M. Potel

New pseudo-one-dimensional metals: M2Mo6Se6 (M = Na, In, K, TI), M2Mo6S6 (M = K, Rb, Cs), M2Mo6Te6 (M = In, TI)

Abstract We present a new series of ternary chalcogenides, derived from divalent molybdenum: M2Mo6X6. These compounds crystallize in a hexagonal lattice with a ≈ 9 A, c ≈ 4.5 A, and space group P6 3 m . The compounds are characterized by clusters (Mo3)1∞ in the form of linear chains, resulting from a linear condensation of Mo6 octahedral clusters. The (Mo3)1∞ clusters are well separated from each other, with the shortest MoMo intercluster distance larger than 6 A. The resulting pseudo-one-dimensional structures show remarkable anisotropy of physical properties.

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.

A new pseudo-one-dimensional superconductor: Tℓ2Mo6Se6

Abstract The properties of a new superconducting pseudo-one-dimensional compound, Tl2Mo6Se6 are presented. The crystal structure of this material contains linear chains in the form of infinite stacks of Mo6 clusters. The anisotropy in the normal state resistivity is found to be of the order of 103 and the parallel upper critical field is 26 times higher than the perpendicular one.

Crystal and magnetic structure of the uranium digermanide UGe2

Abstract The crystal structure of the uranium digermanide UGe 2 was determined and refined from single crystal X-ray diffraction data to R = 0.040, Rw = 0.052. UGe 2 crystallizes with the orthorhombic ZrGa 2 type ( Cmcm ) instead of the ZrSi 2 type ( Cmcm ) as was previously assumed on the basis of rather old and unreliable results. Our magnetic study of a polycrystalline are melted UGe 2 sample confirms the onset of ferromagnetic interactions below ca. 52 K. Neutron powder diffraction studies show that the structure is collinear ferromagnetic, with uranium magnetic moments parallel to the a -axis of the orthorhombic cell: at T = 1.4 K, the uranium magnetic moment value is M 1 = 1.42(4) μ B.

Magnetic relaxation effects in PbMo6S8 superconducting single crystals: A comparison with high-Tc superconductors

Abstract Magnetic measurements performed on PbMo6S8 single crystals show that this classic isotropic superconductor (Tc = 14.5 K) presents surprisingly similar irreversibility and relaxation properties as those generally attributed to layered high-Tc superconductors. An irreversibility line H ∗ = H ∗ 0 (1-t) m with m∼ 3 2 is observed for H ⪅ 1 kOe. From the magnetic relaxation rates measured below this irreversibility line, activation energies U0 around 15 meV are derived within the flux creep picture. The similarity with the high-Tc oxides is explained by the very short coherence length ξ ≈ 25 A of PbMo6S8 which also makes this compound sensitive to disorder and local defects.

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.

Nouveaux chalcogénures et chalcohalogénures à clusters tétraédriques Nb4 ou Ta4

The new chalcogenides GaNb4X8 (X  S, Se), GaTa4Se8 and the selenoiodide NbSeI were obtained by high temperature synthesis from the elements. They crystallize with cubic symmetry (space group, F43m). The structure can be described as being made of [Me4X4] units (Me  Nb, Ta), with Me-Me bonds in tetrahedral Me4 clusters. The compounds are isostructural with the molybdenum chalcogenides MMo4S8 (M  Al, Ga), GaMo4Se8 and the thiohalides Mo4S4Y4 (Y  Cl, Br, I). Their magnetic properties seem to agree well with those of intermetallic compounds with very high densities of states.

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.