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Effects of the Ba-substitution for Sr in the Bi-2201 phase of BiPb(Sr,La)2CuO6+δ

Polycrystalline samples of the Bi-2201 phase of BiPbSr1.15-xBaxLa0.85CuO6+δ have been prepared by the conventional solid-state reaction method. Through the Ba-substitution for Sr, the lattice becomes large while Tc decreases gradually. With increasing x(Ba), the hole concentration in the CuO2 plane decreases despite the increase in the number of excess oxygen. The suppression of the superconductivity may be due to the decrease in the hole concentration in the CuO2 plane and the increase in the local lattice-distortion caused by excess oxygen.


Introduction
Among a lot of superconductors, the Bi-2223 cuprate superconductor is most expected for the practical application of superconductivity at the liquid nitrogen temperature because of its relatively high value of the superconducting transition temperature T c ~110 K. However, it is 15-25 K lower than those of poisonous Tl-2223 and Hg-1223 cuprate superconductors, the crystal structures of which are analogous to that of the Bi-2223 cuprate superconductor. Elucidating why its T c is low and increasing T c up to ~135 K will promote the practical application of the Bi-2223 cuprate superconductor. There have been many efforts since the discovery of the Bi-2223 cuprate superconductor, but the problem has not yet been solved. In order to solve this problem, it is meaningful to investigate the Bi-2201 phase among three kinds of Bi-based cuprate superconductors of Bi 2 Sr 2 Ca n-1 CunO 2n+4+δ (n = 1, 2, 3), because the crystal structure is simple and the synthesis can readily be done, though its T c is low.
The value of T c in the Bi-2201 phase of Bi 2+x Sr 2-x CuO 6+δ at the time of discovery is ~ 8 K [1]. It increases up to ~ 15 K in Bi 2 Sr 2 CuO 6+δ in which Bi does not occupy the Sr site [2]. However, this value is much smaller than 90 -95 K in the Tl-2201 phase of Tl 2 Ba 2 CuO 6+δ and the Hg-1201 phase of HgBa 2 CuO 4+δ [3,4]. One of causes of the low T c in Bi 2 Sr 2 CuO 6+δ is a large amount of excess oxygen introduced into the Bi 2 O 2 layer which supplies the conductive CuO 2 plane with holes excessively to cause the local distortion. Therefore, T c increases up to ~30 K by decreasing the hole concentration through the partial substitution of La 3+ for Sr 2+ [5]. Moreover, T c is improved to ~40 K in (Bi,Pb) 2 (Sr,La) 2 CuO 6 by removing excess oxygen through the partial substitution of Pb 2+ for Bi 3+ with keeping the hole concentration at the optimum [6,7].
According to the empirical rule of high T c in cuprate superconductors, T c increases with increasing distance between Cu and O. The Cu-O distance in Bi 2 Sr 2 CuO 6+δ is shorter than that in HgBa 2 CuO 4+δ and Tl 2 Ba 2 CuO 6+δ , because smaller Sr 2+ ions in Bi 2 Sr 2 CuO 6+δ are replaced by larger Ba 2+ ions in HgBa 2 CuO 4+δ and Tl 2 Ba 2 CuO 6+δ . In this research, we have attempted to increase T c further in the Bi-

Experimental
Polycrystalline samples of BiPbSr 1.15-x Ba x La 0.85 CuO 6+δ were prepared by the solid-state reaction method. Stoichiometric amounts of Bi 2 O 3 , PbO, SrCO 3 , BaCO 3 and CuO powders were mixed, pressed into pellets and heated at 770°C for 24 h in flowing gas of Ar for 24 h. The products were then annealed at 500°C for 3 h in flowing gas of O 2 . The structural analysis was carried out by the powder x-ray diffraction using CuKα radiation. The iodometric titration was carried out to estimate the oxygen content. Measurements of the thermoelectric power at room temperature were carried out to estimate the hole-concentration in the CuO 2 plane [8]. Magnetic susceptibility measurements were performed for the powdered samples using a SQUID magnetometer in a magnetic field of 10 Oe on warming after zero-field cooling in order to determine T c . Figure 1 shows the powder x-ray diffraction patterns of BiPbSr 1.15-x Ba x La 0.85 CuO 6+δ . All the samples are almost of the single phase with the Bi-2201 structure, although a small amount of Ba(Bi,Pb)O 3 is included for x ≥ 0.8. The crystal structure is orthorhombic for all the samples. As shown in Fig. 2, the lattice parameters increase linearly with increasing x(Ba) for x(Ba) ≤ 1.0, suggesting that the Cu-O distance increases through the Ba-substitution.

Results
It is well known that Ba cannot be substituted for Sr at all in Bi 2 Sr 2 CuO 6 . In contrast, Basubstituted samples are successfully synthesized in the present system. This may be explained as follows. In Bi 2 Sr 2 CuO 6 , the Ba-substitution is impossible owing to the enhancement of the size mismatch between the smaller Bi-O layer and the larger Sr-O layer. On the other hand, the size mismatch is reduced in (Bi,Pb) 2 (Sr,La) 2 CuO 6 , because larger Pb is substituted for smaller Bi and smaller La is substituted for larger Sr, which allows for the Ba-substitution for Sr in the present system. x =0  x Ba x La 0.85 CuO 6+δ . A diamagnetic signal due to the shielding effect is observed for all the samples. It is found that T c tends to decrease and the superconducting volume fraction decreases with increasing x(Ba). Figure 4 displays the x(Ba)-dependences of T c , defined at the temperature where χ changes by 0.1% of the value indicating the perfect-diamagnetism from the normal-state value, and the superconducting volume fraction V SC estimated from the value of χ at 5 K. The x(Ba)-dependences of the hole concentration in the CuO 2 plane p and the oxygen content 6+δ are also shown. Although the lattice becomes large monotonically through the Ba-substitution for Sr as shown in Fig. 2, both T c and V SC decreases gradually contrary to the empirical rule of high T c in cuprate superconductors. The x(Ba)dependence of T c is quite similar to that of p. The p is as low as ~0.12 and tends to decrease slightly with increasing x(Ba), though the excess oxygen content increases with increasing x(Ba). This suggests that holes generated by excess oxygen are supplied to the (Bi, Pb) 2 O 2 layer rather than to the CuO 2 plane. Form these results, it is concluded that the suppression of the superconductivity through the Ba-substitution is due to the decrease in p and the increase in the local lattice-distortion caused by excess oxygen. Similarly, the reason why the maximum value of T c ~ 30 K in the present system is lower than the highest T c ~40 K in the Bi-2201 phase reported [6,7] is concluded to be the low holeconcentration ~0.12 and the local lattice-distortion caused by excess oxygen.

Conclusions
Polycrystalline samples of the Bi-2201 phase of BiPbSr 1.15-x Ba x La 0.85 CuO 6+δ have been prepared by the conventional solid-state reaction method. With increasing x(Ba), the lattice becomes large, while T c decreases from ~30 K gradually. The unexpected suppression of the superconductivity may be due to the local lattice-distortion caused by excess oxygen, the number of which increases with increasing x(Ba). Despite the increase in excess oxygen, the hole concentration in the CuO 2 plane decreases slightly, indicating that holes generated by excess oxygen are supplied to the (Bi,Pb) 2 O 2 layer rather than to the CuO 2 plane. This decrease in the hole concentration in the CuO 2 plane also suppresses the superconductivity. Therefore, the removal of excess oxygen through the reduction annealing and the increase in the hole concentration in the CuO 2 plane through the decrease in the La-content will allow for the investigation of effects of the enlargement of the Cu-O distance on the superconductivity in the present system.