To investigate the phonon dynamics of iron in the heavily hydride-ion-substituted region of ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{H}}_x$ ($x>0.2$), ${}^{57}\mathrm{Fe}$ nuclear resonant inelastic scattering measurements were performed over a wide temperature range from 5 to 300 K on two polycrystalline samples with $x=0.35$ and 0.51, which exhibit superconducting and antiferromagnetic ground states, respectively. The resulting inelastic scattering spectra revealed distinct differences between the two compositions. The Fe phonon density of states (PDOS) exhibits a pronounced peak at 15 meV for $x=0.35$, whereas this peak is absent in $x=0.51$. Density functional theory calculations support the interpretation that the PDOS peak at 15 meV is associated with the optical vibrational modes of Fe atoms along the nearest-neighbor direction, mediated by As atoms. The calculations further suggest that the suppression of the PDOS peak at 15 meV for $x=0.51$ originates from in-plane electronic inequivalence. These findings suggest that signatures of electronic nematicity may persist over a wide temperature range in the $x=0.51$ composition, which exhibits magnetic and structural order, whereas such signatures are almost absent in the superconducting $x=0.35$ composition.

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• Doping effects
• Lattice dynamics
• Iron-based superconductors
• Mössbauer spectroscopy
• X-ray scattering