Graeme B. Peacock

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.

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.

Thermal decomposition and chemical stabilization of mercurocuprate high-temperature superconductors

We have investigated the thermal decomposition of the mercurocuprate phases HgBa2 Ca2 Cu3 O8+ , HgBa2 CuO4+ and (Hg0.8 Re0.2 )Sr2 CuO4+ . In each compound, decomposition is initiated by the breaking of weak Hg-O bonds, but is driven to completion by the formation of thermodynamically stable barium carbonate phases (BaCO3 , BaCuO2 .(CO3 )x ). We show that the barium-free mercurocuprate (Hg0.8 Re0.2 )Sr2 CuO4+ is significantly more thermally stable than HgBa2 CuO4+ , allowing heat treatments at temperatures of at least 800 ?C for moderate periods of time and with only very minor decomposition observed. Further, the mechanism of decomposition for the Ba-free compound appears to be kinetically controlled, not suffering from the thermodynamic sinks (highly thermodynamically stable barium carbonates) available to Ba-based compounds.

(Hg, Sb)Ba2Ca2Cu3O8+delta thick films on YSZ substrates

Superconducting thick films of (Hg, Sb)Ba2Ca2Cu3O8+ have been fabricated on polycrystalline yttria-stabilized-zirconia substrates utilizing an Hg-free precursor film reacted with Hg vapour, released from a solid Hg source, in a sealed quartz tube. The resulting films have been studied by x-ray diffraction, scanning electron microscopy, ac susceptibility and resistance measurement techniques. A high quality Hg(Sb)-1223 superconducting thick film on YSZ can be fabricated by using a pre-melted Hg-free precursor film. The zero resistance superconducting transition temperature in the post-growth oxygenated thick film is in excess of 130 K and the transport critical current density for the film is 510 A cm-2 at 77 K.

On the role of transition metal elements as structure-stabilising agents in cuprate superconductors

Abstract We outline here several key aspects of the crystal chemistry of (high-valent) transition metal-stabilised cuprate superconductors. An examination of the chemistry of the mercurocuprate homologous series HgBa 2 Ca n −1 Cu n O 2 n +2+ δ leads us to propose that the inherent instability of the analogous strontium system (HgSr 2 Ca n −1 Cu n O 2 n +2+ δ ) arises from bond-length mismatch at the interface between the rock-salt and perovskite layers. A simple set of principles can now be formalised for the stabilisation of strontium mercurocuprates via partial substitution of mercury by a high-valent transition metal within the crystal structure of interest. The chemistry of three different cuprate systems, in which high-valent transition metal ions adopt a pivotal role as structure-stabilising agents , are illustrated here. First, we demonstrate how a combination of structural stabilisation (by the substitution of rhenium onto the mercury site) combined with control of the copper oxidation state (via judicious control of the Nd 3+ :Ca 2+ ratio), allows the successful synthesis of superconducting strontium analogues of HgBa 2 CaCu 2 O 6+ δ in the (Hg 1− x Re x )Sr 2 (Nd 1− y Ca y )Cu 2 O 6+ δ system. Second, we report the successful preparation of superconducting Hg 1− x Cr x Sr 2 CuO 4+ δ thin films, by laser ablation and ex situ mercuration. Finally, we describe a new family of layered cuprates with the general chemical formula (Cu 1− x TM x )Sr 2 (Y 1.2 Ce 0.8 )Cu 2 O 10− δ , with TM=Ti, Cr, Mo, W, V and Re, in which these transition metal substituents, once again, fulfil a key role in structure stabilisation.

Improvement of critical current density in Sb-doped HgBa2Ca2Cu3O8+δ superconductor prepared by Hg vapour diffusion process

Abstract Antimony-doped Hg-1223 superconductors have been synthesised from the reaction of a Sb-doped precursor with elemental Hg vapour released from a reactant in sealed silica tubes (the Hg vapour diffusion process). The Hg vapour pressure can be controlled by a small addition of Se in the reactant bars. The phase purity and microstructure of the material have been characterised by XRD and SEM and the superconducing properties measured by electrical resistance and AC susceptibility. High quality superconducting material can be fabricated by this Hg vapour diffusion process with a suitable Se addition in the reactant bars. The microstructure reveals dense, plate-like grains with reduced void formation, when compared with material prepared by a solid state reaction route. Resistance and AC susceptibility measurements give a T c of 132 K. The diffusion process route significantly enhances the intergrain critical current density ( J c = 2.5×10 7 Am −2 at 77 K and zero field) and reduces the degradation of J c in a magnetic field.

Synthesis and Structure of Hg 1−x Cr x Sr 2 CuO 4+δ Mercurocuprates

We report preliminary results concerning the synthesis and structural characterisation of the chromium stabilised 1201 phases: Hg1−xCrxSr2CuO4+δ. A systematic study of the formation of phases in this system has been undertaken, together with a combined powder neutron and synchrotron x-ray diffraction study, to address the issue of chromium clustering and ordering in this system.

Fluorination of Underdoped Mercurocuprate Superconductors

We present the results of a study into the incorporation of fluorine into the mercurocuprate superconductors. In all cases, the effect of fluorination has been to increase the Tc of underdoped superconductors to the maxima reported. This has important ramifications for the role of the interstitial oxygen in the mercurocuprate superconductors. To assist us in our study, we have, additionally, developed a novel synthetic procedure for preparing HgBa2CuO4+δ in a highly underdoped state. Remarkably, this material shows an increase in Tc, upon fluorination, of 60K!

Anisotropic penetration depth measurements of High-Tc superconductors

A systematic study of the anisotropic magnetic penetration depth lambda for several high-T-c superconductors is presented. It is shown that the existence of the linear term in the low temperature in-plane penetration depth is independent of the number of

Mercurocuprates: The Highest Transition-Temperature Superconductors

With these words [1] Heike Kamerlingh-Onnes summarised the existence of “a new state” of mercury. This discovery (initially made in 1911 [2]) gave the world the first superconductor — elemental mercury — which entered this new state of matter below a superconducting transition temperature (Tc) of 4.2K. Eight decades later, mercury once again took centre stage, but this time in a chemically combined state, as the [O-Hg-O]2- unitwithin a mixed-metal oxide possessing superconductivity up to a record-high temperature of some 133K.