Melting criterion based on configuron percolation

Crystalline solids melt at well-defined material-specific temperatures Tm via first-order phase transitions, whereas glasses undergo continuous transformations from solid to molten states at glass transition temperatures Tg, resembling second-order transitions. Despite extensive study, the microscopic origin of this distinction remains unveiled. In this work, both melting and glass transition are described within a unified framework based on analysis of thermally activated breakings of chemical bonds, treated as elementary excitations of condensed matter, termed configurons. The increasing concentration of configurons leads to a percolation transition corresponding to loss of mechanical rigidity of an elastic solid whose atoms are connected via chemical bonds. Configurons are delocalized and mobile in crystals, enabling their condensation and consequent latent heat release, whereas in glasses they are localized (Anderson localization), suppressing condensation and yielding a continuous transition from solid to molten states. The proposed framework provides a unified physical interpretation of phase transitions.