Calorimetric and neutron scattering studies on the boson peak of lithium chloride aqueous solution glasses

Abstract

The heat capacities of the glassy and liquid states of three lithium chloride aqueous solutions (LiCl⋅4H2O,LiCl⋅6H2O,LiCl⋅7.5H2O) were measured with an adiabatic calorimeter in the temperature range 5–340 K; the crystalline state was also measured for LiCl⋅4H2O. For all of the glassy samples, a large non-Debye excess heat capacity appeared below 30 K. The neutron scattering spectra of glassy LiCl⋅4H2O and LiCl⋅6H2O were measured with two spectrometers operating with different energy windows (E<20 meV and E<100 meV). A boson peak, corresponding to the excess heat capacity, appeared at around 4 meV in the S(Q,E) spectra of both glassy samples. From the analysis combining the heat capacity and neutron scattering data, the excess density of states due to the boson peak (compared with hexagonal ice) was found to be 0.10 degree of freedom per water molecule for both glassy samples. A librational peak of water molecules appeared at around 60 meV in glassy samples. This energy is about 10 meV lower than that of hexagonal and amorphous ices, indicating that the hydrogen bonds connecting water molecules are broken due to the strong coordination of water molecules to lithium ions.