M. Mair

57Fe, 119Sn and 151Eu Mössbauer studies of (Tl,Pb)(Ba0.2Sr0.8)2Ca2Cu3Oy superconductors

Abstract The introduction of the Mossbauer nuclei 57 Fe , 119 Sn and 151 Eu into Tl-based 12( n −1) n superconductors was investigated. The doped Tl-based superconductors were characterized with respect to critical temperatures, critical currents, microstructure and composition of the superconducting and secondary phases. The samples were then analyzed by Mossbauer spectroscopy. Energy dispersive X-ray (EDX) analysis showed that both 57 Fe and 151 Eu were incorporated in the superconducting crystallites. Sn was found in secondary phases. Incorporation of Eu into the superconducting phase favored the formation of the Tl-1212 phase. The isomer shift and the electric field gradient derived from the 151 Eu spectra were consistent with Eu 3+ at the Ca site. The 57 Fe Mossbauer spectra refer to two different micro-environments for the Fe 3+ . They were assigned to Fe 3+ substituting Cu between the two Ca layers and between the Ca and Ba–Sr–O layers.

Fabrication, phase composition and properties of (Tl, Bi, Pb)(Sr, Ba)2Ca2Cu3Oz superconductors

New Ti-1223 bulk superconductors with high phase purity and Tc(0) between 110 K and 118 K have been synthesized by varying the ratios of Sr to Ba and of Tl to Pb and/or Bi in the (Tl1-(m+n)BimPbn) (Sr2-yBay)Ca2Cu3Oz superconducting system in the composition range m=0-0.4, n=0-0.4 and y=0.2 or 0.4. A decrease in the Ba content to y=0.2 favoured the formation of the 1212 phase accompanied by a drop of the Tc(0) value to about 79 K. Replacement of Pb or Tl by Bi above m=0.1 led to higher contents of the barium-bismuth oxide and calcium-strontium-copper oxide secondary phases. Best results were obtained for specimens with nominal overall compositions of (Tl0.55Pb0.4)(Sr1.6Ba0.4)Ca2Cu3Oz and (Tl0.63Bi0.1Pb0.2)(Sr1.6Ba0.4)Ca2Cu3Oz. The X-ray diffraction spectra for these compositions could almost exclusively be assigned to the 1223 phase. The critical current densities at 77 K were 1600 A cm-2 and 1200 A cm-2 respectively, and the Tc(0) values 111 K with transition widths of 1 K.

Mössbauer spectroscopy of Hg-1223 superconductors

Abstract Doping of (Hg0.3Pb0.4)(Sr0.8Ba0.2)2Ca2Cu3Oy superconductors with the Mossbauer nuclei 57 Fe and 119 Sn was investigated. Energy dispersive X-ray (EDX) analysis showed that 57 Fe entered the superconducting phase. The introduction of Fe into the superconductor led to a decrease in the Tc(0)-values from 112 K for the undoped to 105 K for the Fe-doped phase. Low concentrations of Fe were also present in the Ca–Sr–Cu–O phase. Sn formed a separate secondary phase, predominately barium stannate with some Sr, Ca, Pb, Cu and low amounts of Fe. Small amounts of Sn were also present in the Sr–Ca–Cu–O secondary phase. The 57 Fe Mossbauer spectra were interpreted under the assumption that Fe occupies the square pyramidal fivefold oxygen-coordinated Cu sites and the square planar fourfold oxygen-coordinated Cu sites in the superconducting phases. The 119 Sn Mossbauer spectra mainly reflected the Sn environment in the Sn-rich secondary phase.

57Fe Mössbauer spectroscopy of (Bi, Pb)-2223 and (Tl, Bi)-1223 superconductors

(Bi0.93Pb0.17)2Sr1.9Ca2.05(Cu1.02 57Fe0.01)3Oy and (Tl0.74Bi0.25)(Ba0.2Sr0.8)2Ca2(Cu0.99 57Fe0.01)3Oy superconductors were synthesized and investigated by Mossbauer spectroscopy. The samples were carefully characterized by x-ray diffraction, by scanning electron microscopy in combination with energy dispersive x-ray fluorescence (EDX) and by electrical resistivity measurements. Introduction of small quantities of Fe led to a modest decrease in the critical temperatures but the samples remained superconductors. EDX analysis confirmed that iron entered the superconducting phase. The 57Fe Mossbauer spectra of these compounds could be evaluated based on two doublets. The two doublets were assigned to Fe located in the square planar fourfold oxygen coordinated Cu sites and to Fe in the square pyramidal fivefold oxygen coordinated Cu sites in the 2223 and 1223 lattices. The Mossbauer parameters derived in this study were compared with the results obtained for (Tl, Pb)-1223 and (Hg, Pb)-1223 compounds measured earlier. Based on the combined set of data, information on the different chemical bonding of the apical and basal oxygen in the square pyramids and on the layered electronic structure of these superconductors was derived.

Thin (Hg,Pb)Ba2CaCu2Oy films prepared from thermally evaporated precursors by post annealing in Hg-atmosphere

Abstract Superconducting (Hg,Pb)Ba2CaCu2Oy thin films have been prepared in a two step process. Approximatively 0.2-μm-thick Ba–Ca–Cu–O or Pb–Ba–Ca–Cu–O precursor films have been deposited on MgO (100) monocrystalline substrates by a sequential evaporation method. Hg was then introduced into the precursor in a closed silica tube. The surface morphology was characterized by SEM, the composition of the superconducting phase was determined by wave dispersive X-ray electron microanalysis. Phase identification was carried out by X-ray diffraction. The c-axis-oriented plate-like crystals of the Hg-1212 and (Hg,Pb)-1212 phase were weakly connected. Although coverage of the substrate was uneven, we were able to measure transport superconductivity. The partial substitution of Hg by Pb did not significantly influence the Tc values. For both the Pb-free and Pb-doped Hg-1212 samples Tc(on)-values of about 132 K and the zero resistance temperatures Tc(0) from 105 to 117 K were determined.

