Boson peaks of glassy mono- and polyalcohols studied by inelastic neutron scattering

We have measured the inelastic neutron scattering spectra of the glasses of six mono- and polyalcohols; 1-propanol, ethylene glycol, propylene glycol, 1,3-propanediol, glycerol and threitol. Broad excitation peaks with peak top energy of 3-5 meV appeared in all of the S(2θ,E) spectra. These peaks were identified as boson peaks characteristic of glassy materials from the temperature dependence of the peak intensity. We found a systematic relation among the boson peak energy, the boson peak intensity per molecule and the hydrogen-bond density estimated as the ratio of the number of hydroxyl groups to that of carbon atoms (NOH/NC); the peak energy decreases and the peak intensity increases as hydrogen-bond density decreases. The present result indicates that the origin of the boson peak in network glasses is related to the flexible part in the network structure (e.g., non-hydrogen-bonded alkyl-groups in alcohol glasses). We also measured partially deuterated propanol (CD3CD2CD2OH and CH3CH2CH2OD) and glycerol (CD2(OH)CD(OH)CD2OH and CH2(OD)CH(OD)CH2OD). Both energy and intensity of the boson peak were not affected much by the partial deuteration, indicating that the hydrogen-bonding and non-hydrogen-bonding (alkyl) parts contribute to the boson peak cooperatively. The present result was compared with the predictions from a simple model recently developed by Nakayama et al.