Ayako Tokiwa-Yamamoto

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.

Large enhancement of Tc in the 134 K superconductor HgBa2Ca2Cu3Oy under high pressure

Abstract The onset temperature (Tc) of the Meissner signal for HgBa2Ca2Cu3Oy (which is 134 K under normal pressure) has been measured under high pressure up to 11 Gpa. It is observed that Tc increases with increasing pressure at the rate of dTc/dP = 2. 0. K/ GPa and then dTc/dP becomes relatively small above 6 GPa. The maximum of Tc observed in this experiment is as high as 150 K at 11 GPa. This value is higher than those reported for any high-temperature superconductors.

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

Abstract The composition, crystal structure and superconducting properties of HgBaCuO and HgBaCaCuO 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 HgBaCuO samples were found to be nearly single phases of HgBa2CuO4+δ (Hg-1201), but the HgBa CaCuO samples contained HgBa2Ca2Cu3O8+δ (Hg-1223), HgBa2CaCu2O6+δ (Hg-1212) and other phases. The fraction of the higher-Tc phase in the HgBaCaCuO 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 HgBaCuO samples exhibit 95 K-classs superconductivity, and the Hg BaCaCuO 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.

Substitution of methanol-d4 for the coordinated water in the trinuclear complexes, [M3(μ3-O)(μ-CH3COO)6(H2O)3]+ (M3=Ru3, Rh3 or Ru2Rh) in methanol-d4

The 1H NMR spectra of acetate methyl signals of the titled three complexes in CD3OD change with time due to successive substitution of CD3OD for the coordinated water molecules. The first-order rate constants for the first methanol-d4 substitution of the triruthenium(III) and trirhodium(III) complexes are 7.7×10−4 s−1 (298.2 K) (ΔH‡,=103±6 kJ mol−1 and ΔS‡=+41±12 J K−1 mol−1 at 0–21 °C) and 1.3×10−3 s−1 (298.2 K) (ΔH‡=102±9 kJ mol−1 and ΔS‡=+42±32 J K−1 mol−1 at 0–10 °C) per one metal ion, respectively, which are greater by approximately 2 and 6 orders of magnitude, respectively, than the water exchange reactions of the hexaaqua complexes of these metal ions. The trans effect of the central oxide ion is considered as a major factor responsible for the labilization. The first-order rate constants for the mixed-metal rhodium-diruthenium complex at 298.2 K are 9.9×10−5 (ΔH‡=109±4 kJ mol−1 and ΔS‡=+44±9 J K−1 mol−1) and 7.9×10−5 s−1 (ΔH‡=103±3 kJ mol−1 and ΔS‡=+22±6 J K-1 mol−1 at 10.1–35.3 °C) at ruthenium and rhodium centers, respectively, which are c. 10 times smaller than the corresponding values for the homonuclear complexes. The slower rates in the mixed-metal complex indicate that electronic configuration in the molecular orbital based on (metal-dπ)-(oxygen-pπ) interactions plays some role in controlling the substitution rate. On the basis of the activation parameters, a dissociative mechanism is proposed for all these reactions.

Annealing effects on the Meissner signals from HgBaCaCuO superconductors

Abstract As-sintered samples of the HgBaCaCuO 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.

Superconductivity of (Hg0.7Tl0.3)2 Ba2Y1−xCaxCu2O8−δ (0 ≦ x ≦ 0.8) with a wide range of doping state

Abstract (Hg, Tl)-based cuprates with the nomial compositions of (Hg0.7Tl0.3)2Ba2Y1−xCaxCu2O8−δ (0 ≦ x ≦ 1.0, every 0.1 step) were prepared under high pressure, and nearly single phases were obtained for the samples with 0 ≦ x ≦ 0.8. We systematically investigated their unit cell dimensions, DC susceptibility, electrical resistivity, oxygen content and copper valence as a function of Ca content, x. For the samples with x=0–0.4, Tc, which was determined by DC susceptibility, increased from 12 K to 84 K with increasing x, while the samples with x = 0.5 – 0.8 exhibited a Tc decrease from 84 K to 40 K. This result indicates that the doping state of the samples can be extensively varied from underdoped to overdoped states by the Ca substitution. This variation in the doping states was also supported by the systematic change in the temperature dependence of the electrical resistivity. Oxygen analysis revealed that the copper valence gradually increased from 2.12 to 2.26 with increasing x from 0 to 0.8. The copper valence with the maximum Tc of 84 K in these samples was determined to be 2.18–2.19, which is comparable to the value for the optimum Tc in other high-Tc cuprates.

Effect of Tl doping on the phase stability of Hg2Ba2(Y,Ca)1Cu2O8−δ superconductors prepared by a high-pressure technique

Abstract (Hg, Tl) based cuprates with the nominal compositions of (Hg 1− x Tl x ) 2 Ba 2 Y 1− y Ca y Cu 2 O 8−δ (0 ≦ x ≦ 1.0 and 0 ≦ y ≦ 1.0) were prepared under a high pressure and high temperature. The solubility of Ca ions into the Hg-2212 phase for various Tl contents was examined by X-ray powder diffraction and energy-dispersive X-ray spectroscopy. The substitution of Ca 2+ for the Y 3+ site for the samples with x = 0.3 and 0.5 is realized over a full range of 0 ≦ y ≦ 1.0, while the solubility range is limited for the samples with x ≦ 0.15 and x ≧ 0.75 . The samples with x = 0.3 exhibited a maximum T c of 84 K, which was about 30 K higher than the maximum T c for the samples with x = 0. The significant extension of Ca solubility for the Tl doped samples is probably explained by the compensation of excess negative charges in the (Hg 1− x Tl x ) 2 O 2−δ charge reservoir. The fact that the Y free Hg-2212 phase is obtained by Tl doping suggests the possibility of synthesizing a Hg-22( n − 1) n ( n = 3, 4, …) homologous series.