Doped Tl-1223 Superconductors

Doped Tl-1223 superconductors of the composition TlaBibPbcBanSr2−nCa2Cu3Oy (a = 0.50 − 0.76, b = 0, 0.16 and 0.3, c = 0.5, 0.24 and 0, n = 0.15 − 0.4) were prepared from oxidic Ba-Sr-Ca-Cu precursor material. Upon calcination of the tartrate gels or the nitrates, the respective amounts of Tl2O3, PbO and Bi2O3 were added to the oxidic powders by milling. The samples were sintered in silver foil. The specimens consisted predominantely of the Tl-1223 phase with small amounts of Tl-1212 and secondary phases. Tc(0)-values ranged between 115 K and 119 K with transition widths around 1 K. Transport critical current densities up to 1.4 kA cm−2 at 77 K were measured for the assintered samples. Texturing was obtained by repressing and resintering of the superconducting material followed by annealing at 750°C for 50 h in flowing oxygen. The transport critical current densities could be improved up to 7 kA cm−2 by the thermomechanical treatment. Microstructural studies yielded information on the alignment of the superconducting grains, on secondary phases and the porosities.

Mössbauer Spectroscopy and Preparation of Hg-1223, Tl-1223 and Tl-1212 Superconductors

Introduction of the Mössbauer nuclei 57Fe into Hg-1223 phase and that of 57Fe and 151Eu into Tl-1212 and Tl-1223 superconductors were investigated. Samples of high phase purity were obtained. Scanning electron microscopy and optical microscopy in normal and polarized light were employed to study the microstructure of the specimens. Energy dispersive X-ray analysis showed that 57Fe in the Hg-based samples and 57Fe as well as 151Eu in the Tl-based compounds, entered superconducting phases. Incorporation of Eu3+ into the superconducting phase favored the formation of the Tl-1212 phase. Mössbauer spectroscopy showed that Eu3+ entered the Ca-site. Two doublets found in the 57Fe Mössbauer spectra in both the Hg-1223 and the Tl-1223 phase referred to two different micro-environments of Fe3+. The assignment of the 57Fe Mössbauer spectra was made under the assumption that Fe favored the 1223 phase. Fe3+ may replace Cu in both the square pyramidal, five-fold oxygen coordinated Cu sites between the Ca and Ba-(Sr)-O layers, and in the square planar, four-fold oxygen coordinated Cu sites the Ca layers in the superconducting phases in both the Hg- and the Tl-based materials. From the relative areas of the two doublets, we concluded that the Fe3+ preferred the square planar Cu site.

Influence of the Tl- and Hg-content on magnetic and transport properties of the Pb, Sr-doped Tl-1223 and Hg-1223 superconductors

Abstract The effects of the thallium and mercury content x on the as-sintered and post annealed samples of M x Pb 0.4 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O 8+ δ {M: Tl (0.32≤ x ≤0.74) or Hg (0.18≤ x ≤0.68)} have been studied by magnetization and transport measurements. For Tl-1223 we have found the optimum Tl doping level to be x =0.53 regarding the grain properties, the content of superconducting phase, the first penetration field H pl wl , the transport ( J c tr ), magnetic intergrain ( J c M ) and intragrain ( J c g ) critical current densities. The critical temperature T c of the as-sintered Tl-1223 sample decreased with increasing Tl content. Post-annealing in oxygen improved the T c for Tl contents of x ≥0.53 and had generally positive effects on the critical current densities. The intergrain properties of the Hg-1223 samples were much worse than those of the Tl-based superconductors.

Preparation and characterization of Tl–Ba–Ca–Cu–O 2212 superconducting thick films

Abstract Tl 2 Ba 2 CaCu 2 O y superconducting films of high phase purity with thicknesses from 3 to 4 μm have been prepared on polycrystalline 3 mol% Y 2 O 3 -doped ZrO 2 substrates. The precursor material was deposited from aerosols prepared by an ultrasonic nebulizer. Thallination was carried by wrapping a Tl-source with the precursor material in a silver foil and annealed at 830 and 815°C. The resistance measurements showed T c(0) -values of 98 K with transition widths of 6–7 K. The microstructure of the film and the chemical composition of the well-developed crystallites were investigated by scanning electron microscopy in combination with EDX. The superconducting films showed good adhesion to the substrate and preferred c -axis orientation. Isolated particles (0.3–1 μm) of secondary phases, especially barium–copper-oxides were found at the interface of the superconducting film and the substrate.

Specific heat and electron photoemission of Pb- and Bi- doped Tl(Ba,Sr)CaCuO (1223) high-temperature superconductors

Abstract Specific heat of Tl 0.58 Pb 0.4 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O y , Tl 0.79 Bi 0.25 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O y and Tl 0.74 Bi 0.3 Sr 1.8 Ba 0.2 Ca 2 Cu 3 O y has been measured and the electronic specific heat coefficient γ and the specific heat jumps ΔC at T C have been calculated. The specific heat jumps are about three times smaller than for the YBa 2 Cu 3 O x samples. The core level and valence band spectra for Tl 0.58 Pb 0.4 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O y have been detected at room, at liquid nitrogen and at 125 K temperatures. The Fermi edge has not been observed though pronounced changes of the valence band shapes have been registered. This is most likely due to the larger anisotropy of electronic band structure of the thallium samples in comparison to the yttrium superconductors.