S. LeFloch

Single crystal X-ray diffraction study of the HgBa2CuO4+δ superconducting compound

Abstract A high precision X-ray diffraction analysis up to sin θ / λ = 1.15 of a HgBa 2 cuO 4+δ single crystal having a T c of ≈ 90 K is presented. The cell parameters are a = 3.8815(4), c = 9.485 (7) A . The refinements indicate the existence of a split barium site due to the presence of excess oxygen in the mercury layer. The position of this excess oxygen might be slightly displaced from the (1/2 1/2 0) position. A 6% mercury deficiency is observed. Models, including mercury defects, substitution by copper cations, or carbonate groups, are compared. However, we obtain no definite evidence for either of the three models. A possible disorder of the Hg position, due to the formation of chemical bonds with neighbouring extra oxygen anions, could correlate to the refinements of mixed species at the Hg site. A low temperature single crystal x-ray diffraction study, and comparison of refinements for the same single crystal with different extra oxygen contents, are in progress to help clarify this problem.

AuBa2(Y1−x, Cax)Cu2O7: a new superconducting gold cuprate with Tc above 80 K

Abstract We have synthesised by the high pressure-high temperature technique a new layered gold cuprate of chemical formula AuBa2(Y1−x,Cax)Cu2O7 (0 ≤ × ≤ 0.4) (hereafter denoted as Au-1212). The structural properties of this compound are investigated by means of electron microscopy and powder X-ray diffraction. The average structure (orthorhombic Pmmm, a = 3.8298(5) A , b = 3.8420(5) A , c = 12.111(1) A for nominal composition x = 0.4) is very similar to that of YBa2Cu3O7, with Au3+ cations in square planar coordination sharing corners to form chains running along the b axis. Electron microscopy reveals the presence of a superstructure with double b parameter, which is attributed to a long range ordered zig-zag conformation of the square chains. For x = 0, the formal copper valence is 2 + and the compound is insulating. Doping of the conducting CuO2 planes is achieved by the partial substitution of yttrium by calcium, up to x ≈ 0.4. Upon substitution, the compound becomes superconducting with a maximum Tc at ∼ 82 K for x ≈ 0.4.

Reaction mechanism in the high-pressure synthesis of Hg-cuprates: an in-situ synchrotron diffraction study

Abstract The high pressure-high temperature (HP-HT) synthesis of the Hg-1201 and Hg-1223 superconducting cuprates was analysed by real-time, in-situ synchrotron diffraction. The main parameter controling the synthesis is the temperature which in turn strongly depends on pressure. The formation of Hg-1223 involves the apparition of Hg-1212, which then reacts with Ca 2 CuO 3 and CuO to yield Hg-1223.

High pressure synthesis and structure of the superconducting mercury cuprates (Hg1−xMx)Ba2Can−1CunO2+2n+ϖ with M = C, S.

Abstract In mercury superconducting cuprates synthesized at high pressure, partial carbon substitution on the mercury site occurs when the precursors are not absolutely carbon free. Hg-12(n−1)n samples made from carbonate-containing precursors show lower T c s and smaller c parameter. A model of the structural arrangement of the CO 3 −2 groups is proposed from neutron diffraction data and crystal-chemistry considerations. The third oxygen of the oxyanion is situated in the (Hg,C)O ϖ layer and hinders the incorporation of extra-oxygen atoms. The (Hg 1−x C x )Ba 2 CuO 4+ϖ phase diagram shows three different phases, namely HgBa 2 CuO 4+ϖ , Hg 0.5 C 0.5 Ba 2 CuO 4+ϖ and CBa 2 CuO 4+ϖ . The latter two phases are not superconducting due to the insufficient hole concentration on the superconducting layer. Sulphur can also partially replace mercury in Hg 1−x S x Ba 2 CuO 4+ϖ and forms a solid solution up to x=0.15. Neutron diffraction shows that the sulphur atoms arrange as SO 4 −2 oxyanions. The larger SO distances induce steric limitations to the presence of other sulphate anions in the same mesh.

Pressure-induced polyamorphism in TiO{sub 2} nanoparticles

Two different nanometric (6 nm) TiO{sub 2} compounds, anatase polycrystals and amorphous particles, were investigated under high pressure using Raman spectroscopy. Nanoanatase undergoes a pressure-induced amorphization. The pressure-induced transformations of this mechanically prepared amorphous state are compared with those of a chemically prepared amorphous particles. In the mechanically prepared amorphous state, a reversible transformation from a low-density amorphous state to high-density amorphous state (HDA1) is observed in the range 13-16 GPa. In the chemically prepared sample, a transformation to a new high-density amorphous state (HDA2) is observed at around 21 GPa. Further compression leads to the transformation HDA2{yields}HDA1 at {approx}30 GPa. We demonstrate that depending on the starting amorphous material, the high-pressure polyamorphic transformations may differ. This observation indicates that pressure is a suited tool to discriminate between nanomaterials apparently similar at ambient conditions.