Showing papers by "Colin Greaves published in 1997"

Anion substitutions and insertions in copper oxide superconductors

Abstract: After the discovery of high- superconductivity in multinary cuprates, a large number of different superconductors have been synthesized by varying the chemical composition and structural features of the insulating layers which connect the superconducting Cu - O planes. Although experimental studies initially focused on aliovalent cationic substitutions in these layers, it was subsequently discovered that oxyanion groups - - can also be located on some cationic sites to provide a further means of controlling the electronic properties of the superconducting regions. Even more recent developments have allowed the study of oxide - halide superconductors, synthesized by `soft chemistry' or high pressure techniques aimed at overcoming the thermodynamic instability of most of these materials. Superconductivity in these new families of oxide halides is of particular interest since critical temperatures appear to be at least as high as oxide analogues, and it provides a valuable guide to the importance of structural features, e.g. the apical anion, for superconducting properties.

Fluorination of the Ruddlesden–Popper type cuprates, Ln2–xA1+xCu2O6–y (Ln=La, Nd; A=Ca, Sr)

Abstract: The low-temperature (200–350 °C) fluorination of the Ruddlesden–Popper type cuprates, Ln2–xA1+xCu2O6–y(Ln=La, Nd; A=Ca, Sr) using F2 gas, CuF2 , and NH4F is reported. The incorporation of large levels of fluoride ions is observed for each of these fluorinating agents. The general characteristics of each method are discussed, and it is shown that for this system, fluorination mainly occurs by insertion of fluorine for reaction with F2 , substitution of fluorine for oxygen for NH4F, and a mixture of the two processes for CuF2 . For A=Sr, it is assumed that fluorine inserts mainly between the two CuO2 layers, since large expansions of the unit cell along the c direction are observed. No evidence for bulk superconductivity has so far been observed after fluorination.

Synthesis, structure, and XPS characterization of the stoichiometric phase Sr 2 CuO 2 F 2

Abstract: Thermogravimetric analysis has revealed that reduction of ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{F}}_{2+\mathrm{\ensuremath{\delta}}}$ in ${10%\mathrm{}\mathrm{H}}_{2}{/90%\mathrm{}\mathrm{N}}_{2},$ occurs in two distinct steps on heating up to 930 \ifmmode^\circ\else\textdegree\fi{}C. Whereas reduction to give ${\mathrm{SrF}}_{2},$ SrO, and Cu is complete above 800 \ifmmode^\circ\else\textdegree\fi{}C, a stable intermediate forms in the region between 250 and 450 \ifmmode^\circ\else\textdegree\fi{}C. This has been identified as a new tetragonal $(I4/mmm)$ phase, ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{F}}_{2},$ with unit-cell dimensions $a=3.967(1)\AA{}$ and $c=12.816(2)\AA{}.$ The structure has been determined from powder x-ray-diffraction data and is related to ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}$ (${T}^{\ensuremath{'}}$-type). Madelung energy, bond valence sum calculations, and F $1s$-XPS data clearly indicate that the ${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ ions occupy sites within the fluorite block $({\mathrm{Sr}}_{2}{\mathrm{F}}_{2})$ insulating layers. This contrasts with the ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ ($T$-type) structure which is adopted by ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{2}{\mathrm{F}}_{2+\mathrm{\ensuremath{\delta}}}.$

Substitution of SO42− anions in La1.85Sr0.15CuO4: structure and superconductivity

Abstract: The possibility of substituting S 2− and SO 4 2− ions into the superconducting cuprate, La 1.85 Sr 0.15 CuO 4 ( T c ≈ 40 K) has been investigated. In contradiction to previous reports, no evidence has been found for the successful incorporation of S 2− into this material, but SO 4 2− groups show limited solubility, and are situated at the copper sites. A change in the cell symmetry from tetragonal to orthorhombic occurs at a substitution level of between 5% and 10%. The presence of sulphur in the form of SO 4 2− groups has been confirmed by X-ray emission spectroscopy for samples prepared from both sulphides and sulphates. Structure refinements based on X-ray powder diffraction data indicate that the SO 4 2− groups are substituting at the copper sites. SQUID measurements have also been performed and show that superconducting properties are adversely affected as the level of substitution is increased.