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 (Ca1-xNax)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. (C) 2008 The Japan Society of Applied Physics.