Structure and dynamic properties of twist boundaries in NaCl through a molecular dynamics study

NaCl Σ = 5 [001] twist boundaries, in which the ions interact through the rigid-ion model of Tosi and Fumi, have been studied by a constant-pressure molecular dynamics simulation. The Tasker-Duffy structure, in which Schottky defects are introduced on the boundary plane, was found to be stable up to the bulk melting temperature T_m. Excess grain-boundary thermodynamic properties, including the excess entropy and excess free energy, were calculated from low temperatures up to the bulk melting temperatures. The boundary stress changes significantly and even changes sign as the system is heated. An analysis of the temperature dependence of the local structural and thermodynamic properties near the boundary revealed that the thickness of the region affected by the boundary is very narrow, for the pure stoichiometric boundaries studied. Although significant disorder due to vacancy migration occurs above about 1013 K (0-95T_m), no grain-boundary melting transition was observed well below T_m. The values of the diffusion coefficients calculated for both the cations and the anions near the boundary region approach those corresponding to the supercooled liquid as disordering at the boundary progresses.