Search Results (5)

New radical-cation salts based on tetramethyltetrathiafulvalene (TMTTF) and tetramethyltetraselenefulvalene (TMsTSF) with metallacarborane anions (TMTTF)[3,3′-Cr(1,2-C₂B₉H₁₁)₂], (TMTTF)[3,3′-Fe(1,2-C₂B₉H₁₁)₂], and (TMTSF)₂[3,3′-Cr(1,2-C₂B₉H₁₁)₂] were synthesized by electrocrystallization. Their crystal structures were determined by single crystal X-ray diffraction, and their electrophysical properties in a wide temperature range were studied. The first two salts are dielectrics, while the third one is a narrow-gap semiconductor: σ_RT = 5 × 10⁻³ Ohm⁻¹cm⁻¹; E_a ≈ 0.04 eV (aprox. 320 cm⁻¹). Full article

Tetramethyl-tetraselenafulvalene (TMTSF) and bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) are flagship precursors in the field of molecular (super)conductors. The electrocrystallization of these donors in the presence of (n-Bu₄N)TaF₆ or mixtures of (n-Bu₄N)TaF₆ and (n-Bu₄N)PF₆ provided Bechgaard salts formulated as (TMTSF)₂(TaF₆)_0.84(PF₆)_0.16, (TMTSF)₂(TaF₆)_0.56(PF₆)_0.44, (TMTSF)₂(TaF₆)_0.44(PF₆)_0.56 and (TMTSF)₂(TaF₆)_0.12(PF₆)_0.88, together with the monoclinic and orthorhombic phases δ_m-(BEDT-TTF)₂(TaF₆)_0.94(PF₆)_0.06 and δ_o-(BEDT-TTF)₂(TaF₆)_0.43(PF₆)_0.57, respectively. The use of BEDT-TTF and a mixture of (n-Bu₄N)TaF₆/TaF₅ afforded the 1:1 phase (BEDT-TTF)₂(TaF₆)₂·CH₂Cl₂. The precise Ta/P ratio in the alloys has been determined by an accurate single crystal X-ray data analysis and was corroborated with solution ¹⁹F NMR measurements. In the previously unknown crystalline phase (BEDT-TTF)₂(TaF₆)₂·CH₂Cl₂ the donors organize in dimers interacting laterally yet no organic-inorganic segregation is observed. Single crystal resistivity measurements on the TMTSF based materials show typical behavior of the Bechgaard phases with room temperature conductivity σ ≈ 100 S/cm and localization below 12 K indicative of a spin density wave transition. The orthorhombic phase δ_o-(BEDT-TTF)₂(TaF₆)_0.43(PF₆)_0.57 is semiconducting with the room temperature conductivity estimated to be σ ≈ 0.16–0.5 S/cm while the compound (BEDT-TTF)₂(TaF₆)₂·CH₂Cl₂ is also a semiconductor, yet with a much lower room temperature conductivity value of 0.001 to 0.0025 S/cm, in agreement with the +1 oxidation state and strong dimerization of the donors. Full article

In highly anisotropic organic superconductor (TMTSF)${}_2$ClO${}_4$, superconducting (SC) phase coexists with metallic and spin-density wave phases in the form of domains. Using the Maxwell-Garnett approximation (MGA), we calculate the volume ratio and estimate the shape of these embedded SC domains from resistivity data at various temperature and anion disorder, controlled by the cooling rate or annealing time of (TMTSF)${}_2$ClO${}_4$ samples. We found that the variation of cooling rate and of annealing time affect differently the shape of SC domains. In all cases the SC domains have oblate shape, being the shortest along the interlayer z-axis. This contradicts the widely assumed filamentary superconductivity along the z-axis, used to explain the anisotropic superconductivity onset. We show that anisotropic resistivity drop at the SC onset can be described by the analytical MGA theory with anisotropic background resistance, while the anisotropic $T_c$ can be explained by considering a finite size and flat shape of the samples. Due to a flat/needle sample shape, the probability of percolation via SC domains is the highest along the shortest sample dimension (z-axis), and the lowest along the sample length (x-axis). Our theory can be applied to other heterogeneous superconductors, where the size d of SC domains is much larger than the SC coherence length $\xi$, e.g., cuprates, iron-based or organic superconductors. It is also applicable when the spin/charge-density wave domains are embedded inside a metallic background, or vice versa. Full article

Three novel organic conductors (TMTSF)${}_8$(I${}_3$)${}_5$, (TMTSF)${}_5$(I${}_3$)${}_2$, and (TMTSF)${}_4$(I${}_3$)${}_4$·THF (THF = tetrahydrofuran) were synthesized and their crystal structures were characterized by X-ray diffraction analyses, where TMTSF denotes tetramethyltetraselenafulvalene. The crystals of both the (TMTSF)${}_8$(I${}_3$)${}_5$ and (TMTSF)${}_5$(I${}_3$)${}_2$ are composed of one-dimensional stacks of TMTSF trimers separated by TMTSF monomers. The crystal of the (TMTSF)${}_4$(I${}_3$)${}_4$·THF is composed of the TMTSF tetramers and I${}_3^{-}$ tetramers; and regarded as the elongated rock-salt structure. The electrical conductivity of the (TMTSF)${}_8$(I${}_3$)${}_5$ and (TMTSF)${}_5$(I${}_3$)${}_2$ is about 60 and 50 S·cm${}^{-1}$ at room temperature, respectively. The electrical resistivity of (TMTSF)${}_8$(I${}_3$)${}_5$ is weakly metallic below room temperature and rapidly increases below 88 and 53 K on cooling suggesting two possible phase transitions. The electrical resistivity of (TMTSF)${}_5$(I${}_3$)${}_2$ is semiconducting below room temperature but shows an anomaly around 190 K, below which the activation energy becomes small. The application of hydrostatic pressure up to 1.7 GPa do not change these behaviors of (TMTSF)${}_8$(I${}_3$)${}_5$ and (TMTSF)${}_5$(I${}_3$)${}_2$ very much. A method to evaluate the non-integer valence of crystallographically independent TMTSF molecules is developed by using the DFT (density-functional-theory) and MP2 (Hartree–Fock calculations followed by Møller–Plesset correlation energy calculations truncated at second order) calculations. It is shown that the method gives the valence of the TMTSF molecules of the I${}_3$ salts consistent with their electrical properties. Full article