Chemical bonding of HF, HCl, and H 2 O onto YF 3 surfaces: Quantification by first principles.

The surfaces of waimirite β- YF 3 have been studied for their fluorine and chlorine versus water affinity. Bonding patterns of HF, HCl, and H 2 O chemically adsorbed onto surfaces of (010), (100), (011), and (101) have been quantified by density functional theory applying energy decomposition analysis. We found that the adsorption of H 2 O is dominated by about 65% of electrostatics, which causes a low surface sensitivity and weak interactions. On the contrary, the adsorptions of HF and HCl are driven by strong hydrogen bonds resulting in a highly surface-dependent ratio of 30-60% electrostatic versus orbital contribution. Among the stoichiometric surfaces, the shortest and strongest hydrogen bonds and consequently most covalent bonding patterns are found within YF 3 · HCl. However, when including the preparation energy, each surface favors the adsorption of HF over HCl, which reproduces the higher affinity of yttrium towards fluoride over chloride, previously known for solutions, also for the solid state.