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In the CeO2, ThO2, and NpO2 crystals, the acoustic branches of the phonon spectra correspond predominantly to the vibrations of metal ions, and the optical branches correspond mainly to the vibrations of oxygen ones, due to a significant difference in the masses of these components of the compounds. An important crystallographic phenomenon is that metal and oxygen are located in sublattices corresponding to different Bravais types, which makes it possible to analyze vibrations in the corresponding Brillouin zones (BZ). This analysis was carried out by unfolding the optical branches of the phonon spectrum from the BZ of the crystal into the BZ of the oxygen sublattice. The number of optical branches in this BZ is half that in the BZ of the crystal. The effect of anharmonicity on the thermal broadening of the spectral lines of the phonon frequencies is studied in the approximation of two-phonon densities of states. For all three compounds, the calculated two-phonon densities of states represent two broad structured peaks. The temperature dependences of the upper peak show thermal broadening of the upper part of the optical branches and the lower peak shows thermal broadening of the low-energy optical and acoustic branches. [ABSTRACT FROM AUTHOR]

The different translational symmetry of the cuprite crystal sublattices is taken into account by unfolding the phonon spectrum from the Brillouin zone of the crystal into the Brillouin zones of the anion and metal. Such a representation of the spectrum is reasonable because 6 optical branches are essentially caused by the vibrations of anions and 12 acoustic branches are due to the vibrations of metal owing to the great value difference between their masses. The volumes of the Brillouin zones of the anion and metal sublattices are two and four times, respectively, as large as those of the crystal resulting in the qualitative differences in the corresponding unfolded spectra because of the metal tetrahedron vibrations of the distortion type in the acoustic part of the spectrum. These vibrations located on the Х′(1,0,0)–W′(1,0,1/2) and Х′(0,1,0)– W′(0,1,1/2) lines of the metal Brillouin zone are responsible for the negative thermal expansion. The unfolded spectra have only three branches in the corresponding Brillouin zones and this simplifies the selection rules for the physical processes with the participation of the sublattice particle vibrations. [ABSTRACT FROM AUTHOR]

The density functional theory is used to calculate vibrational spectra, suggest dependencies of total energies and frequencies on BaF2 and PbF2 crystal lattice constants, and obtain the density of two phonon states in PbF2 crystal weighted by thermal occupation numbers. Due to the order-of-magnitude difference between metal and fluorine masses, acoustic and optical branches are determined by metal and fluorine vibrations, respectively. Therefore, phonon spectra can be demonstrated in two Brillouin zones, namely fluorine lattice and sublattice, the number of optical branches being decreased twice. Based on the suggested dependencies of total energies in the crystal and squared frequencies on the lattice parameter, anharmonicity constants are identified. It is shown that with increasing temperature the frequency attenuates. For PbF2 crystal, anharmonicity constants are shown to be higher than for BaF2 crystal, and this fact is supported by the experiment. The model of the density of two phonon states in PbF2 crystal shows that the temperature broadening of Raman frequency is more substantial than that of the infrared active phonon frequency. [ABSTRACT FROM AUTHOR]

The phonon spectrum of hypothetical superhard cp-BC2N is calculated based on ab initio method of density functional in the center of the Brillouin zone and interpolated over the entire Brillouin zone using the Keating phenomenological model. The interaction parameters are determined by optimization of the IR- and Ramanactive frequencies for a phenomenological model by their comparison with the results of ab initio calculations. Numerical values of short-range interaction constants and charges are in agreement with the characteristics of the chemical bond calculated ab initio. These parameters have transparent physical meaning and chemical nature and can further be used for both qualitative estimations of any physical and physico-chemical quantities and quantitative calculations of the phonon spectra of a number of isostructural compounds. The Keating phenomenological model is used to study the genesis of the phonon spectrum from the spectra of sublattices. [ABSTRACT FROM AUTHOR]

In the recent decade, the spatially localized large-amplitude vibrational modes in defect-free crystals, referred to as discrete breathers (DBs), are intensively investigated in the materials science. This review reports the main results on gap DBs in crystals with the NaCl structure. The experimental proof of their existence in a NaI crystal is described. A number of molecular dynamics simulation results are presented, including new so far unpublished data. The properties of crystals potentially affected by the DBs are discussed. [ABSTRACT FROM AUTHOR]