Calculations of Li adsorption and diffusion on MgO(100) in comparison to Ca

Abstract

Density-functional theory calculations are used to study the adsorption and growth of small Li clusters on MgO(100). The binding of Li is found to be similar to that of Ca in many respects. Monomers bind to oxygen sites on the terrace and diffuse at room temperature to defect sites. A range of defect sites will bind monomers with increasing energies from oxygen vacancies, steps, kinks, divacancies, and magnesium vacancies, as well as to peroxo species on step edges. The binding strength correlates with the amount of Bader charge transferred from the adsorbing metal to the substrate. Small clusters are found to be highly mobile on the surface at room temperature. In two cases, we have found qualitative differences in the binding energies of Ca and Li which lead to different growth modes. First, Ca binds less strongly to charged oxygen vacancy defects (${\text{F}}^{+}$ centers) than it does on the terrace whereas Li forms a bond with the electron in the vacancy, and is trapped at room temperature. Second, the gas phase Ca dimer distance is longer than the O-O distance on the MgO surface so that epitaxial Ca islands are strained whereas Li is not. These two differences have a profound effect on the growth of clusters. At room temperature, a Ca monomer will not trap at defect sites occupied by another Ca monomer. Thus, Ca atoms diffuse on the terrace until the defects are saturated before nucleating three-dimensional islands. On the other hand, two-dimensional epitaxial Li island nucleate at Li-bound defects. These calculations explain the lower initial heat of adsorption of Li and higher surface coverage as compared to Ca in adsorption microcalorimetry measurements by Farmer et al. [J. Am. Chem. Soc. 131, 3098 (2009)].