Atomistic Modeling of the Low-Temperature Atom-Beam Deposition of Magnesium Fluoride

We model the deposition and growth of MgF2 on a sapphire substrate as it occurs in a low-temperature atom-beam-deposition experiment. In the experiment, an (X-ray) amorphous film of MgF2 is obtained at low temperatures of 170-180 K, and upon heating, this transforms to the expected rutile phase via the CaCl2-type structure. We confirm this from our simulations and propose a mechanism for this transformation. The growth process is analyzed as a function of the synthesis parameters, which include the substrate temperature, deposition rate of clusters, and types of clusters deposited. Upon annealing an initially amorphous deposit, we observe the formation of two competing nanocrystalline modifications during this process, which exhibit the CaCl2 and CdI2 structure types, respectively. We argue that this joint growth of the two nanocrystalline polymorphs stabilizes the kinetically unstable CaCl2-type structure on the macroscopic level long enough to be observed in the experiment.