  La-substituted CaFeAsH,(Ca1-xLax)FeAsH, were synthesized by the solid state
reaction at 1173 K under high pressure of 2.5 GPa. The r-T anomaly
corresponding to tetragonal to orthorhombic structural transition was
suppressed by La-substitution and then the superconductivity was observed at an
electron doping content of Ne > 0.05. The maximum onset Tc of 47.4 K was
reached for Ne = 0.18, which is significant increase compared to 23 K achieved
by direct electron doping to the FeAs-layer via Co-substitution to the Fe-site.
This value of Tc value fits well to the phenomenological relation between Tc
and the bond angle of As-Fe-As in indirectly electron doped LnFeAsO (Ln =
Lanthanide). These results indicate that the electron was doped via the
indirect mode through the aliovalent ion substitution to CaH-layer (Ca2+ La3+ +
e-).
