H.-C.I. Kao

Superconductivity and hole concentration of La3−xCa2xBa3−xCu6Oy compounds

Abstract The relationship between superconductivity and hole concentration was studied in a series of tetragonal triple-perovskite La 3− x Ca 2 x Ba 3− x Cu 6 O y (0 ≤ x ≤ 1.00) compounds. It was found that its superconductivity was dependent on the hole concentration of the CuO layers, but not on the hole concentration of the compound. It was semi-empirically demonstrated that superconductivity occurred in the CuO layers but not in the CuO chains in the La 3− x Ca 2 x Ba 3− x Cu 6 O y compounds. Moreover, this explained the reason why La 3 Ba 3 Cu 6 O y was not superconducting in terms of the degree of its hole concentration.

The effect of the Ca/Ba ratio on the superconductivity of the La3CaχBa4−χCu7Oy system

Abstract A series of samples with nominal composition of La 3 Ca χ Ba 4−χ Cu 7 O y (0≤χ≤4.00) was prepared by the conventional powder reaction method. Single-phase materials with triple-perovskite structure were obtained for χ≤1.25 samples of which the lattice parameters monotonically decreased with the increase of Ca substitution. Superconductivity was observed for χ≤3.50 samples with 37 K≤ T conset ≤86 K. A T c plateau above 80 K was found in the 1.00≤χ≤1.25 samples with a -axis length of 3.879≥ a ≥3.872 A. In other words, an optimal r Cu-O distance of ca. 1.94 A in the Cu-O planes was the most appropriate value for superconductivity in the La 3 Ca χ Ba 4−χ Cu 7 O y system.

Superconductivity of the single phase La4-xCaxBa3Cu7Oy system

Abstract A series of single phase materials with tetragonal triple-perovskite structure and nominal compositions of La 4- x Ca x Ba 3 Cu 7 O y (0.50⩽ x ⩽1.375) was prepared by the solid state reaction method. Due to the differences in the size and valence between tha Ca ions and La ions, increasing the Ca substitution for La resulted in a decrease in unit cell axes, volume and the oxygen stoichiometry. A rapid decrease in the T c was observed when the a -axis was larger than 0.38 nm indicating the importance of the r Cu-O distance in the copper-oxygen planes for the superconductivity. Evidence of hole doping was observed by Ca substitution till x =1.00 which had the highest T c and a maximum hole concentration in the La 4- x Ca x Ba 3 Cu 7 O y superconductors.

Hole localization in Pr-doped RBa2Cu3O7−y systems

Abstract The hole concentration per unit cell (p) was measured by using an iodometric titration technique for bulk Gd1xPrxBa2Cu3O7−y (x = 0.1−0.9) and R0.8Pr0.2Ba2Cu3O7−y (R = Yb, Er, Dy, Gd and Nd) samples. These data were compared with the Hall number per unit cell (nH) at 100 K and superconducting transition temperature (Tc) reported in the literature. We suggest that the hybridization between 4f states of the Pr ion and the conduction band states in CuO2 planes, leading to the hole localization are the mechanism for the suppression of superconductivity in these systems. The correlation between Tc and p cannot be described by the empirical relation observed in many high-Tc oxides

Superconductivity in the triple-perovskite La-Ca-Ba-Cu-O system

Three series of single-phase La-Ca-Ba-Cu-O compounds with tetragonal triple-perovskite structure and respective nominal compositions of La4-xCaxBa3Cu7Oy (LC series, 0.50<or=x<or=1.15), La3CaxBa4-xCu7Oy (CB series, 0<or=x<or=1.25) and La3.5-0.5xCaxBa3.5-0.5xCu7Oy (LCB series, 0<or=x<or=1.25) were prepared by either a solid state reaction method or a polymeric citrate precursor method. All the single-phase materials were superconducting except for samples containing small x in the LCB series. Unit-cell parameters, such as a axes, c axes and cell volume, decreased with increasing Ca substitution. In general, the Tc and hole concentration (p) increased with increasing x from x=0 to x=1.00. In other words, evidence of hole doping was observed during Ca substitution that was the main reason for the increase of the Tc from 0 to 80 K. The optimal Tc was observed in a region close to the formula of La3CaBa3Cu7Oy which had a formula per unit celt of La1.29Ca0.43Ba1.29Cu3Oz where z=7.104, p=0.308, a axis =0.3879 nm and Tc(zero)=80 K.

