Nature of boson peak and associated heat capacity anomaly caused by structural and proton disorder in I and LDA Ices.

In this work, the nature of the boson peak associated with the interaction between acoustic and optical phonons is revealed in low-density amorphous LDA ice and hexagonal crystalline ice (I) using the lattice dynamics method within the SPC/E water model. The appearance of these vibrations is associated with proton disordering in ice I and with both protons and oxygen atoms in LDA ice. Based on calculation of the eigenfrequencies and eigenvectors of vibrations, dispersion curves were constructed, the dynamics of water molecules involved in various vibrations was characterized, and the spatial molecular distribution was visualized. This enabled the identification of the structural inhomogeneities of the molecules participating in the vibrations. The presence of a boson peak in both amorphous LDA ice and crystalline ice I and its relation to proton or molecular disorder are shown. The collective nature of the vibrations that form the boson peak is discussed. The distances between neighboring water molecules involved in high-amplitude boson peak vibrations in both LDA and I ices are below the average value and are located more closely to each other. Analysis of the obtained data showed that the appearance of the boson peak can be associated with the interaction of acoustic vibrations with the vibrations of structural inhomogeneities, which form low-energy optical vibrations. This paper shows for the first time a boson peak-like anomaly in the heat capacity of the structure of crystalline ice I, as well as amorphous LDA ice. Our work confirms that the mechanism of formation of the boson peak is associated with acousto-optical resonance, leading to pseudo van Hove singularities of the acoustic phonon branches, and is also associated with the densification of low-frequency optical branches, which are quasi-degenerate with longitudinal acoustic vibrations, and leads to the appearance of excess low-frequency vibrations.