Struture Stability and Compressibility of Iron-Based Superconductor Nd(O_(0.88)F_(0.12))FeAs under High Pressure

摘要

Very recently, La(O_(1-x)F_x)FeAs (x = 0.05-0.12) was found to be an electron-doped superconductor with the transition temperature T_c at about 26 K, by doping with F ions at the O-site in LaOFeAs. Substituting other small-radius lanthanide ions for La ions in La(O_(1-x)F_x)FeAs evidently increases the T_c. It was reported the T_c is ～55 K for Sm(O_(0.9)F_(0.1))FeAs, which clearly shows the chemical inner pressure effect on the superconducting property. All these compounds adopt the ZrCuSiAs type structure with the space group P4mm. Arranged alternately along the c-axis, the Ln_2(O, F)_2 layer (Ln = lanthanide elements) and Fe_2As_2 layer are the charger reservoir layer and conduction layer, respectively. There is a relatively large interspace between the two neighbor layers along the c-axis, indicating that high pressure will remarkably adjust their structures and consequently tailor their physical behaviors.rnSome in situ high-pressure experiments on these compounds have been performed. For example, the T_c of F-doped LaOFeAs was reported to increase with pressure and reach a maximum of ～43 K at 4 GPa, obtained from electrical resistance measurements under high pressure. However, the behavior of structural stability and the equation of state of these iron-based superconductors under high pressure still remain unknown. It is important to understand the structural evolution of these new iron-based superconductors under pressure. Among these new iron-based superconductors, Nd(O_(1-x)F_x)FeAs is a good candidate for this study since it is a sister compound with La(O_(1-x)F_x FeAs and crystallizes into the similar structure with a larger T_c of near 50 K at x = 0.12. In this report, we present the discovery of pressure-induced structural transition on this iron-based superconductor by using a diamond anvil cell (DAC) technique combined with an in situ angle-dispersive synchrotron X-ray diffraction (AD-XRD) experimental technique.