The superconducting transition temperature T_c of the Bi-2212 phase of Bi₂Sr₂CaCu₂O_8+δ should be higher than 100 K for the practical use of Bi-2212 at liquid N₂ temperature. Considering an empirical rule in the cuprate superconductors that T_c increases with increasing distance between Cu²⁺ and the apical O^2-, the substitution of larger Ba²⁺ for smaller Sr²⁺ is effective way. Recently, we have succeeded in the Ba²⁺-substitution for Sr²⁺ by the heavily Pb²⁺-substitution for Bi³⁺ [1]. In BiPbBa₂NdCu₂O₈, however, the hole doping is not realized. This may be because Nd³⁺ with the larger valence than Ca²⁺ prevents hole doping into CuO₂ phase. Therefore, it is expected that superconductivity appears by the hole-doping in the F^--substituted BiPbBa₂CaCu₂O₇F. In this study, we have attempted the synthesis of BiPbSr₂CaCu₂O_8-xF_x, which is expected to be easier to be obtained than BiPbBa₂CaCu₂O_8-xF_x.

Polycrystalline samples of BiPbSr₂CaCu₂O_8-xF_x (0 ≤ x ≤ 1) were prepared by the solid-state reaction method. As shown in Fig. 1(a), the Bi-2212 phase appears as the main phase by the F^--substitution although the impurity phases still remain. As shown in Fig. 1(b), the low T_c of ~80 K may be due to the cation nonstoichiometry caused by the formation of the impurity phases. Concludingly, the F-substitution is useful for the formation of the heavily Pb-doped Bi-2212 phase.

[1] T. Watanabe et al., Jpn. J. Appl. Phys. 59, 050902 (2020).

Fig. 1. (a) Powder X-ray diffraction patterns of BiPbSr₂CaCu₂O_8-xF_x (0 ≤ x ≤ 1). (b) Temperature dependence of the magnetic susceptibility for BiPbSr₂CaCu₂O_8-xF_x (x=0.8, 1.0).

Keywords: Bi-2212, superconducting transition temperture, Pb-substitution, F-substitution