I. Bryntse

New complex copper oxides: HgBa2RCu2O7 (R = La, Nd, Eu, Gd, Dy, Y)

Abstract A new family of complex copper oxides, HgBa 2 R Cu 2 O 7 ( R = La, Nd, Eu, Gd, Dy, Y), has been synthesized and studied by X-ray powder diffraction, transmission electron microscopy and EDS analysis. The crystal structure of one of these compounds was determined. The Hg-atoms are in rock salt like blocks alternating with perovskite like ones where the copper atoms are located. This family can be considered as a first example of Hg-containing complex copper oxides with intergrowth structures (“1212” structural type). Magnetic measurements down to 12 K did not show noticeable diamagnetic effects.

Sol-gel synthesis and superconducting properties of HgBa2CaCu2O6+δ

Abstract Composition and superconducting properties of powder samples of Hg-1212, HgBa 2 CaCu 2 O 6+δ , were investigated. The specimens were prepared under various synthetic conditions by reacting the precursor Ba 2 CaCu 2 O 5±δ , synthesized by the sol-gel technique, and HgO in closed ampoules at two different temperatures. The initial ratio (HgO:precursor) was in most cases larger than 1.0, which gave a Hg-1212 phase of better purity. An excess of 40% HgO gave the purest product. The thermal decomposition of the precursor gel and the subsequent formation of the superconducting phases are described. The synthesized materials were characterized by X-ray powder diffraction, IR spectroscopy, and scanning and transmission electron microscopy. Common impurities were Hg-1201, CaHgO 2 and unreacted precursor. After annealing in flowing oxygen at 300°C for 6 h the samples became superconducting at a T c onset of 124–127.5 K, determined by AC susceptibility measurements. The purest superconducting powder contained 95% of Hg-1212, judged from XRD, and had a superconducting volume χ ′ ≈ −2 × 10 −4 m 3 /kg (−20 × 10 −3 emu/g) corresponding to approximately 100 vol.% of the sample.

(Hg23Tl43)Ba2CaCu2O8+δ, a 2212 type superconductor

A new Hg, Tl based superconductor, Hg23Tl43Ba2CaCu2O8+δ, has been synthesized in sealed quartz ampoule s at temperatures 720–820°C. The structure is similar to that of Tl-2212, Tl2Ba2CaCu2O8+δ, with a tetragonal unit cell (a=3.8544(4) A, c=29.385(5) A, space group 14/mmm). The composition of the phase was determined by energy-dispersive spectrometry analysis and it was studied with X-ray powder diffraction, as well as scanning and transmission electron microscopy. By AC susceptibility measurements the onset of the superconducting transition temperature was found to be 100 K.

Fluctuation conductivity of polycrystalline Hg, Tl-1223

Detailed resistivity measurements were made for polycrystalline Hg1-xTlxBa2Ca2Cu3O8+delta (x = 0 and 0.2) in zero field and above the superconducting T-c. The fluctuation conductivity Deltasigma is analyzed. With decreasing temperature two crossover temperatures could be identified for each sample; i.e. T* from two to three dimensional fluctuations in the mean field region (MFR), and T-G from the MFR to the critical region. All results are in agreement with the Lawrence-Doniach model. The c-axis coherence length and the interlayer coupling factor were obtained

Effect of thallium oxide, Tl2O3 on the formation of superconducting HgBaCaCuO films

Abstract We show that a small doping of Tl 2 O 3 in the bulk material significantly promotes the formation of superconducting HgBaCaCuO films during encapsulated post-annealing. We also demonstrate that by using this method good quality superconducting HgBa 2 CaCu 2 O 6+δ films can directly be prepared using Hg-free thin film precursors. As-prepared films display a strong (00 l ) texture, layer-by-layer growth surface morphology and exhibit superconducting transition temperatures, T c,zero s as high as 117 K as determined from magnetic and electric transport measurements.

Superconductivity in HgBa2Ca2Cu3O8 + δ synthesized by different methods

The composition and superconducting properties of powder samples of Hg-1223, HgBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+{delta}}, were investigated. The specimens were prepared by a single-step method using individual metal oxides, and by a two-step method in which the precursor Ba{sub 2}Ca{sub 2}Cu{sub 3}O{sub 7{+-}{delta}} was first synthesized by sol-gel method and then mixed with HgO. The final annealing was performed in sealed quartz ampoules at 665 C or 860 C, and the purity of the obtained samples was compared. After oxygen treatment at 300 C for 6 h the samples showed superconductivity at a {Tc}{sub (onset)} of approximately 134 K, as determined from AC susceptibility measurements. The materials were also studied by X-ray powder diffraction and scanning and transmission electron microscopy.

