EVALUATION OF THE EFFECT OF POINT DEFECTS ON THE PHONON SPECTRUM OF A CRYSTAL AND THE POSSIBILITY OF EXCITATION OF NONLINEAR MODES

10.25712/ASTU.1811-1416.2022.04.002

Abstract

The crystal lattice dynamics is determined by many factors, including structural defects and external conditions. In this work, we have considered the behavior of the density of phonon states of a crystal at different concentrations of point defects and temperatures. A crystal of A₃B stoichiometry is considered, using Pt₃Al as an example, in which the existence of a band gap in the phonon spectrum of the crystal is possible. The model was a computational cell containing from 10³ to 10⁵ atoms. The particles interacted by means of a many-particle potential obtained by the embedded atom method. In this case, periodic boundary conditions were imposed in order to avoid edge and surface effects. Next, the required fraction of Al and/or Pt atoms was removed, followed by relaxation of the model for at least 20 picoseconds and heating to the required temperature. The crystal spectra were calculated directly from the data obtained in the calculations, i.e. the frequency of each atom was calculated, and the entire range of the obtained values was divided into 100 intervals to determine the density of their distribution. This approach is less accurate than theoretical calculations, but it makes it possible to track the dynamics of the spectrum; moreover, the theoretical calculation of complex lattice configurations with defects is extremely difficult. Thus, qualitative results have been obtained in the work, which make it possible to estimate the change in the density of phonon states of the lattice in the presence of point defects. The dependences obtained indicate that with an increase in temperature and defect concentration, the probability of excitation of modes increases, the frequencies of which are close to the natural frequencies of discrete breathers in a given crystal.