TOPOLOGICAL DEFECTS AT THE (001) SURFACE OF MGO - ENERGETICS AND REACTIVITY

We present the results of a theoretical study in which we investigated the properties of topological defects on the (001) MgO surface. We employed a recently developed method that provides an accurate description of local properties near a defect while taking into account the interactions between the defect and the surrounding crystal. We report calculated formation energies for steps and islands of various sizes on this surface and compare those energies to the formation of similar islands in vacuum. In addition, we evaluated the energetic contribution of various low-coordination sites to the total crystal energy. By combining these data and extrapolating the results, we were able to determine the average size of the islands. These results, in conjunction with calculations that evaluated the electronic properties of different defect sites, allowed us to conclude that there is little interaction between low-coordinated sites in a block of adatoms and the ions that comprise the ''surface'' below. The electrostatic potential acting on low-coordinated sites in a given island is mainly generated by the other co-planar ions in the island. The chemistry of these steps and islands differs considerably from that of the perfect (001) surface. In part, we attribute the enhanced reactivity of the low-coordinated sites to anisotropy in the electrostatic potential. In particular, we note the presence of ''lobes'' in the surface electrostatic potential that are oriented approximately 45 degrees off the surface normal (this region of space is also associated with the largest electric field magnitude). This finding suggests that the adsorption of small polarizable molecules would preferentially take place at about 45 degrees from the surface normal in the vicinity of certain topological defects.