Makoto Itoh

Encapsulation method for the synthesis of nearly single-phase superconducting HgBa2Ca2Cu3O8+δ with Tc⩾ 135 K

Nearly single-phase polycrystalline samples of HgBa2Ca2Cu3O8+δ were successfully synthesized under relatively simple and unprecedented synthesis conditions, i.e. at a “low” temperature (<700°C) and for a “long” period of time (5–168h), using mono-oxide powders of Hg, Ba, Ca, and Cu as starting materials. A pressed powder mixture was heated at 665°C for ≈75 h in an evacuated fused quartz tube, and then post-annealed in flowing O2 gas to exhibit a Meissner signal below 135.4 K. Zero resistance of this sample was observed at 133.9 K.

The effect of Pb doping in HgBa2Ca2Cu3O8+δ superconductor

Abstract Polycrystalline samples with the starting compositions of (Hg 0.8 Pb 0.2 ) x Ba 2 Ca 2 Cu 3 O 8+δ (1.0⩽ x ⩽2.5) were prepared under various synthetic conditions. Both powder X-ray-diffraction analysis and energy dispersive X-ray spectroscopy revealed that the Pb ions were successfully doped into the Hg-1223 phase. High-quality Pb-doped Hg-1223 samples for the starting composition with x =1.5 were obtained when heated at 675°C only for 1 h. The resistivity (ϱ) of such a sample was low, being ∼8×10 3 Ω cm at 300 K, and the ϱ versus T curve was linearly extrapolated to zero resistance at 0 K, being similar to the cases of high-quality high- T c cuprates. The zero resistance due to superconductivity started at 122.2 K. When the sample was reduced in flowing argon gas, it exhibited a Meissner signal below 132 K and its Meissner volume fraction exceeded 20% at 5 K. According to thermoelectric power measurements, the carrier density in the sample was reversibly dependent on the oxygen partial pressure, and the sample post-annealed in flowing oxygen gas was likely overdoped.

Increase of Tc in HgBa2CuOy by reducing treatment

Abstract The superconductivity of HgBa 2 CuO y annealed in various reducing as well as oxidizing atmospheres was investigated. The assintered HgBa 2 CuO y specimens prepared by solid state reaction starting with mono-oxides of Hg, Ba and Cu exhibited superconductivity at 90 K. It was found that the superconductivity critical temperature ( T c ) increased up to 96 K after a reducing treatment of the as-sintered specimen, whereas annealing of the specimen in high-pressure oxygen gas resulted in a decrease of T c .

Dependence of superconducting properties on the Cu-valence determined by iodometry in HgBa2CuO4+δ

Abstract We have synthesized nearly single-phase HgBa2CuO4+δ samples with various oxygen contents, and systematically investigated the relationship between the lattice parameters, the superconductivity transition temperature (Tc) and the oxygen content determined by iodometry. Furthermore, the nominal copper valence values were calculated from the oxygen content data by assuming constant valence values of +2.00 and −2.00 for mercury and oxygen, respectively. Both the a and c lattice parameters were found to decrease monotonically with the Cu valence over a wide range of +2.05–2.28. The obtained Tc versus Cu valence curve exhibits a “bell shaped” correlation with the maximum in Tc at the Cu valence of ∼ +2.18.

Composition, crystal structure and superconducting properties of HgBaCuO and HgBaCaCuO superconductors

Abstract The composition, crystal structure and superconducting properties of Hgue5f8Baue5f8Cuue5f8O and Hgue5f8Baue5f8Caue5f8Cuue5f8O superconductors were investigated. Samples with various nominal compositions were prepared from the mono-oxides of Hg, Ba, Ca and Cu using various sintering processes. The samples were examined by X-ray powder diffraction and energy dispersive X-ray analysis (EDX). The Hgue5f8Baue5f8Cuue5f8O samples were found to be nearly single phases of HgBa2CuO4+δ (Hg-1201), but the Hgue5f8Baue5f8 Caue5f8Cuue5f8O samples contained HgBa2Ca2Cu3O8+δ (Hg-1223), HgBa2CaCu2O6+δ (Hg-1212) and other phases. The fraction of the higher-Tc phase in the Hgue5f8Baue5f8Caue5f8Cuue5f8O sample varied in accordance with the nominal composition. High-resolution transmission electron microscopic (HRTEM) studies revealed the existence of Hg-1201 (tetragonal cell with a=3.85 and c=9.5 A ), Hg-1212 ( c=12.6 A ). and Hg-1223 structures ( c=15.7 A ). Most of the Hgue5f8Baue5f8Cuue5f8O samples exhibit 95 K-classs superconductivity, and the Hgue5f8 Baue5f8Caue5f8Cuue5f8O samples showed 130 K-class superconductivity. The highest Meissner onset temperature, Tcm determined from DC susceptibility was 134 K. The same sample showed superconductivity with a ≈ 16% diamagnetic volume fraction at 5 K.

