ON THE THERMODYNAMIC STABILITY OF AMORPHOUS INTERGRANULAR FILMS IN COVALENT MATERIALS

The thermodynamic origin, structure, and stability of the thin amorpho us films commonly found in grain boundaries in covalent ceramics are i nvestigated by molecular-dynamics simulation, To focus on the purely t hermodynamic aspects, any kinetic effects associated with impurity-con trolled interface chemistry are excluded by investigating pure silicon (described by the Stillinger-Weber three-body potential), For this si ngle-component covalent model material, we demonstrate that all high-e nergy boundaries exhibit a universal amorphous structure, with a width of similar to 0.25 nm, whereas low-energy boundaries are crystalline and much sharper, We also demonstrate that introduction of an amorphou s film into a crystalline interface lowers the excess energy to a leve l similar to the energy of two bulk crystal-amorphous interfaces, The competition between a narrow crystalline boundary structure and a wide r amorphous boundary structure is shown to be governed by the relative excess energies of the atoms in the grain boundaries and in the bulk amorphous phase, These observations suggest that, in principle, amorph ous grain-boundary films do not require impurities for their stabiliza tion and that, as first proposed by Clarke, an equilibrium grain-bound ary phase of uniform thickness can be the result of purely thermodynam ic rather than kinetic factors.