I. Gameson

Structural studies of trigonal lamellar particles of gold and silver

The results of detailed structural studies of trigonal lamellar particles of both gold and silver are presented. The particles have been characterized both in sol by means of optical spectroscopy and powder X-ray diffraction and ex sol using high resolution electron microscopy in both plan view and profile imaging modes. The results of these studies have indicated that the particles have a trigonal outline and are shortened along a ≺111≻ direction to give a plate-like morphology. The presence of small numbers of parallel {111} twin planes has also been confirmed and used to explain the presence of the formally forbidden ⅓{422} reflections observed in plan view. The precise structural requirements for the observation of such reflections has also been confirmed using multislice calculations. Possible growth mechanisms for these particles are also discussed.

Superconductivity up to 64 K in the copper oxyfluorides Sr2−xAxCuO2F2+δ(A = Ca, Ba) prepared using NH4F as a fluorinating reagent

Abstract We report here a simple and efficient synthetic route to high-temperature superconducting oxyfluorides. Solid phase fluorination of Sr 2 CuO 3 with NH 4 F at relatively low temperatures produces superconducting Sr 2 CuO 2 F 2+δ ; this simple preparation route eliminates the need for F 2 gas. Importantly, as-synthesised samples from the NH 4 F route appear to have the optimally doped value of the superconducting transition temperature, T c , of 46 K. Moreover, Ba doping for Sr results in an increase in T c , for both F 2 gas and NH 4 F routes, to a maximum value of 64 K for Sr 1.4 Ba 0.6 CuO 2 F 2+δ ; this represents the highest ever recorded value for a material with the confirmed La 2 CuO 4 structure.

An improved route to the synthesis of superconducting copper oxyfluorides Sr2−xAxCuO2F2+δ (ACa, Ba) using transition metal difluorides as fluorinating reagents

Abstract We report here a simple and efficient synthetic route to high temperature superconducting oxyfluorides. Solid phase fluorination of Sr 2− x A x CuO 3 (ACa, 0 ⩽ x ⩽ 2.0; ABa, 0 ⩽ x ⩽ 0.6) with the transition metal difluorides, CuF 2 , ZnF 2 , AgF 2 , NiF 2 , at relatively low temperatures (225–250°C) produces superconducting Sr 2− x A x CuO 2 F 2+δ with a maximum T c of 64 K (for ABa, x = 0.6). This simple preparation route eliminates the need for F 2 gas, and at the same time produces negligible (Sr/A)F 2 impurity, unlike the corresponding fluorination with NH 4 F. Using this route the synthesis of the oxyfluoride, Ba 2 CuO 2 F 2+δ , is also reported for the first time. These studies and the examination of the fluorination of other cuprate systems have shown that this is a powerful and versatile fluorination route.

Fluorination of mercury-based high-temperature superconductors

Abstract There are numerous reports of cationic substitutions on the mercurocuprate very high-temperature superconductors of general formula HgBa 2 Ca n−1 Cu n O 2 n +2+δ . Such investigations have been carried out in an attempt to mimic, within chemical substitution at room pressure, the spectacular effect of external pressure, which has raised the superconducting transition temperature, T c , to record-high values above 160 K. In the present work, we report the post-synthetic incorporation of elemental fluorine as an interstitial anion into the first five congeners of the mercurocuprate family of superconductors. The effect of fluorine incorporation by this route appears to mimic the more usual oxygenation process in giving maximum T c values of 96, 128 and 135 K for the single, double and triple CuO 2 -layer materials, respectively, while the T c of the five CuO 2 -layer compound are increased to an extent only previously observed by high-pressure oxygenation. We comment upon a derived linear correlation of T c with the crystal a -lattice parameter and compare this behaviour against that of high-pressure studies. Interestingly, it appears that intrinsic ‘chemical factors’ dictating the average copper oxidation state have the more marked effect upon T c than the application of external hydrostatic pressure.

