On the directivity of three-phonon interactions

The quantum-mechanical conceptions of the phonon gas are applied for analysis of the three-phonon interaction. It is shown that the directivity of the processes in this interaction depends on the occupation numbers of the phonon branches and is determined by the tendency of the system to maximum entropy. At equilibrium, the energy is, in the general case, distributed nonuniformly among the branches, indicating that three-phonon interactions are insufficient to establish complete thermodynamic equilibrjum. Cases of "coalescence" and decay of phonons are an­ alyzed.