Different modes of water aggregation on (111) surfaces of fluorite structures

Moleclar-dynamics studies of water aggregation on (111) surfaces of ${\mathrm{BaF}}_2$ and ${\mathrm{CaF}}_2$ show that at a critical coverage of about 0.5 monolayer, regular chain and ring patterns develop on ${\mathrm{BaF}}_2$ and ${\mathrm{CaF}}_2$, respectively, governed by the free energy of the system. At one monolayer, the structure on ${\mathrm{CaF}}_2$ randomizes, whereas on ${\mathrm{BaF}}_2$ it forms a two-dimensional pattern, commensurate with the substrate ${\mathrm{F}}^{\mathrm{-}}$ sublattice. Beyond one bilayer, regular ice-Ih grows on ${\mathrm{BaF}}_2$, stable even at room temperature. Vibrational frequencies at low coverages and low-energy electron diffraction patterns for high coverages are predicted.