Superconductivity at 54 K in F-free NdFeAsO1-y

We have synthesized a series of oxygen-deficient, fluorine-free polycrystalline samples with nominal chemical formula NdFeAsO1-y using high-pressure technique. The ZrCuSiAs type crystal structure, as expected for NdFeAsO, is formed as the main phase for the composition 0 <= y <= 0.6. Sharp superconducting (SC) transition with their transition temperature (T-c) as high as 54K is observed for 0.3 <= y <= 0.8 samples, which is higher than that of F-doped NdFeAsO. While T, does not depend on y, SC volume fraction shows maximum at y = 0.4. Around the same y, the lattice parameters show discontinuous shrinkage. These experimental results suggest the existence of a stable superconducting phase with a fixed atomic ratio close to Nd : Fe : As : O = 1 : 1 : 1 : 0.6, presumably associated with a regular array of oxygen deficiency.