Stabilization of a non-superconducting, orthorhombic phase by over-hydrogenating LaFeSiH
Chemical composition provides a powerful route to tune the electronic ground state of iron-based superconductors and other quantum materials, yet access to highly doped phases remains limited. Here we demonstrate that high-pressure thermal decomposition of hydrogen-rich precursors enables over-hydrogenation of LaFeSi. Using anthracene, we synthesize tetragonal superconducting LaFeSiH, including a single hydrogen site, while ammonia borane yields a structurally distorted over-hydrogenated phase, LaFeSiH1+x, with an orthorhombic structure. Chemical analysis reveal excess hydrogen (x ~ 0.6), implying a second H site in LaFeSiH1.6 whose localization and occupancy are determined by neutron diffraction. In contrast to metallic LaFeSi and superconducting LaFeSiH, orthorhombic LaFeSiH1.6 exhibits semiconductor-like behavior. Upon hydrogen release near 100 {\deg}C, it transforms into tetragonal superconducting LaFeSiH1+{\delta} ({\delta} << 0.6). These results establish the chemical flexibility of the layered LaFeSiX (X = H, O, F) family and provide access to a high hydrogen-doping regime, creating new opportunities to investigate superconductivity in Fe-based silicides.