  Here we demonstrate the technique to stabilize the ErFeAsO-based
superconductor with the smallest lattice constants in LnFeAsO1-y (Ln =
lanthanide) series using hydrogen doping method. Polycrystalline samples were
synthesized by heating pellets with nominal compositions of ErFeAsO1-y (1-y =
0.75 ~ 0.95) sandwiched between pellets of LaFeAsO0.8H0.8 compositions at 1100
{\deg}C under a pressure of 5.0 - 5.5 GP. The sample with lattice constants of
a = 3.8219 {\AA} and c = 8.2807 {\AA} shows the highest superconducting
critical temperatures (Tc) of 44.5 K and 41.0 K determined by onset transitions
of resistivity and susceptibility, respectively. We discuss phase diagram of Ln
dependence of Tc in LnFeAsO-based superconductors.
