S.F. Hu

A new 92 K high-Tc superconductor: Hg-containing Tl-based 1212 phase

Abstract Bulk superconductivity up to 92 K has been achieved in the mercury-containing curprate (Tl 0.5 Hg 0.5 )Sr 2 (Ca 1− x Y x )Cu 2 O 7−δ when x =0.3. Based on X-ray and electron diffraction and electron probe microanalyzer results, we identify the phase responsible for the superconductivity to be similar to that of (Tl 0.5 Pb 0.5 )Sr 2 CaCu 2 O 7 (so-called 1212 phase) with a space group of P4/mmm and lattice constants of a ∼ 3.8 A and c ∼ 12.0 A. Moreover, neither superlattice nor intergrowth along the a ∗ and c ∗ directions were observed by electron diffraction. An increase in Ca 2+ doping, 0.5⩽ x ⩽0.3, into the Y 3+ sites results in a maximum superconducting transition temperature ( T c ) at around 92 K for x =0.3. For compositions x T c was observed.

An enhancement of Tc, from 45 K to 70 K, via Cd substitution in (Pb, Cu)Sr2(Ca, Y)Cu2O7−δ

Abstract An enhancement in the superconducting transition temperature, T c , from 45 K to 70 K, has been discovered during the course of our investigation of Cd-doping of the Pb-based 1212 superconductors. Specifically, we have attempted to synthesize a material with nominal composition (Pb 0.5 Cd 0.5 )Sr 2 (Ca 0.5 Y 0.5 )Cu 2 O 7−δ , derived from the lead cuprate superconductor (Pb 0.7 Cu 0.3 )Sr 2 (Ca 0.5 Y 0.5 )Cu 2 O 7−δ . From X-ray and electron diffraction, coupled with energy-dispersive X-ray emission spectrometry, we can identify the phase responsible for the 70 K superconductivity to be (Pb 0.67(2) Cd 0.33(5) )Sr 2.1(1) (Cd 0.20(3) Ca 0.33(3) Y 0.48(4) )Cu 2 O 7−δ . Thus, we propose substitution of Cd both for Cu, in the rock salt-type (Pb, Cu)O layers, and Ca, in the Ca 0.5 Y 0.5 lattice sites. No superlattice spots or streaks were observed in the electron diffraction experiments along the a ∗ , b ∗ and c ∗ directions in the Cd-substituted material.

Superconductivity at 124 K in (Tl, Pb)Sr2Ca2Cu3O9

Abstract We report the preparation of almost phase pure (Tl 0.5 Pb 0.5 )Sr 2 Ca 2 Cu 3 O 9 , which is characterized by high temperature superconduc tivity with T c onset =130 K, T c midpoint =124 K, T c zero =122.5 K as measured by electrical resistance, and a diamagnetic onset temperature, T c mag of 124 K determined by DC magnetic susceptibility. The new procedure involves synthesizing a material with nominal stoichiometry (Tl 0.5 Pb 0.5 )Sr 1.6 Ca 2.4 Cu 3 O 9 , which is encapsulated in gold foil and sintered at 950°C for 3 h in oxyge n, then sealed in an evacuated quartz tube and finally annealed at 700∼750°C for 5∼10 days. From the results of X-ray diffraction and energy-dispersive X-ray spectrometry (EDS), we have identified the phase responsible for the superconductivity in the annealed sample to be Tl 0.58(3) Pb 0.38(2) Sr 1.9(2) Ca 2.1(1) Cu 3 O 9 . No superlattice and intergrowth along the a ∗ and c ∗ directions were observed but streaks along the c ∗ -axis were found from microcrystallites of the annealed sample.

Composition-induced superconductivity (up to 55 K) in the system (Pb0.75Cu0.25)Sr2(Ca1−xYx)Cu2O7−δ

The authors demonstrate a compositionally induced superconductor-to-semiconductor transition in the system (Pb{sub 0.75}Cu{sub 0.25})Sr{sub 2}(Ca{sub 1{minus}x}Y{sub x})Cu{sub 2}O{sub 7-{delta}}. Superconducting behavior is observed over the composition range 0.1 {le} x {le} 0.6. Samples with 0.1 {le} x {le} 0.3 are multiphasic, but the superconducting transition temperature is a maximum (55 K) over this range of compositions. The sample having x = 0.5 gives rise to an acceptable phase purity, a superconducting transition temperature of 45 K, and the maximum superconducting (Meissner) volume fraction (20.2%). A superconductor-to-semiconductor transition occurs at x = 0.7.

