The excited state model and an elementary act of softening of glassy solids

A new elementary act of the glass-liquid transition is proposed, and the softening criterion is calculated for different classes of glassy systems. The transition of an amorphous substance from a glassy solid state to a liquid state is explained by defreezing of the process of excitation of the kinetic unit responsible for viscous flow when the average energy of thermal lattice vibrations becomes equal to or higher than the work of the ultimate elastic deformation of the interatomic bond, which corresponds to the maximum of the quasi-elastic force. The excitation of the kinetic unit is considered to mean its critical displacement from the equilibrium position, which corresponds to the maximum of the attractive force between particles. The kinetic unit (an atom or a group of atoms) capable of being displaced over a critical distance is referred to as an excited atom, and the approach under consideration is termed the excited state model. The nature of excited atoms in silicate glasses and amorphous polymers is discussed. The nature of fluctuation holes in liquids and glasses is considered in the excited state model. The known Frenkel exponential formula for the hole concentration acquires the meaning of the probability of excitation of the kinetic unit responsible for viscous flow. The elementary act of softening of glasses can serve as a molecular mechanism of their plastic deformation.