Boson peaks in hydrogen-bonded molecular glasses

Abstract

We have measured the inelastic neutron scattering spectra of the following three hydrogen-bonded molecular glasses: (1) mono- and polyalcohols with carbon numbers smaller than 4, (2) the mixture of 1,3-propanediol and 1,2-propanediamine, and (3) aqueous solutions of LiCl. A broad excitation peak identified as a “boson peak” appeared at 2–5 meV in all of the S( Q, E) spectra. To characterize the three hydrogen-bond systems, “hydrogen-bond density” was defined for each of the substance groups: (1) the ratio of the number of hydroxyl groups to that of carbon atoms ( N OH/ N C), (2) the deviation of the composition from x=0 or 1 (the hydrogen-bond density becomes maximum at the center composition x=0.5), and (3) the concentration of water (electrolytes will destroy hydrogen bonds). We found a systematic relation among the boson peak energy, the boson peak intensity per molecule, and the hydrogen-bond density; the peak energy decreases and the peak intensity increases as hydrogen-bond density decreases.