Thermodynamic criterion for the stability of amorphous intergranular films in covalent materials

The thermodynamic origin of disordered intergranular films commonly found in covalent ceramic materials is studied by molecular-dynamics simulation. Our studies show that all high-energy grain boundaries in a model covalent material, silicon, exhibit a universal amorphous structure, whereas low-energy boundaries are crystalline. This allows the identification of a thermodynamic criterion for the existence of stable disordered intergranular films based on the relative energies of the atoms in the grain boundaries and in the bulk amorphous phase.