A Theoretical Study of Structural, Electronic and Vibrational Properties of Small Fluoride Clusters

Alkaline earth metal fluorides are an interesting family of ionic crystals having a wide range of applications in solid state lasers, luminescence, scintillators, to name just a few. In this work, small stoichiometric clusters of (MF$_{\mathrm{2}})_{\mathrm{n}}$ (M$=$ Mg, Ca Sr, Ba, n$=$1-6) $_{\mathrm{\thinspace }}$were studied for structural, vibrational and electronic properties using first-principles methods based on density functional theory. A clear trend of structural and electronic structure evolution was found for all the alkaline earth metal fluorides when the cluster size n increases from 1 to 6. Our study reveals that these fluoride clusters mimic the bulk-like behavior at the very small size. Among the four series of metal fluorides, however, (MgF$_{\mathrm{2}})_{\mathrm{n\thinspace }}$clusters stands out to be different in its preference of equilibrium structures owing to the much smaller ionic radius of Mg and the higher degree of covalency in the Mg-F bonding. The calculated binding energy, highest stretching frequency, ionization potential, and HOMO-LUMO gap decrease from MgF$_{\mathrm{2}}$ to BaF$_{\mathrm{2}}$ for the same cluster size. These variations are explained in terms of the change in the ionic radius and the basicity of the metal ions.