Structure and properties of potassium iodide nanoparticles. A molecular dynamics study

Properties of solid and liquid clusters ranging in size from 108 to 500 KI pairs have been investigated in computer simulations. Melting points, densities or, for the liquid, a density profile, coefficients of thermal expansion, heat capacities, heats of fusion and diffusion coefficients were determined, for the most part as functions of particle size. Coefficients of thermal expansion showed the greatest differences from the bulk, particularly for the liquid where the coefficient was twice as large. Effects of the Laplace pressure could be discerned in both the liquid and solid densities. Although the solid particles retained their well-developed facets and exhibited little surface diffuseness until very warm, the molten clusters had very diffuse liquid–vapor interfaces. Unlike the case of liquid argon clusters, the surface diffuse layer was considerably thicker than could be accounted for by classical capillary waves. As had been found in the liquid state of bulk salts, the closest cation–anion internuclear distances were appreciably shorter than in the solid, a consequence of the lower coordination numbers. The solid clusters possessed what is nominally the face centered cubic rock salt structure but deviated from it in that the cations tended not to be midway between the anions. This tendency resembles that in the liquid, though to a smaller extent. It also can be attributed to the diminished coordination number of those ions at the surface. This asymmetry causes alternations in K–I distances to be propagated into the interior.