Annealing effects on the Meissner signals from HgBaCaCuO superconductors

Abstract As-sintered samples of the Hgue5f8Baue5f8Caue5f8Cuue5f8ue5f8O superconductors were annealed in flowing argon, flowing oxygen and high-pressure oxygen gases. The superconducting critical temperature, T c , of the sample increased after annealing in either flowing oxygen or a high-pressure oxygen gas, while it decreased after annealing in flowing argon. These results indicated that the as-sintered samples were in slightly “under-doped” states.

High-pressure synthesis of superconducting (Ca, K)2 CuO2Cl2

Abstract We synthesized a halo-oxocuprate superconductor, (Ca, K) 2 CuO 2 Cl 2 , from Ca 2 CuO 2 Cl 2 and KClO 4 using a high-pressure technique. The products exhibited a superconductivity below ∼ 24 K and a large Meissner volume. From compositional analyses, we confirmed that the monovalent potassium ions are incorporated in the superconducting grains and the Cl/Cu atomic ratio is approximately two. It is likely that the potassium substitution for calcium is the dominant hole-doping process in this superconductor.

Thermogravimetric study on HgBa2Can−1CunO2(n+1)+δ (n = 1, 2, 3) superconductors at various oxygen partial pressures

Abstract The oxygen non-stoichiometry of HgBa2Can−1CunO2(n+1)+δ (Hg-12(n−1)n, n = 1, 2, 3) superconductors was studied by thermogravimetric analysis (TGA) in a flowing gas atmosphere with various partial pressures of oxygen ranging from 10−5 to 10 atm. In 1 atm oxygen, irreversible reactions, probably corresponding to vaporization of Hg, were observed above about 420, 480 and 510°C for Hg-1201, 1212 and 1223 samples, respectively. Reversible reactions with oxygen absorption and desorption were confirmed below these temperatures. By controlling the oxygen partial pressure during the heating process up to 300°C and a subsequent cooling process, we obtained Hg-12(n − 1)n samples with various oxygen contents. The superconducting properties of these samples were also studied. It was found that the oxygen non-stoichiometry of Hg-12(n − 1)n superconductors is very sensitive to the oxidizing atmosphere when they are annealed, even below 300°C.

Pb‐doping effect on irreversibility fields of HgBa2Ca2Cu3O8+δ superconductors

We prepared nearly single‐phase polycrystalline samples of HgBa2Ca2Cu3O8+δ(Hg‐1223) without and with Pb doping, i.e., Pb‐free Hg‐1223 and (Hg0.8Pb0.2)‐1223, by means of a sample encapsulation technique. According to quantitative energy dispersive x‐ray spectroscopy analyses and iodometric oxygen titration, the Pb‐free Hg‐1223 and the (Hg0.8Pb0.2)‐1223 sample were identified to be Hg0.97Ba2.00Ca2.04Cu3.00O8.33 and (Hg0.79Pb0.29)Ba2.00Ca1.98Cu2.94O8.42, respectively. The highest Meissner‐signal onset temperatures for the Pb‐free Hg‐1223 (after post‐annealing in O2 gas) and the (Hg,Pb)‐1223 (as‐sintered) sample were observed at 135.4 and 132.0 K, respectively. The difference in the irreversibility fields (Hirr) for both samples was very small at temperatures above 80 K, while, at temperatures below 70 K, the Hirr versus T curve for the (Hg,Pb)‐1223 sample shifted toward the higher field side. Thus, Pb doping was effective for the improvement of magnetic property of Hg‐1223 superconductors, at least, at tempe...

Synthesis of HgBa2Ca3Cu4Oy under ambient pressure

Abstract The mercury-based oxide superconductor, HgBa 2 Ca 3 Cu 4 O y phases was synthesized by a controlled vapor/solid reaction technique using high-purity powders of HgO, BaO, CaO and CuO as starting materials. Two types of barsm, with different cation ratios of Hg:Ba:Ca:Cu=0:2:2:3 and 1:2:2:3, were heated in an evacuated quartz tube. The resulting HgBa 2 Ca 3 Cu 4 O y sample had a T c of 123 K and lattice parameters a=3.852(2) A and c = 19.01(4) A with a volume fraction of 83% of the total shielding signal. The formation condition was widely investigated, showing that the higher heating temperature and longer heating time lead to the formation of the HgBa 2 Ca n Cu n −1 O y phase with a higher number n of Cuue5f8O 2 planes in a unit cell.