We compare the atomic dynamics of the glass to that of the relevant crystal. In the spectra of inelastic scattering, the boson peak of the glass appears higher than the transverse acoustic (TA) singularity of the crystal. However, the density of states shows that they have the same number of states. Increasing pressure causes the transformation of the boson peak of the glass towards the TA singularity of the crystal. Once corrected for the difference in the elastic medium, the boson peak matches the TA singularity in energy and height. This suggests the identical nature of the two features.

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Abstract. Ab initio calculations and a direct method have been applied to derive the phonon dispersion curves and phonon density of states for the TiC crystal. The results are compared and found to be in a good agreement with the experimental neutron scattering data. The force constants have been determined from the Hellmann-Feynman forces induced by atomic displacements in the 2×2×2 supercell. The calculated phonon density of states suggests that vibrations of Ti atoms form acoustic branches, whereas the motion of C atoms is confined to optic branches. The elastic constants have been found using the deformation method and compared with the results obtained from acoustic phonon slopes. The transition-metal carbide compounds, of which TiC is a representative, are of big scientific and technological interest because of their striking mechanical properties, extreme hardness combined additionally with metallic electrical and thermal conductivity. They have usually a rock-salt crystal structure -like some ionic crystalswhile having properties typical for materials with covalent bonding. It suggests that the bonding have some mixed nature, which makes these materials very interesting as an object of study. PACSOver a large range of fractional carbon content from TiC to TiC 0.5 [1] the TiC is stable in the NaCl structure, with space group Fm3m. In practice, it is usually nonstoichiometric and contains carbon vacancy defects. The phonon dispersion curves of nearly stoichiometric TiC 0.95 have been measured along main symmetry directions by Pintschovius et al [2] using the inelastic neutron scattering technique. In the same paper, the influence of the carbon content on the phonon dispersion curves has been additionally checked by measuring a sample with lower carbon concentration TiC 0.89 . The results shown that the optic modes could differ by 3.5%, while the acoustic modes are less sensitive to carbon content. The elastic constants derived from the measured acoustic dispersion curves agree very well with those obtained by ultrasonic measurements for TiC 0.91 crystal [3].Very interesting properties and simple structure of TiC caused that efforts to understand its properties using firstprinciple calculations have been undertaken several times [4,5,6,7]. The electronic structure, bulk modulus, and elastic constants have been found [8] by means of accurate first-principles total-energy calculations using the full potential linear muffin-tin orbital method. The calculated In this paper we intend to extend first-principle calculations to describe the phonon dispersion curves and phonon density of TiC. The method which was is based on the total energy calculation and Hellmann-Feynman (HF) forces. Dispersion relations are calculated by direct method [9,10,11,12,13], in which the force constants of the dynamical matrix are calculated from HF forces. Elastic constants and bulk modulus have been estimated by straight evaluation of energy derivatives with respect to deformation.The energies and forces of the TiC crystal were calcu...

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The first principles plane-wave pseudopotential calculations based on density functional theory were used to determine structural and electronic properties of CuInS2 semiconducting chalcopyrite. The Born effective charges were obtained using Berry phase method. Phonon dispersion relations were evaluated from Hellmann–Feynman forces with the direct method. The elastic constants were derived from the stress–strain relationships. The calculated quantities agree well with available experimental data.