Theoretical Study of Relaxation Processes in Phonon Spectrum of Dielectrics

Theoretical definition of dependence of the phonon lifetime in dielectrics on their type and crystal lattice temperature is considered. General expressions are deduced which define the one-particle Green's function of phonon at finite temperatures with account for three- and four-phonon processes for different types of mass operator approximation which allow one to perform numerical estimations of phonons' lifetime and renormalize their energy spectrum when elaborating the force tensors of the third and fourth orders.