Synthesis of LnFeAsO1?y superconductors (Ln=La and Nd) using the high-pressure technique

The high-pressure synthesis technique is utilized for synthesizing a series of oxygen (O)-deficient LnFeAsO (Ln=La and Nd) polycrystalline samples ranging from the undoped parent compound to the highest Tc superconductor. Introduction of the O vacancy causes a decrease of lattice parameters, and superconductivity appears when the a- and c-axis lattice parameters shrink by 0.1% compared with the undoped compounds. The maximum Tc, 28?K for Ln=La and 54?K for Ln=Nd, is achieved when the a-axis lattice parameter is shrunk by 0.2%. Upon further introduction of O-deficiency, Tc goes down to less than 20?K for Ln=La, while the maximum Tc value is sustained for Ln=Nd. The difference indicates that the superconducting region is much narrower for Ln=La than for Ln=Nd.