Configurational Entropy Induced Superconductive Phase Transition Temperature Dome in La 1.8 Sr 0.15 K x Na 0.05‐ x CuO 4‐ δ

The data for the temperature-dependent resistance and magnetic susceptibility of La1.8Sr0.15KxNa0.05-xCuO4-δ indicate that a superconductive phase transition temperature dome (Tc-dome) forms with the isovalent doping of Na+ by K+. This is something unusual because Tc-dome is typically formed by the aliovalent doping in a superconductor, where the charge concentration increases from under-doped, to optimal-doped, and further to over-doped state with Tc first increasing and then decreasing. X-ray diffraction data show that La1.8Sr0.15KxNa0.05-xCuO4-δ crystallizes in I4/mmm space group with a monotonic increase of the lattice parameters a and c when x increases, which excludes that the appearance of the Tc-dome is due to the appearance of impurity, the phase transition, or the nonmonotonic change of lattice parameters. The first-principles calculations suggest that the change of configurational entropy can induce the change of the ratio Cu2+:Cu3+ along with the change of oxygen vacancies in La1.8Sr0.15KxNa0.05-xCuO4-δ, which may cause the change of Tc. Therefore, it seems reasonable to imply that the dome of the change of configurational entropy in La1.8Sr0.15KxNa0.05-xCuO4-δ may induce its Tc-dome.