A novel synthesis route to the mercury-containing superconductor HgBa2CaCu2O6+δ partly based on the sol–gel technique

A sol–gel-based preparation route of precursors to be used in the synthesis of the high-Tc superconductor HgBa2CaCu2O6+δ(Hg-1212) has been investigated. The samples were synthesized by reacting the precursor ‘Ba2CaCu2O5+δ’, made from acetate-tartrate solutions containing appropriate amounts of Ba, Ca and Cu ions, and HgO in closed ampoules at 630–730 °C and yielded the 1212 phase as the major phase together with the starting precursor and Hg(I). The advantages of this method are that the impurity CaHgO2 did not form and that the finegrained powders reacted only slightly with the silica tubes. The onset superconducting transition temperatures were 111 and 127 K, for the as-synthesized and oxygen-treated samples, respectively.

Synthesis and structure of a perovskite related oxide, Bi23Cu3Ti4O12

The phase Bi23Cu3Ti4O12 has been synthesized and studied with electron and powder diffraction. The crystallographic study was performed by full-profile analysis of powder diffraction data obtained with a STADI/P diffractometer, using the mode of symmetric transmission geometry. The structure is of the perovskite related CaCu3Mn4O12 type. The title compound crystallizes in the cubic space group Im3 with a = 7.4175(3) A and z = 2. Bismuth occupies the icosahedral A-sites of ideal perovskite. The eight nearest oxygen neighbours of copper form two perpendicular rectangles of different size, and titanium occupies the B-sites of ideal perovskite.

Synthesis and investigation of Sr-containing 223 and 1222 complex oxides in the R-Sr-Ce-Cu-O system (R = Pr, Nd, Sm-Gd, Dy-Yb and Y)

Abstract An extended family of layered complex copper oxides has been synthesized in the system R 2 O 3 -SrO-CeO 2 -CuO (R = Pr, Nd, Sm-Gd, Dy-Yb and Y). The compounds were studied by X-ray powder diffraction, electron microscopy and EDS analysis. Eu 1.6 Sr 1.8 Ce 0.6 Cu 3 O 8.6 ( a = 3.8439 (2) A, c = 28.09 (1) A) and Y 1.5 Sr 1.8 Ce 1.0 Cu 2.7 O 8.6 ( a = 3.8160 (7) A, c = 29.120 (5) A), belonging to two structural groups of 223- and 1222-type, respectively, were chosen for Rietveld structure refinement. Their structures can be described as 14-layered stackings along the c -axis. The compounds with larger lanthanides (R=Pr-Gd) are isostructural with Ba-containing 223 phases, e.g. (R,Ba) 2 (R, Ce) 2 Cu 3 O 8+δ . Among the synthesized compounds only the ones with R=Sm and Eu have been shown to be superconductors ( T c =30 and 36 K, respectively) after high-pressure oxygen treatment (120 atm). 1222 phases with smaller rare earth elements (R=Dy-Yb and Y) differ from 223 phases in composition and lattice parameters, having structures which are more similar to (Pb, Cu)-1222, e.g. (Pb, Cu) (Eu, Ce) 2 (Sr, Eu) 2 Cu 2 O 8+δ . Compounds of 1222-type did not become superconducting even when treated with high-pressure oxygen.

A new procedure for the preparation of Tl2Ba2CaCu2O8-δ characterization by neutron diffraction, electron diffraction and HREM

Abstract A new procedure for the preparation of Tl 2 Ba 2 CaCu 2 O 8-δ is presented. In the first step stoichiometric amounts of Cu(NO 3 ) 2 ·3H 2 O, Ba(OH) 2 ·H 2 O, CaO and Tl 2 O 3 are reacted in a closed alumina crucible at 760°C to form a mixture of Tl- 2212, CuO, CaO and other oxide phases. In the second step this material is milled, pelletized and fired in a gold-sealed alumina crucible at 860–900°C for three to eight hours. Three batches of Tl-2212 prepared by this method were investigated showing less than 1% loss of thallium while T c was close to 110 K. Rietveld refinements using time-of-flight neutron-diffraction data demonstrate that the method produces pure material in a reproducible way and that the materials are substoichiometric with respect to oxygen (δ=0.3). Investigations using electron diffraction and high-resolution electron microscopy indicate the absence of stacking faults.