P. Gougeon

Ternary molybdenum chalcogenides: a route to new extended clusters

Within the last decade, the ternary molybdenum chalcogenides have played a major role in superconductivity: opening of the ternary superconductor range, competition between superconductivity and magnetism. It is mainly due to the pseudomolecular nature of the Mo 6 X 8 unit. In these materials, the Mo 6 cluster plays the electron-acceptor role and the number of valence electrons per molybdenum atom is related to the Mo-Mo intracluster bonding. The increase of this valence electron concentration (VEC) per molybdenum in new ternary chalcogenides corresponds to larger and larger clusters. These new extended clusters belong to a very large family containing the general Mo 3n X 3n+2 ( n ≥2) block unit. The tridimensional stacking of these building blocks provides more and more large channels and leads to a more and more anisotropic character of their physical properties.

Ultrasound study on YBa2Cu3O7−δ and GdBa2Cu3O7−δ single crystals

Abstract Sound velocity results on YBa 2 Cu 3 O 7-δ and GdBa 2 Cu 3 O 7-δ single crystals are reported. Discontinuities in the temperature derivatives of the longitudinal and shear sound velocities are observed at the superconducting transition T c . Hardening in both longitudinal (C 33 ) and shear (C 44 ) modes is observed below T c . Estimations of the second stress derivatives of T c are evaluated using a mean field thermodynamics analysis. A second phase transition is observed around 108 K.

Neutron scattering study of the antiferromagnetic ordering in YBa2Cu3O6+x powder and single crystal samples

Abstract Neutron diffraction experiments have been performed on powder and single crystal (2×5×2×1.5 mm 3 ) samples of YBa 2 Cu 3 O 6+x compounds. The reduced compound YBa 2 Cu 3 O 6 is found to order antiferromagnetically belowT N = 420 ± 10K. The magnetic moment is about 0.6 ± 0.05 μ B on the Cu 2+ sites and aligned perpendicular to the tetragonal c-axis. The magnetic structure is characterized by an in-plane wave vector|1/2 1/2 0| and an antiferromagnetic coupling between the two CuO 2 planes of the unit cell. The addition of oxygen induces only a decrease of T N and the moment value (290 ± 10 K, 0.45 ± 0.05 μ B and 245 ± 10 K, 0.28± 0.05 μ B , for x = 0.25 ± 0.03 and 0.38 ± 0.03, respectively). The long range order disappears abruptly aroundx = 0.40.

Ultrasound study of YBa2Cu3O7−δ single crystals

Abstract Sound velocity measurements on YBa 2 Cu 3 O 7− δ single crystals are reported. No anomalous behaviour within the experimental resolution (≊5 10 −5 ) was observed in the superconducting state. Present results confirm that the elastic properties of this system can be related to the thermodynamic model which predicts a very small velocity change (≊ 10 −5 ) at the superconducting transition temperature.

Synthesis, crystal structure and electrical properties of the first compound containing uniquely Mo12Se14 cluster units: Cs2Mo12Se14

Abstract The study of the Cs2Mo6Se6Mo6Se8 system has afforded the new structural type Cs2Mo12Se14 containing only Mo12Se14 cluster units. This compound crystallizes in the R3 space group with a stacking of the Mo12Se14 similar to that of the Mo6X8 and Mo12X14 unit in Mo6X8 and Tl2Mo9X11 (X = S, Se). Electrical resistivity measurements performed on single crystal indicate that Cs2Mo12Se14 is a metallic conductor with an unusual temperature dependence of its resistivity. Furthermore, it exhibits a superconducting transition at 4.7 K.

Ag3.6Mo9Se11: Premier compose´a`clusters Mo9 dans des motifs Mo9Se11

Abstract The synthesis, the crystal structure and the electrical properties of Ag 3.6 Mo 9 Se 11 are presented. The structure is orthorhombic (space group Cmcm ) and can be well described as a stacking of Mo 9 Se 11 units. The tridimensional arrangement of these units leaves a certain number of cavities in the chalcogen atom network. These cavities are partially occupied by silver atoms.

Promising thermoelectric properties in AgxMo9Se11 compounds (3.4 ≤ x ≤ 3.9)

Polycrystalline Mo9 cluster chalcogenides AgxMo9Se11 (3.4≤x≤3.9) have been prepared by powder metallurgy techniques, sintered by spark plasma sintering and characterized by x-ray diffraction. Their thermoelectric properties (electrical resistivity, thermopower, thermal conductivity) have been determined in the 300–800 K temperature range. The AgxMo9Se11 compounds show p-type conduction characteristics. The outstanding low lattice thermal conductivities give rise to a rather high value of the dimensionless thermoelectric figure of merit ZT of ∼0.65 at 800 K for x=3.8–3.9, making this family of materials particularly promising for thermoelectric power generation applications.

Magnetization jumps in YBa2Cu3O7 single crystal, up to 18T

Abstract The magnetization of a large single crystal (27 mg) of YBa 2 Cu 3 O 7 exhibits several new features associated to the anisotropy (and sample size) : for H // c axis large magnetization jumps are observed below about 15 K. The magnetization jumps are smaller, less spaced and observed only below 7 K for H ⊥ c. At high temperature the hysteresis loops, M(H), are convex lens shaped which gives a larger density of currents in high fields. The critical currents (6.8 10 5 A/cm 2 or 2.8 10 5 A/cm 2 for H parallel or perpendicular to the c axis, at 4.2 K) can be compared with values obtained for much smaller crystals.

Anisotropy of the superconducting magnetic field Hc2 of a single crystal of TmBa2Cu3O7−δ

Abstract Magnetoresistance of a single crystal of TmBa2Cu3O7−δ with a superconducting critical temperature of 86 K has been measured at nitrogen temperature with the magnetic field applied perpendicular and parallel to the (a,b) plane. At this temperature we estimate the coherence lengths to be: ξ ⊥ = 74 A and ξ // = 9 A . The strong anisotropy of the superconducting properties reveal the two-dimensional character for electron transport in these high Tc superconductors.

Structure and properties of Rb4Mo18Se20: First structural type containing the new Mo18Se20 cluster unit

Abstract The authors present the synthesis, crystal structure and electrical properties of Rb 4 Mo 18 Se 20 containing for the first time the new Mo 18 Se 20 cluster unit. This compound crystallizes in the R3 space group with a stacking of the Mo 18 Se 20 similar to that of the Mo 6 X 8 (X=S, Se) and Mo 12 Se 14 units in Mo 6 X 8 and Cs 2 Mo 12 Se 14 . The relationship of the Rb 4 Mo 18 Se 20 structure to those of the latter two compounds is discussed. Electrical resistivity measurements performed on single crystals show that Rb 4 Mo 18 Se 20 is a metallic conductor down to 4.4 K where a superconducting transition takes place.