A correlation between the oxygen stoichiometry and Tc of Bi2CaSr2(1−x)Cu2(1−y)O8+δ−2x−2y superconductors

Abstract A series of superconductors with a nominal composition of Bi 2 CaSr 2(1−x) Cu 2 O 8−2x+δ or Bi 2 CaSr 2 Cu 2(1−y) O 8−2y+δ , where 0≤ x , y ≤0.6, was prepared under identical conditions at 830°C. Their superconductivity degraded rapidly if x or y was larger than 0.4. from X-ray diffraction studies, it was suggested that vacancies were created when part of the SrO or CuO was removed from the Bi 2 CaSr 2 Cu 2 O 8+δ . The extra oxygen stoichiometry, δ, decreased with increasing x but increased with increasing y . When δ was in between 0.04 and 0.18, T c(zero) of Bi 2 CaSr 2 Cu 2 O 8+δ was not dependent on δ and it was found to be about 60 K. Above this range, T c(zero) decreased rapidly.

Preparation, structure, and peritectic transition of RBa1.5Sr0.5Cu3Oy (R = La, Nd, Sm, Eu, Gd, Dy, Ho, and Y) superconductors

A series of single-phase RBa1.5Sr0.5Cu3Oy (R = La, Nd, Sm, Eu, Gd, Dy, Ho, and Y) compounds with a triple-perovskite unit cell has been prepared by solid-state reaction method. Samples with a larger radius of the R ion (R = La and Nd) are tetragonal, whereas those with a smaller radius of the R ion (R = Sm, Eu, Gd, Dy, Ho, and Y) are orthorhombic. Unit-cell parameters of RBa1.5Sr0.5Cu3Oy decrease monotonically with a decrease of the radius of the R ion. A maximum at R = Nd, Sm, and Eu is found in the peritectic transition temperature (tp), the amount of oxygen loss (Δw) in this transition, and the enthalpy of fusion (ΔHf), respectively. An increase in the ΔHf is correlated with increasing tp for the respective orthorhombic and tetragonal phases of RBa1.5Sr0.5Cu3Oy compounds. Tc of the title compounds gradually increased from 52 to 86 K with a decrease in the radius of the R ion.

Superconductivity of La1.5−x2SrxBa1.5−x2Cu3Oy compounds

Abstract The superconductivity of single phase La 1.5− x 2 Sr x Ba 1.5− x 2 Cu 3 O y , 0 ≤ x ≤ 0.80 (LSB) compounds with tetragonal triple-perovskite structure was studied. Samples with 0.10 ≤ x ≤ 0.80 are superconducting and T c(zero) is found between 28 and 48 K. With increasing x , both the hole concentration in the Cu2O layers (the copper oxygen layers in the center perovskite layer) and T c increase. It is interesting to find that although the hole concentration and oxygen stoichiometry of the LSB series are similar to those of the La 1.5− x 2 Ca x Ba 1.5− x 2 Cu 3 O y (LCB) series, the maximum T c of the LSB series is about 30 K lower than that of the LCB series. The difference in T c is probably a result of difference in locations of the Ca 2+ and Sr 2+ ions in the unit cell. The smaller divalent Ca 2+ ion prefers the smaller trivalent Y 3+ site and acts as a hole dopant for the superconducting Cu2O layers. On the contrary, the larger divalent Sr 2+ ion presumably prefers the larger divalent Ba 2+ site, and thus hole doping on the Cu2O layers is not as prominent as the Ca 2+ ion in the Y 3+ site.

Rietveld analysis on Gd(Ba2-xAx)Cu3Oy (A = Ca, Sr) superconductors

Abstract Two series of single phase triple-perovskite superconducting samples with nominal compositions of Gd(Ba 2−x Ca x )Cu 3 O y (Ca series, 0 ≤ x ≤ 0.30) and Gd(Ba 2−x Sr x )Cu 3 O y (Sr series, 0 ≤ x′ ≤ 1.0) were prepared. Orthorhombicity decreases with increasing amount of Ca or Sr substitution. Part of Ca ions occupy R site and push the same amount of Gd 3+ ion into Ba site, attracting negative oxygen ions to randomly occupy O(4) and O(5) sites on the basal plane which leads to a further reduction of orthorhombicity in Gd(Ba 2−x Ca x )Cu 3 O y series compared with the same amount of substitution in Sr series, in which, Sr only occupies the Ba site. T c is dependent on the orthorhombicity of these samples, no matter wheater it is Ca or Sr substitution.

Preparation of submicrometre superconducting powder with a polymeric precursor method using different polyprotic acids

Submicrometre (La-3137) superconducting powder is prepared by a polymeric polyprotic acid (PA)/ethylene glycol (EG) derived precursor method. Six different PAs, oxalic acid, succinic acid, adipic acid, malic aid, tartaric acid and citric acid, are selected. It is found that of the six PAs citric acid gives the best results in terms of the particle size, homogeneity, carbon residue and values of the resulting La-3137 superconducting powder.