Synthesis of ErFeAsO-based superconductors by the hydrogen doping method

Here we demonstrate the technique to stabilize the ErFeAsO-based superconductor with the smallest lattice constants in LnFeAsO_1-y (Ln=lanthanide) series using the hydrogen doping method. Polycrystalline samples were synthesized by heating pellets with nominal compositions of ErFeAsO_1-y (1-y=0.75-0.95) sandwiched between pellets of LaFeAsO_0.8H_0.8 compositions at 1100 ^°C under a pressure of 5.0-5.5 GP. The sample with lattice constants, a=3.8219 Å and c=8.2807 Å, shows the highest superconducting critical temperatures (T_c), 44.5 K and 41.0 K, determined by onset transitions of resistivity and susceptibility, respectively. We discuss the phase diagram of the Ln-dependence of T_c in LnFeAsO-based superconductors.