (ESR) these phase transitions cannot be observed.

BEDT-TTF salts with square platinates(II) as counterions : [BEDT-TTF]4[Pt(C2O4)2], a new organic metal

Abstract The title compound, [BEDT-TTF] 4 [Pt(C 2 O 4 ) 2 ], is obtained by electrocrystallization, which yields black platelets of stoichiometry C 44 H 32 O 8 PtS 32 , M r = 1909.87. Crystal structure determination at room temperature gives a triclinic cell P 1 , with a = 8.678(2) A, b = 11.878(5) A, c = 15.757(7) A, α = 105.49(3)°, β = 91.05(3)°, V = 1563.64 A 3 , d c = 2.03 g/cm 3 , Z = 1. The solid contains sheets of BEDT-TTF cations separated by sheets of [Pt(C 2 O 4 ) 2 ] 2− counterions. The BEDT-TTF sheets are made up of columns that interact strongly through S-S contacts. The columns are built up by pairs of BEDT-TTF molecules. There are strongly varying bond distances in the two different BEDT-TTF moieties that make up a pair. This fact is especially clearly expressed in the surprisingly short Cue5f8C distances of the central Cue5fbC double bond of the two BEDT-TFF ions: 1.273 A and 1.327 A respectively. These are by far the shortest distances for the central Cue5fbC bonds observed so far in conductive BEDT-TTF salts. The compound behaves like a metal down to about 60 K. Temperature-dependent d.c. conductivity, e.s.r. and thermopower measurements show metallic behaviour at room temperature ( σ 300 = 20 S/cm) with a metal-to-metal phase transition around 200 K. At about 60 K a second broad phase transition occurs and the crystals become semiconducting.

FT-IR investigations of BEDO-TTF and radical salts of BEDO-TTF

Abstract We investigated powder absorption spectra of the donor BEDO-TTF and some radical salts of BEDO-TTF, which are metals even at low temperatures. After an assignment of the bands in the neutral donor we found in the radical salts different vibrational frequencies of bands correlated with Cue5f8O vibrations. These frequencies are directly related to the average charge on the donor molecule. Additionally frequencies of CH 2 -stretching vibrations exhibit differences, which can be ascribed to a varying strength of donor-anion interaction depending on the respective anion.

NMR investigations of the organic metals and superconductors α-(BEDT-TTF)2I3, ß-(BEDT-TTF)2I3 and αt-(BEDT-TTF)2I3

Abstract 1 H and 13 C spin lattice relaxation rates as well as 13 C Knight shifts of the quasi two dimensional organic metals α-, β- and α t -(BEDT-TTF) 2 I 3 are reported. In β- and α t -(BEDT-TTF) 2 I 3 at temperatures below 100 K the Korringa relation is fulfilled. Near the critical temperature of 8K of the stable superconducting state of α t -(BEDT-TTF) 2 I 3 an enhancement of the proton spin lattice relaxation rate due to superconducting fluctuations is observed. The observed individual 13 C Knight shifts, as measured by magic angle sample spinning and cross polarisation methods range between -6 and +244 ppm for α- phase and -10 and +125 ppm for β- and α t - phase crystals.

The double-stack structure of di(3,4-ethylenedithio-3',4'-dimethyl-2,2',5,5'-tetrathiafulvalenium) perchlorate, (DIMET)2ClO4

ClOHIOs~n+.CI04 , M r= 744·62, triclinic, PI, a ~ 7·000 (2), b ~ 7·824 (3), c ~ 27·010 (14) A, • Alternative nomenclature: di(4,5 -ethylenedithio -4,5-dimethyl- 2,2 - bi·l .3-dithioly liden)ium perchlorate.

Radical cation salts of an unsymmetrical BEDT-TTF derivative: DIMET☆

Abstract Crystal structures, temperature dependent conductivity measurements and thermopower data of several DIMET radical salts are presented and discussed.

RADICAL CATION SALTS OF BEDT-TTF AND AN UNSYMMETRICAL BEDT-TTF DERIVATIVE

Investigations of several BEDT-TTF and DIMET radical salts are presented. Measurements on β-(BEDT-TTF)2IAuI crystals show that their conducting and superconducting properties might vary strongly depending on the decomposition of (IAuI)− anions during the electrochemical process.

