Synthesis, structure, and superconductivity of La7+2xSr1−4xKxNaxCu4O16−δ

Abstract

Sodium and potassium have been doped into La₇SrCu₄O_16−δ to form La_7+2xSr_1−4xK_xNa_xCu₄O_16−δ solid solutions by a solid-state reaction, which crystallizes in the I4/mmm space group when 0 ≤ x < 0.16 and Fmmm when 0.16 < x ≤ 0.25. With the equivalent replacement of four Sr²⁺ by one Na⁺, one K⁺ and two La³⁺, it is very interesting to find that T^zero_c (the definition is presented in the introduction part) of La_7+2xSr_1−4xK_xNa_xCu₄O_16−δ does not remain constant with an increase in x but first increases from 14.8 K for x = 0.00 to 33.9 K for x = 0.10 and then decreases to 18.5 K for x = 0.175.