How Proton Incorporation Reshapes Lattice Dynamics In BaSnO ₃ ‐Type Proton Conductors

ABSTRACT Proton conduction in acceptor‐doped perovskites is fundamentally a vibronic process: mobile and do not move independently, but dynamically co‐vibrate with the surrounding oxygen–metal framework. Direct experimental evidence for this behavior is presented using in situ nuclear resonance vibrational spectroscopy (NRVS) on hydrated, deuterated, and dry . Hydration induces systematic redistributions in the Sn‐projected phonon density of states (PDOS), including an upshift of the first spectral moment by about 0.4 meV, indicating a stiffening of the extended Sn–O network. H/D isotopic substitution leaves the Sn‐projected PDOS largely unchanged, with only subtle isotope‐dependent spectral reweighting, demonstrating that protonic degrees of freedom are not localized oscillators but are embedded in collective lattice modes. These results are rationalized using a classical coupled proton–phonon oscillator model that links the observed PDOS variations to changes in effective force constants and vibrational mass terms. The model captures how and participate in cooperative lattice dynamics rather than forming isolated OH/OD entities. Overall, NRVS probes proton–lattice coupling in ceramic proton conductors and quantitatively describes how protonic defects modulate host lattice dynamics to enable phonon‐assisted long‐range proton transport.