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 NdFeAsO 1- 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 54 K is observed for 0.3≤ y ≤0.8 samples, which is higher than that of F-doped NdFeAsO. While T c 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.