Large enhancement of $T_{c}$ (= 35 K) in a sample synthesized from a starting composition of LaFeAsO$_{1-y}$H$_{x}$

Large enhancement of superconducting transition temperature ($T_{c})$ has been found in a sample synthesized from a nominal composition of LaFeAsO$_{1-y}$H$_{x}$. A hydroxide powder of La(OH)$_{3}$ is mixed with starting materials and the sample is synthesized under high-pressure. The sample shows as high as $T_{c}$ of 35 K (susceptibility onset) and 38 K (resistivity onset), while the maximum $T_{c}$ of an oxygen-deficient LaFeAsO$_{1-y}$ is 28 K. A large contraction of lattice is observed in the hydroxide-added samples.