Crystal structure of the (Pb, Hg)Sr2(Ca, Y)Cu2O7-δ superconductor

Abstract Bulk superconductivity up to 90 K has been achieved in the Hg containing cuprate (Pb, Hg)Sr 2 (Ca 1- x Y x )Cu 2 O 7- δ when x =0.3, which has the highest T c among the Pb based materials. The crystal structure of the newly discovered Hg containing Pb based 1212 phase has been investigated by Rietveld analysis of X-ray powder diffraction (XRD) data, selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM). We have identified the phase responsible for superconductivity in the so-called 1212 phase with a space group of P4/mmm and lattice constants of a=3.8166(1) A and c=11.9484(4) A for the x =0.5 sample. A disordering of the hole reservoir layer (Pb, Hg) due to the displacement of the oxygen ions and Pb/ Hg ions from their ideal positions of (0.5, 0.5, 0) to (0.39, 0.5, 0) and from (0, 0, 0) to (0.044, 0, 0), respectively, was found in the XRD refinement of the x =0.5 sample. This displacement may have given rise to the appearance of superlattice spots along the [104] ∗ direction with a commensurate modulation factor of 8 in the electron-diffraction pattern viewed down [010]. Moreover, mutual substitutions of cations (≈19% Cu in the Pb/Hg sites and ≈9% Hg in the Ca/Y sites) have been found in the sample.

Superconductivity up to 32 K in a new family of the Hg-containing (Pb, Hg) (Sr, La)2CuO5−δ (1201) system

Abstract A new family of superconductors with a maximum Tc of 32 K were observed in the Hg-containing (Pb0.5Hg0.5) (Sr2−xLax) CuO5−gd (1201) system for the composition range 0.8 ⩽x⩽ 1.0. The chemical composition and crystal structure of this system have been investigated by X-ray powder diffraction including the Rietveld refinement, electron diffraction, and high-resolution transmission electron microscopy. We have identified the phase responsible for the superconductivity to be similar to (Pb, Cu) (Sr, La)2CuO5 or (Ti, Pb) (Sr, La)2CuO5 (the so-called 1201 phase) with a space group of P4/mmm and lattice constants of a=3.7908(2) A and c=8.6915 (6) A for the x=0.8 sample in (Pb0.5Hg0.5 (Sr2−xLax)CuO5−δ. The disordering of the hole reservoir layer of (Pb, Hg)O due to the displacement of the oxygen ion from its ideal position (0.5, 0.5, 0) to (0.33, 0.5, 0) was found by Rietveld refinement for powder X-ray diffraction data of the x=0.8 sample. This displacement may give rise to the appearance of the superlattice spots along the [102]∗ direction with a modulation factor close to 4 in the electron diffraction pattern viewed down [010].

Improvement of phase purity and accelerated formation of the Tl-1223 phase from the stoichiometric compositions (Tl0.6Pb0.2Bi0.2) (Sr2-xBax)Ca2Cu3O9 (x=0.2-0.3)

Abstract We report an efficient and highly reproducible method for the preparation of the almost monophasic Tl-1223 phase by partial substitution of Ba 2+ into Sr 2+ sites in the stoichiometric compositions of (Tl 0.6 Pb 0.2 Bi 0.2 )(Sr 2- x Ba x )Ca 2 Cu 3 O 9 ( x =0.2-0.3). The as-synthesized samples have a superconducting transition temperature around 115 K which can be further increased up to 122 K after post-annealing at 820°C for 20 h in oxygen.