Single-step synthesis and crystal structure of HgBa2CuO4+δ with a Tc of 97 K

Abstract Single-phase samples of HgBa 2 CuO 4+δ (Hg-1201) have been synthesised from a reaction between a pre-synthesised precursor Ba 2 CuO 3 and HgO, using a rapid-heat rapid-cool technique. This method yields high-quality samples with (as-synthesised) superconducting transition temperatures, T c up to 97 K, as defined by the onset of diamagnetism. The phase purity and crystal structure were identified by powder X-ray diffraction. In addition, powder neutron diffraction was employed to elucidate the structure, particularly the interstitial O3 site, which provides the “holes” required for inducing superconductivity in this material. We observe some disorder in this material, including the displacement of the O3 site from its ideal (0.5, 0.5, 0) position; this has not been observed in previous studies. A fourth oxygen site, labelled here O4, was located between two copper sites at (0.086, 0.5, 0).

The surface chemistry of HgBa2CuO4+σ an investigation by high-resolution electron microscopy

Abstract The surface chemistry of the high- T c superconductor HgBa 2 CuO 4+σ has been studied using high-resolution electron microscopy. The material was found to be sensitive to both acetone and ethanol, but was quite stable under the electron beam. When crushed in air, the initial surface of the crystals was found to be coated by disordered Ba and Cu oxides with a Ba : Cu ratio 2:1 as determined by energy dispersive X-ray microanalysis. Following annealing under the electron beam, these disordered coating layers of approximately 20 A thickness recrystallized into Ba 2 CuO 3 crystallites which intergrew with the bulk HgBa 2 CuO 4+σ on a variety of surface planes.

Structure of the Li1+xTi2−xO4 superconducting system: A neutron diffraction study

The location of the Li+ ions in the superconducting Li1+xTi2−xO4 solid solution (0≤x≤0.33) has been determined by refinement of time-of-flight powder neutron diffraction data. The spinel structure is maintained throughout, with the substituting Li+ directly replacing the Ti ions on the spinel B-sites (16d). AC susceptibility measurements of the superconducting transitions revealed a slight reduction in Tc and a rapid reduction in the apparent volume fraction of superconductor at the metal-insulator boundary (x ≈ 0.15), whilst for x<0.1 little difference in the superconducting properties was observed.

Fabrication of high-temperature superconducting HgBa2CuO4+δ within silver-sheathed tapes

Abstract We report, here, the successful synthesis of the high-temperature superconductor HgBa 2 CuO 4+δ within a silver-sheathed tape, showing a superconducting transition temperature of up to 96 K. We have also identified several control parameters — most notably thermal treatment and cycling — which lead to enhanced superconducting volume fractions within the tapes. The most dramatic effect is that of air-quenching of the sealed tapes from the ceramic synthesis temperature, leading to enhancement of the superconductor phase-purity. In addition, the more subtle, but equally important, factor of thermal cycling at progressively increasing temperatures leads to an increase in the inter-granular connectivity and a stronger magnetic signal. The fabrication of the HgBa 2 CuO 4+δ phase in a silver tape, in this manner, is an important step towards the goal of high-temperature superconducting tapes of the mercury-cuprate materials.

Induction of superconductivity in pulsed laser ablated La2CuO4 thin films by post-deposition fluorination

Abstract Pulsed laser ablation has been used to fabricate La2CuO4 thin films. Superconducting properties have been successfully induced in the films by an ex-situ, post-ablation annealing process in F2 gas resulting in a Tc (onset) of 36 K. The presence of two slightly different c-axis expanded phases in the X-ray diffraction data of the fluorinated films implies a degree of inhomogeneity in F2 uptake. Critical current densities (Jc) and the irreversibility line have been established from hysteresis cycles. A Jc of ∼ 106 A cm−2 for a typical film was observed at 10 K in zero field.

Rapid synthesis of colossal magnetoresistance manganites by microwave dielectric heating

Colossal magnetoresistance manganites have been synthesised by the microwave dielectric heating of metal nitrates, which act both as effective microwave susceptors, and a ready source of oxidizing NO2 to induce mixed valency.