Jean-Pierre Legros

Pressure-temperature phase diagram of α′-TTF [Pd(Dmit)2]2

Abstract Temperature dependence of resistivity (up to 24 Kbar) and of E.S.R. (at 1 bar) measurements suggest that centered monoclinic TTF[Pd(Dmit) 2 ] 2 could be an anisotropic and weak antiferromagnetic fluctuating metal. At low pressure, gradual charge and spin localizations at different temperatures are the signature of a Mott-Hubbard insulator. Above 20 Kbar, “high” superconducting onset temperatures (6.5 K) are observed.

Structural Evidence of Charge Density Waves in the Series of Molecular Conductors and Superconductors: TTF(M(dmit)2)2 (M = Pd, Ni)

We present an x-ray study at ambient pressure of the metal complexes TTF(Pd(dmit)2)2 and TTF(Ni(dmit)2)2 as a function of temperature. At room temperature one-dimensional (1D) fluctuations are observed under the form of diffuse lines on the x-ray patterns. The Ni-derivative exhibits one type of scattering which condenses below about 40 K into satellite reflections. The Pd-derivative shows two kinds of scattering which condense into satellite reflections, respectively, at T1 ≈ 150 K and T2 ≈ 105 K. These observations provide evidence of charge density wave instabilities related to the 1D electronic anisotropy of these materials, and which compete with superconductivity under pressure.

Synthesis, Electrical Conductivity, and Crystal Structure of (AsPh4)0.25 [Ni(dmit)2]

Abstract The synthesis, conductivity behavior and crystal structure of the ion radical salt (AsPh4)0.25 [Ni(dmit)2] are described. This compound crystallizes in the monoclinic system, space group Pn or P2/n, a = 13.507(7), b = 6.610(1), c = 44.536(7) A, β = 96.08(2)[ddot]. The structure consists of thick layers of stacked Ni(dmit)2 entities parallel to (001) and separated by AsPh4 + cations. Short S… S interstack distances are observed such that two-dimensional networks of closely spaced Ni(dmit)2 are formed in the (001) plane of this structure. The two-dimensional character of the structure is reflected in the low anisotropy in the conductivities measured in the (001) plane (σ = 1 : 1 : 10−3. The room-temperature four-probe conductivities measured along the needle axis (b axis) is 10-15 ω−1 cm−1. Temperature-dependent measurements show a thermally activated conductivity with very low activation energies (ca. 0-0.01 eV).

Crystal and molecular structure and magnetic properties of a new µ-oxalato binuclear copper(II) complex containing mepirizole

The crystal and molecular structure of the new µ-oxalato binuclear copper(II) complex [(mpym)(H2O)(NO3)Cu(C2O4)Cu(NO3)(H2O)(mpym)]·2H2O 1[mpym = mepirizole = 4-methoxy-2-(5-methoxy-3-methyl-1H-pyrazol-1-yl)-6-methylpyrimidine] has been determined by X-ray diffraction methods. It consists of discrete binuclear entities where the copper atoms lie in a strongly elongated octahedral environment, surrounded by two nitrogen atoms (one from each ring of a mepirizole molecule) and two oxygen atoms of the bridging oxalato group in the equatorial plane and oxygen atoms of water molecules and nitrate ions in the axial positions. The binuclear entities are not centrosymmetric and the difference in the ligand environments of the copper(II) ions induces and energy separation between the two magnetic orbitals large enough to weaken the antiferromagnetic interaction (J=–142 cm–1) by approximately 60 cm–1 compared to that of symmetrical µ-oxalato binuclear copper(II) compounds. ESR, vibrational and electronic spectra are consistent with the above results.

Crystal and molecular structure and properties of the first characterized copper(II) one-dimensional polymer containing mepirizole [4-methoxy-2-(5-methoxy-3-methyl-1H-pyrazol-1-yl)-6-methylpyrimidine]

The crystal and molecular structure of the complex [Cu(mpym)(H2O)(C4O4)]·2H2O [mpym = mepirizole = 4-methoxy-2-(5-methoxy-3-methyl-1H-pyrazol-1-yl)-6-methylpyrimidine] has been determined by X-ray methods. It crystallizes in the orthorhombic space group Pbcn, with Z= 8, a= 18.592(3), b= 14.723(2) and c= 13.755(3)A, R= 0.053 and R′= 0.070 for 2697 reflections. The structure comprises infinite chains parallel to [001] built up of 1-bis(monodentate) squarate-O1O3 C4O4 anions bridging [Cu(mpym)(H2O)]2+ units. The copper(II) ions display distorted trigonal-bipyramidal geometry. Both electronic and ESR spectra are indicative of an essentially dz2 ground state for the CuII. No exchange coupling has been detected down to 4.2 K by means of magnetic susceptibility measurements. This feature is discussed on the basis of the relative orientation of the co-ordination polyhedron around the metallic atom and the bridging ligand.

Crystal Structure of a Non-Conductive Non-Stoichiometric Tetrathiafulvalenium Salt: (TTF)3(BF4)2

(TTF)3(BF4)2 crystallizes in the triclinic system, space group P1, a = 8.017(3), b = 8.601(1), c = 11.635(2) A, α = 108.79(1), β = 100.96(2), γ = 99.09(2)°, Z = 1. The TTF entities are stacked in p...

