Особенности фононных спектров кристаллов CeO2, ThO2, NpO2, обусловленные структурой их подрешеток

Abstract

In CeO2, ThO2, and NpO2 crystals, the acoustic branches of the phonon spectrum correspond predominantly to the vibrations of metal ions, the optical ones mainly to the vibrations of the oxygen due to a significant difference in the mass values of these components of the compounds. An important crystallographic phenomena - 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 sublatftice,the number of optical branches in this BZ is twice as small than 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. The calculated two-phonon densities of states for all three compounds represent two broad structured peaks. The temperature dependences of the upper peak show the thermal broadening of the upper part of the optical branches, and the lower peak - low-energy optical and acoustic branches.