BEDT-TTF-Salze mit quadratischen Platinaten(II) als Gegenionen: [BEDT-TTF]4[Pt(CN)4] / BEDT-TTF-Salts with Square Platinates(II) as Counter Ions: [BEDT-TTF]4[Pt(CN)4]

The compound [BEDT-TTF]4[Pt(CN)4], BEDT-TTF = Bis(4.5-ethylendithiolo)tetrathiafulvalen = C10H8S8, stoichiometry C44H32N4S32Pt, Mr = 1837.9 (g/mol), was obtained by electrocrystallization in at least three different modifications. Two of them could be identified by single crystal X-ray structure elucidation. 1 = β-modification, is isolated in black, lustrous platelets: triclinic, P 1̄, Z = 1, α = 9.7280(30) Å, b = 10.9237(27) Å, c = 16.5432(60) Å, a = 95.820(25)°, β = 98.296(26)°, γ = 115.215(24)°, V = 1547.56 A3, dc = 2.0 (g/cm3), room temperature, Rw = 0.082 for 7322 observed reflections. 2 = y-modification, is isolated in black not very regular crystals: monoclinic, C 2/c, Z = 4, a = 11.163(6) Å, b = 33.56(2) Å, c = 16.56(1) Å, β = 91.39(4)°, V = 6202.04 A3, dc = 1.97 (g/cm3), room temperature, Rw = 0.053 for 1404 observed independent reflections. Both solids contain sheets of BEDT-TTF cations separated by sheets of the counter anions. 1 contains two crystallographically independent BEDT-TTF moieties while 2 contains four independent donor units per cell. Of interest are the very different lengths of the central C=C bonds of the BEDT-TTF moieties which are assumed to reflect the charge density on these ions. In 1 these distances are 1.350(14) Å and 1.306(14) A, respectively, for the two different BEDT-TTF cations in the lattice, in 2 they are 1.372(34) A, 1.401(42) A and 1.289(54) Å respectively. The remarkable differences in these bond lengths indicate, according to published concepts, quite different charges on the different BEDT-TTF units. Two of the distances are shorter than in the neutral donor. Measurements of the specific resistance and the thermopower show that 1 is a metal with a room temperature conductivity of 10 S/cm. At around 200 K 1 becomes semiconducting. 2 turns out to be a semiconductor with a room temperature conductivity of 1—2 S/cm. Around 225 K a structural phase transition occurs. ESR measurements show an antiferromagnetic coupling between the spins in the solid. A third modification (δ) (semiconducting with a room temperature conductivity around 2 × 10-2 S/cm) was identified by ESR investigations.

Transport properties of single crystals and polycristalline pressed samples of (BEDT-TTF)2X salts and related coordination polymers

Ten years ago, in 1979, superconductivity was observed for the first time in an organic metal [1]. Today, about 30 different organic metals are known, which become superconducting under pressure or ambient pressure. The organic superconductors with the highest transition temperatures are all radical salts of the donor bis(ethylenedithiolo)-tetrathiafulvalene (BEDT-TTF), namely at ambient pressure (BEDT-TTF)2Cu(NCS)2 (Tc = 10.4 K) [2] and αt-(BEDT-TTF)2I3 (Tc = 8 K) [3] and under isotropic pressure βH - (BEDT-TTF)2I3 (0.5 kbar, Tc = 7.5 K) [4,5]. The latter βH-phase can even become superconducting at 8 K and ambient pressure, after a special pressure-temperature cycling procedure [6], i.e. pressurization up to 1 kbar at room temperature, and release of the helium gas pressure at temperatures below 125 K. Nevertheless, this superconducting state at 8 K in βH-(BEDT-TTF)2I3 is only metastable [6,7], since warming up the crystal above 125 K, and cooling down again under ambient pressure, results only in superconductivity at 1.3 K, the so-called βL - or β-phase.

Preparation, Structure and Investigations of BEDT-TTF Trihalides

Abstract Crystals of the β-phases of (BEDT-TTF)2Br-I-Br (1) and (BEDT-TTF)2I-I - Br (2) have been isolated by electrocrystallization. The solids can be obtained using different tetrabutylammonium trihalides containing iodine and bromine as supporting electrolytes. Cyclovoltam metric results show clearly that the trihalide anions are involved in the oxidation reactions near the anode which finally lead to the crystals. 1 C20H16Br2IS16, Mr = 1056.01, and 2 C20H16BrI2S16, Mt = 1103.01 are isom orphous, crystallizing in the triclinic space group P1̄, with Z = 1. Unit cell parameters for 1: a = 6.5979(6) A , b = 8.998(1) Å, c = 15.138(3) Å , a = 94.03(1)°, β = 95.13(1), γ = 110.28(1), V = 834.7 Å and for 2: α = 6.606(2) Å , b = 9.047(6) A , c = 15.229(6) Å, α = 94.28(2)°, β = 95.34(2), v = 109.92(2), V = 846.5 Å3. The different sizes of the Br2I- and Brl2- counterions lead to different intra- and interstack S -S contacts in the solids thus influencing the physical properties. An additional Br2I- phase with a modulation of the above-described structure could be found. Some relevant physical properties of 1 and 2 are reported