First Principle Surface Analysis of YF₃ and Isostructural HoF₃

The trifluorides of the two high field strength elements yttrium and holmium are studied by periodic density functional theory. As a lanthanide, holmium also belongs to the group of rare earth elements (REE). Due to their equivalent geochemical behavior, both elements form a geochemical twin pair and consequently, yttrium is generally associated with the REE as REE+Y. Interestingly, it has been found that DFT/DFT+U describe bulk HoF₃ best, when the 4f-electrons are excluded from the valence region. An extensive surface stability analysis of YF₃ (PBE) and HoF₃ (PBE+Ud/3 eV/4f-in-core) using two-dimensional surface models (slabs) is performed. All seven low-lying Miller indices surfaces are considered with all possible stoichiometric or substoichiometric terminations with a maximal fluorine-deficit of two. This leads to a scope of 24 terminations per compound. The resulting Wulff plots consists of seven surfaces with 5-26% abundance for YF₃ and six surfaces with 6-34% for HoF₃. The stoichiometric (010) surface is dominating in both compounds. However, subtle differences have been found between these two geochemical twins.