Superconductivity in TTF[Ni(dmit)2]2 films

We report on the observation of a superconducting transition in a fiber-like film of the TTF[Ni(dmit)2]2 phase electrodeposited on silicon substrates. Superconductivity is evidenced by a broad drop of the resistance below 0.8 K under the application of a hydrostatic pressure of 7.7 kbar. Zero resistance is not reached due to the contribution of inter-fiber resistance. Superconductivity is confirmed by the application of a magnetic field perpendicular to the plane of the film. The critical field determined on the film is in agreement with that obtained in bulk single crystals.

Synthesis and characterization of new nickel and platinum complexes derived from the 5,6-dihydro-1,4-dithiin-2,3-dithiolate (dddt) ligand

Abstract Electrochemical studies shows that the M(dddt)2 (M = Ni,Pt) complexes exist as dianionic, monoanionic, neutral and fractional oxidation state cationic species. Associating the M(dddt)2 complexes with the Ni(dmit)2 acceptor yields a new type of bisdithiolene compounds containing a bisdithiolene ligand simultaneously in the cationic and in the anionic part. The two phases [Ni(dddt)2]x[Ni(dmit)2]y and [Pt(dddt)2][Ni(dmit)2] have been synthesized; both compounds are insulators (σpowder = 10−5 S/cm). The crystal structure of [Pt(dddt)2][Ni(dmit)2] is reported. The TTF[Ni(dddt)2]x salt has been also prepared; its room-temperature single crystal conductivity is 10−5 S/cm.

Molecular structure and solid-state properties of the two-dimensional conducting mixed-valence complex [NBu4]0.29[Ni(dmit)2] and the neutral [Ni(dmit)2](H2dmit = 4,5-dimercapto-1,3-dithiole-2-thione); members of an electron-transfer series

Single crystals of the mixed-valence radical salt [NBu4]2[Ni(dmit)2]7·2CH3CN {ignoring the solvent molecules, the formula may be written as [NBu4]0.29[Ni(dmit)2]; H2dmit = 4,5-dimercapto-1,3-dithiole-2-thione} were obtained by galvanostatid oxidation of [NBu4][Ni(dmit)2] in acetonitrile. The crystal structure of this compound has been determined at 118 K by X-ray diffraction studies: crystals are triclinic, space group P1 or P(assumed), with Z= 2 and unit-cell dimensions a= 13.425(2), b= 22.791(3), c= 24.183(4)A, α= 108.49(1), β= 103.02(1), and γ= 89.82(1)°. The structure was solved by direct methods and refined by least squares to R= 0.038 for 7 170 unique, observed, diffractometer data. It consists of thick layers of stacked Ni(dmit)2 entities parallel to (001), and separated by sheets of [NBu4]+ cations and CH3CN molecules. Strong π interactions within a stack are evidenced by the short stacking distances (3.48–3.57 A). There is also an extensive interleaving of the Ni(dmit)2 which involves close S ⋯ S interstack contacts. As a result, this structural arrangement is nearly two-dimensional. Conductivity measurements, carried out using a four-probe technique, show a high (σ= 1–10 Ω–1 cm–1 at 300 K) thermally activated (Ea= 0.1–0.02 eV) conductivity. Measurements of the conductivity along the b and a axes, using the Montgomery method, show a low anisotropy (σb/σaca. 1), consistent with the two-dimensional nature of this compound. The related neutral member of the [Ni(dmit)2]n– electron-transfer series (n= 0) has been isolated for the first time and its crystal structure determined: crystals are monoclinic, space group P21/a, with Z= 2, a= 17.108(9), b= 5.302(4), c= 7.720(4)A, and β= 77.09(4)°. The structure was solved by direct methods and refined by least squares to R= 0.041 for 772 unique, observed, diffractometer data. The structure of this semi-conducting compound consists of regular stacks of Ni(dmit)2 along the [010] direction with short interstack S ⋯ S distances.

Thin films and nanowires of molecule-based conductors and magnets

Abstract Nanowires of tetrathiafulvalene (TTF)[Ni(dmit)2]2 (dmit2−: 2-thioxo-1,3-dithiole-4,5-dithiolato) are obtained by successively dipping SSCC in acetonitrile solutions of (Bu4N)[Ni(dmit)2] and (TTF)3(BF4)2, observed by S.E.M. and AFM, and characterized by Raman micro-probe. Thin films of (TTF)[Ni(dmit)2]2 are obtained by the same process but using a new original silicon conversion coating (SiCC). The deposit is observed by S.E.M., and characterized by Raman micro-probe and transport measurements. TTF·tetracyanoquino-dimethane (TCNQ) individual nanowires and (TTF)[Ni(dmit)2]2 nano-needles scattered on the surface of the substrate or aggregated in wads, are obtained by the dipping process on oxide-free micro-rough silicon (O-f.Mr.Si). Adherent thin films of the V(TCNE)2 (TCNE: tetracyanoethylene) magnet are deposited on (O-f.Mr.Si) substrates by chemical vapor deposition from bis(benzene)vanadium and TCNE as precursors.