Carolyn S. Cariss

The crystal structures and physical properties of polymeric (BEDT-TTF)-metallothiocyanates

The crystal structures of two new ET metallothiocyanates [ET or BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] with polymeric anions are presented: (ET)Cu2(SCN)3 and (ET)Agx(SCN)2 (x = ⋍ 1.6). The former is a second, semiconducting, phase in the ETCuSCN system which also yields superconducting κ-(ET)2Cu(NCS)2 (Tc = 10.4 K).

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.

Temperature dependence of conductivity of κ(BEDTTTF)2Cu(SCN)2; Resolution into two components; Small polaron

Abstract The variation of resistance with temperature of κ(BEDTTTF)2Cu(NCS)2 displays a maximum or a suppressed maximum near 90 K. These unusual variations can be interpreted in terms of a model which resolves the variation into metal-like and thermally activated components. The resolution suggests the applicability of the small polaron model of Bottger and Bryksin with tunneling and hopping conductivities (Hopping Conduction in Solids, VCH Verlagsgesellchaft, 1985).

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.

Crystal structure and electrical conductivity of (TTF)5Hg6(SCN)16

Abstract The crystal structure and electrical properties of (TTF)5Hg6(SCN)16 have been determined. The crystals are monoclinic, space group P21/a, with unit cell dimensions a = 15.032(3) , b = 16.846(5) , c = 17.364(3) A and β = 108.92(1)°. The TTF units are stacked along the c axis. Within the stacks, dimers and trimers alternate. The complex anion contains bridging as well as non-bridging SCN− ligands. The crystals exhibit semiconducting behavior with a room temperature conductivity of 8.7 × 10−4 S·cm−1.

Synthesis, conductivity, and X-ray photoelectron spectrum of Bi2Sr2CuO7+X. A new ternary bismuth-oxide system exhibiting metallic conductivity

Abstract The preparation and some of the properties relating to the superconductive state of the newly discovered ternary bismuth oxide, Bi2Sr2Cu2O7+x, are described. Conductivity behavior ranging from semiconductive to metallic is observed when four-probe AC resistivity measurements are carried out on pressed pellet specimens that have been annealed under different conditions. From a determination of the total oxygen present by an iodometric titration, it was found that metallic conductivity was associated with a higher oxygen content. An X-ray photoelectron experiment was carried out in order to determine whether bismuth or copper was present as the mixed-valent species. The XPS spectrum of the Bi 4f orbital electrons in the oxide was nearly identical to that observed in Bi2O3, with no evidence of any Bi5+.

Preparation and characterization of two structural phases of (EPT)2ICI2

Abstract Two 2:1 salts of the unsymmetrical organic donor EPT (4,5-ethylenedithio-4′,5′-propylenedithiotetrathiafulvalene) and the ICI 2 − anion have been prepared by electrocrystallization. The monoclinic phase contains stacks of EPT donors with short S…S interstack contacts, whereas the triclinic phase contains chains of side-by-side EPT dimers. Crystals of both phases exhibit semiconductor behavior in measurements of their resistivity.

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.