Optimization of superconducting transition temperature in (Pb0.7Cu0.3)Sr2(Ca0.5Y0.5)Cu2O7+δ by postannealing

We demonstrate systematic studies of the optimization of the superconducting transition temperature in (Pb 0.7 Cu 0.3 )Sr 2 (Ca 0.5 Y 0.5 )Cu 2 O 7+δ by annealing in an oxygen atmosphere. As-sintered samples with a superconducting midpoint transition temperature, T c(midpoint) , of 18 K were prepared by solid state reaction at 1000°C for 1 hr in oxygen and then quenched in air. Subsequently, the samples were annealed in the temperature range between 700 and 1000°C for 12 hr in oxygen and then rapidly quenched in air. The ultimate T c(midpoint) increased from 30 to 48 K with an increase in the annealing temperatures from 700 to 800°C, but a further increase in the annealing temperature of the samples from 800 to 1000°C resulted in a decrease in T c(midpoint) . Based on our studies, we believe that the optimum T c(midpoint) of 48 K arises from the removal of excess oxygen in the insulating slabs of (Pb,Cu)O in (Pb 0.7 Cu 0.3 )Sr 2 (Ca 0.5 Y 0.5 )Cu 2 O 7+δ and this leads the material to approach an optimum hole concentration.

New high Tc superconductor : (In,Pb,Cu)Sr2(Ca,Y)Cu2O7-δ

Bulk superconductivity was observed in the new system (In{sub {ital x}}Pb{sub 0.75{minus}{ital x}}Cu{sub 0.25})Sr{sub 2}(Ca{sub 0.5}Y{sub 0.5})Cu{sub 2} O{sub 7{minus}{delta}}. An increase in indium doping, 0{le}{ital x}{le}0.2, into the Pb sites results in an increase in the superconducting transition temperature {ital T}{sub {ital c}} from 45 K for {ital x}=0 to 60 K for {ital x}=0.2. For nominal compositions {ital x}{gt}0.2, a decrease in {ital T}{sub {ital c}} was observed, and an increase in the concentration of an impurity phase. From the results of the x-ray diffraction and energy- dispersive spectrometry, we identify the phase responsible for the superconductivity to be (In{sub 0.06(1)}Pb{sub 0.69(1)}Cu{sub 0.25})Sr{sub 1.99(7)}(Ca{sub 0.47(1)}Y {sub 0.53(2)})Cu{sub 2}O{sub 7{minus}{delta}}; this phase resulted from the synthesis of a sample of nominal composition (In{sub 0.2}Pb{sub 0.55}Cu{sub 0.25})Sr{sub 2}(Ca{sub 0.5}Y{sub 0.5})Cu{sub 2} O{sub 7{minus}{delta}}.Bulk superconductivity was observed in the new system (InxPb0.75−xCu0.25)Sr2(Ca0.5Y0.5)Cu2 O7−δ. An increase in indium doping, 0≤x≤0.2, into the Pb sites results in an increase in the superconducting transition temperature Tc from 45 K for x=0 to 60 K for x=0.2. For nominal compositions x≳0.2, a decrease in Tc was observed, and an increase in the concentration of an impurity phase. From the results of the x‐ray diffraction and energy‐ dispersive spectrometry, we identify the phase responsible for the superconductivity to be [In0.06(1)Pb0.69(1)Cu0.25]Sr1.99(7)[Ca0.47(1)Y 0.53(2)]Cu2O7−δ; this phase resulted from the synthesis of a sample of nominal composition (In0.2Pb0.55Cu0.25)Sr2(Ca0.5Y0.5)Cu2 O7−δ.

Bulk new high-Tc superconductors in the (Pb,Cu)Sr2(Ca,Y)Cu2O7−δ and (In,Pb,Cu)Sr2(Ca,Y)Cu2O7−δ systems

Bulk superconductivity was observed in the systems (Pb 0.75 Cu 0.25 )Sr 2 (Ca 1− x Y x )Cu 2 O 7−δ and (In y Pb 0.75− y Cu 0.25 )Sr 2 (Ca 0.5 Y 0.5 )Cu 2 O −δ . With decreasing x in (Pb 0.75 Cu 0.25 )Sr 2 (Ca 1− x Y x )Cu 2 O 7−δ , T c increases up to 55 K at x =0.1; however this specimen is multiphasic with superconducting Meissner volume fraction of ca. 1.5%. In contrast, the sample with x =0.5 is nearly single phase and has a lower T c (45 K) and a high superconducting Meissner volume fraction ca. 20.2%. An increase of the indium doping in (In y Pb 0.75− y Cu 0.25 Sr 2 (Ca 0.5 Y 0.5 )Cu 2 O 7−δ , 0 ≤ y ≤0.2, results in an increase in the superconducting transition temperature from 45 K for y =0, to 60 K for y =0.2.