| Summary: | 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 HoF<sub>3</sub> best, when the 4f-electrons are excluded from the valence region. An extensive surface stability analysis of YF<sub>3</sub> (PBE) and HoF<sub>3</sub> (PBE+U<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mi mathvariant="normal">d</mi></msub></semantics></math></inline-formula>/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 YF<sub>3</sub> and six surfaces with 6–34% for HoF<sub>3</sub>. The stoichiometric (010) surface is dominating in both compounds. However, subtle differences have been found between these two geochemical twins.
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