Surface reconstruction of fluorites in vacuum and aqueous environment

Abstract

Surfaces and interfaces of bulk materials with liquids are of importance for a wide range of chemical processes. In this work, we systematically explore reconstructions on the (100) surface of calcium fluoride $({\mathrm{CaF}}_{2})$ and other fluorites $(M{\mathrm{F}}_{2})$, $M={\text{Sr},\text{Cd},\text{Ba}}$ by sampling the configurational space with the minima hopping structure prediction method in conjunction with density functional theory calculations. We find a large variety of structures that are energetically very close to each other and are connected by very low barriers, resulting in a high mobility of the topmost surface anions. This high density of configurational states makes the ${\mathrm{CaF}}_{2}$ (100) surface a very dynamic system. The majority of the surface reconstructions found in ${\mathrm{CaF}}_{2}$ are also present in ${\mathrm{SrF}}_{2},{\mathrm{CdF}}_{2}$, and ${\mathrm{BaF}}_{2}$. Furthermore, we investigate in detail the influence of these reconstructions on the crystal growth of ${\mathrm{CaF}}_{2}$ in solvents by modeling the fluorite-water interface and its wetting properties. We perform a global structural search both by explicitly including water molecules and by employing a recently developed soft-sphere solvation model to simulate an implicit aqueous environment. The implicit approach correctly reproduces both our findings with the explicit-water model and the experimentally reported contact angles for the partial-hydrophobic (111) and hydrophilic (100) surfaces. Our simulations show that the high anion mobility and the low coordination of the (100) surface atoms strongly favors the adsorption of water molecules over the (111) surface. The aqueous environment makes terminations with low-coordination surface atoms more stable, promoting (100) growth instead of the (111).