Superconductivity at 26 K in (Ca1-xNax)Fe2As2

The ternary iron arsenide CaFe₂As₂ becomes superconducting by hole doping which was achieved by the partial substitution of Ca by Na. We have found bulk superconductivity at 26.0 K in a sample of (Ca_1-xNa_x)Fe₂As₂ with a nominal composition of x = 0.4. The a-parameter shrank and the c-parameter expanded by the Na substitution. The parent compound CaFe₂As₂ forms the tetragonal ThCr₂Si₂-type structure, which is a poor metal and decrease of resistivity started below 160 K, presumably associated with the spin-density wave order. By substituting Ca with Na, holes are introduced to the FeAs layers, which weakens the resistive anomaly and induces the superconductivity.