Emergence of the Boson Peak in a Hybrid Framework Mediated by Hydrogen Bonds

The Boson peak (BP)─an excess of low-frequency vibrational states─is long regarded as a definitive signature of structural disorder. Its recent observation in nominally ordered crystals challenges this view, yet a conclusive understanding is impeded by complexities such as phase transitions, orientational glassiness, dipole disorder, or strong anharmonicity in previous model systems. Here, we introduce the layered organic-inorganic hybrids DAPCdX₄ (DAP = 1,3-diaminopropane; X = Cl, Br) as a clean platform free of phase transitions, glassiness, and any form of disorder. Low-temperature specific heat reveals a pronounced boson-peak-like anomaly with tunneling-state contributions down to 0.5 K. Crucially, the anomaly's temperature and intensity are directly modulated by hydrogen-bond strength, tuned via halide substitution (Cl → Br). Combined Raman spectroscopy and first-principles calculations trace its origin to an excess of low-frequency vibrations in the inorganic framework, induced by hydrogen-bond-mediated coupling between low-energy optical and acoustic phonons. Our work demonstrates that the boson peak can emerge intrinsically in a harmonic crystal lattice, providing a definitive structure-property relationship for controlling ∼1 THz vibrations via noncovalent interactions.