J. Padiou

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.

Nouveau type structural dans la chimie des phosphures : Le compose ternaire Ni20Zr6P13. Preparation, structure et proprietes

Abstract Two new compounds Ni 12 Zr 2 P 7 and Ni 20 Zr 6 P 13 were synthesized in the NiZrP system by reacting the constituent elements. Ni 12 Zr 2 P 7 is of the Fe 12 Zr 2 P 7 -type while Ni 20 Zr 6 P 13 appears as a new structural type in the chemistry of transition metal phosphides. Its unit cell is hexagonal with space group P6 and contains one formula unit. The X-ray structure was studied from three-dimensional single-crystal counter data and was refined down to R = 0.040 for 221 independent reflections. The structure of Ni 20 Zr 6 P 13 can be described as built up by two groups of three phosphorus trigonal prisms occupied by the zirconium atoms. In each group, the |ZrP 6 | prisms are linked together by common edges in order to generate triangular phosphorus sites occupied by nickel atoms. In addition, nickel atoms are also in tetrahedral and square-planar pyramidal phosphorus sites. A comparative study with the Fe 2 P- and Co 4 Hf 2 P 3 -type structures having the same metal/non metal ratio as in Ni 12 Zr 2 P 7 and Ni 20 Zr 6 P 13 is also discussed. A nearly temperature independent paramagetism and a metallic conduction deducted from magnetic and electrical measurements exhibit the metallic behavior for these new compounds.

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.

Polymorphism and physical properties of LnNi2As2 compounds (Ln = La → Gd)

Abstract Ternary BaAl 4 -type derivative structures are frequently found with lanthanoid transition metal silicides, germanides and pnictides with the Ln atoms occupying the Ba sites and the other atoms distributed in an ordered fashion on the Al sites. The ternary phase diagrams LnNiAs were investigated and the compounds LnNi 2 As 2 were obtained with Ln = La → Gd. They crystallize either with the body-centered tetragonal ThCr 2 Si 2 -type (L.T. form), space group I4/mmm, or primitive tetragonal CaBe 2 Ge 2 -type structure (H.T. form), space group P4/nmm. The X-ray structures of LaNi 2 As 2 and CeNi 2 As 2 were refined down to residual values of 0.059 and 0.053 in their L.T. and of 0.028 and 0.060 in their H.T. forms. Electrical and magnetic measurements were performed on single crystals; they exhibit a metallic conductivity and a magnetic ordering at low temperature when Ln = Ce, Pr, Sm, Eu. Above 50 K, the compounds have a normal C.W. behavior with magnetic moments consistent with those of the free Ln 3+ ions, except for Eu which is divalent. LaNi 2 As 2 (L.T. and H.T.) is paramagnetic slightly temperature dependent.

Surface and in-plane characterization of YBa2Cu3O7 thin films grown by laser ablation

Abstract We have used reflection high energy electron diffraction (RHEED) and oscillating crystal X-ray diffraction to analyze the surface and in-plane structural characteristics of YBa 2 Cu 3 O 7 (YBCO) thin films that were grown in-situ by laser ablation onto substrates of (100) MgO and (100) SrTiO 3 . We obtain RHEED streaks, characteristic of atomically smooth surfaces and indicative of epitaxial order, for these thin films. Oscillating crystal diffraction also indicates that the YBCO film axes are collinear with the substrate axes. The films grown on (100) substrates are solely c -axis oriented (perpendicular to the plane of the film) as determined by θ-2θ diffractometry and the FWHM of rocking curves about the (005) reflection are 0.3° to 0.5° for the best films on SrTiO 3 substrates and 0.4° to 0.6° for films grown on MgO. Weissenberg photography allows us to determine the in-plane structure of some films grown on MgO that display anomalous surface features seen by RHEED and by scanning electron microscopy.

Crystal structure and normal state physical properties of HoxMo6S8 (0.88≤x≤1.0) single crystals

Abstract A new approach to the crystal growth of rare earth molybdenum chalcogenides resulted in the production of large well-shaped single crystals of the Ho x Mo 6 S 8 system. Crystal structure refinement showed the existence of a homogeneity domain at x ⩽ 1.0. The resistivity of Ho 1.0 Mo 6 S 8 was measured, and residual resistivity ratios close to 20 were obtained. Preliminary susceptibility measurements were performed on stoichiometric and non-stoichiometric samples. All samples were superconductors at about 2.0 K.

Structure de PbU2Se5

PbU2Se5 crystallizes in a monoclinic unit cell a = 8,605 A; b = 7,788 A; c = 12,27 A; β = 90° of space group P21c, Z = 4. The crystal structure has been determined for 1251 independant reflections and refined to a final R value of 0,068. Two kinds of uranium sites are found; the average U-Se distances are 2,97 A for eight-coordination and 2,88 A in seven-coordination. The average Pb-Se distance for eight-coordination is 3,20 A. This structure explains the solid solution between PbU2Se5 and U3Se5.

YBa2Cu3O7 films epitaxially grown on MgO, LaAlO3, SrLaAlO4 and Al2O3 substrates structural and superconducting properties in correlation with the microwave surface resistance and the far-infrared transmittance

YBa2Cu3O7 epitaxial films have been grown by laser ablation on MgO, LaAlO3, SrLaAlO4 and Al2O3 substrates. Films on LaAlO3 have excellent structural, superconducting and microwave properties, reflected by surface-resistance (RS) values systematically in the range 0.5–1 mΩ (10 GHz, 77 K). On MgO substrates, such low RS (<0.50 mΩ) values can be reached on both fully “normal-0°” and “45°” in-plane oriented YBa2Cu3O7 films, whereas in mixed (0°–45°) films the presence of the high-angle grain boundaries leads to a surface resistance larger than 100 mΩ. On SrLaAlO4 and bare Al2O3 substrates, despite good superconducting properties, obtained for sapphire due to the specific substrate surface treatment, the structural quality and surface resistance are moderate. The latter results are in complete agreement with the correlation we have demonstrated between structural quality and surface resistance as well as far-infrared transmittance. We also point out the importance of a RS determination as an intrinsic superconducting parameter: the absence of a direct correlation with low-frequency measurements shows that the latter are not sufficient to fully characterize the superconducting thin films.