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 overhydrogenation of LaFeSi. Using anthracene, we synthesize tetragonal superconducting LaFeSiH, including a single hydrogen site, while ammonia borane yields a structurally distorted overhydrogenated phase, ${\mathrm{LaFeSiH}}_{1+x}$, with an orthorhombic structure. Chemical analysis reveals excess hydrogen ($x\approx$ 0.6), implying a second H site in ${\mathrm{LaFeSiH}}_{1.6}$ whose localization and occupancy are determined by neutron diffraction. In contrast to metallic LaFeSi and superconducting LaFeSiH, orthorhombic ${\mathrm{LaFeSiH}}_{1.6}$ exhibits semiconductorlike behavior. Upon hydrogen release near 100 °C, it transforms into tetragonal superconducting ${\mathrm{LaFeSiH}}_{1+\delta }$ ($\delta \ll 0.6$). These results establish the chemical flexibility of the layered $\mathrm{LaFeSi}X$ ($X$ = H, O, F) family and provide access to a high hydrogen-doping regime, creating new opportunities to investigate superconductivity in Fe-based silicides.

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• Superconductivity
• Hydrogenated materials
• Iron-based superconductors
• Polycrystalline materials
• Electron diffraction
• Magnetization measurements
• Mass spectrometry
• Nuclear magnetic resonance
• Resistivity measurements
• Sample preparation
• Thermogravimetric analysis
• X-ray powder diffraction