Unusual doping dependence of superconductivity in NayFeAs

Superconductivity and phase relationships were explored in the Na-Fe-As system. The PbFCl-type 111 phase is stable only within a Na stoichiometry range of 1.00 to similar to 0.85, and exhibits bulk superconductivity within an even narrower range around 0.90 in Na0.9FeAs. In particular, stoichiometric NaFeAs is not a bulk superconductor. The onset of the superconducting transition varies in a totally different way and the highest T-c occurs in multiphase samples with a nominal composition of Na:Fe:As=0.5:1:1, where the superconductive volume-fraction is almost zero. Such doping dependency is rather surprising and in disagreement with most expectations.