The cuprate superconductor YBa₂Cu₄O₈ (Y-124) has the double Cu-O chains without oxygen nonstoichiometry as shown in Fig. 1(a), while YBa₂Cu₃O_7-δ (Y-123) has the single Cu-O chains with oxygen nonstoichiometry. Due to this high stability of oxygen content, Y-124 is a promising material for the practical application of superconductivity. However, T_c ~ 80 K of Y-124 is too low for the practical application at 77 K. The relatively low T_c of Y-124 is due to the low hole-concentration in the CuO₂ planes. It is reported that its T_c increases up to ~90 K, which is almost the same as 92 K of Y-123, by increasing the hole concentration through the partial substitution of Ca²⁺ for Y³⁺ [1]. More interestingly, T_c is greatly enhanced up to 108 K by application of a high pressure of 12 GPa [2]. On the other hand, T_c increases only up to about 90 K by applying the chemical pressure through the partial substitution of Sr²⁺ for Ba²⁺ [3]. In this study, we have investigated the effects of co-substitution of Ca²⁺ for Y³⁺ and Sr²⁺ for Ba²⁺ on T_c in Y_1-xCa_xBa_2-ySr_yCu₄O₈ to explore the possibility of T_c ~ 100 K at ambient pressure.

Samples of Y_1-xCa_xBa_2-ySr_yCu₄O₈ were prepared by the molten KOH method. As shown in Fig. 1(b), Y-124 phase was obtained for x(Ca) ≤ 0.2 and y(Sr) ≤ 0.2, although small amounts of impurity phases are included.We will report the co-substitution effect on T_c.

 [1] T. Miyatake et al., Nature 341, 41(1989). [2] E. N. van Eenige et al., Physica C 168, 482 (1990). [3] Y. Miyachi et al., Jpn. J. Appl. Phys. 58, 070906 (2019).

Fig. 1 (a) Crystal structure of Y-124. (b) Powder x-ray diffraction pattern of Y_0.8Ca_0.2Ba_1.8Sr_0.2Cu₄O₈.

Keywords: Y-124, elemental substitution, superconducting transition temperature, hole concentration