| Summary: | The two elements, yttrium and holmium, form a geochemical twin pair as their cations possess equivalent ratios of charge to radius. However, despite their equal electrostatics, a subtle difference in their fluoride or chloride affinity is known within solutions. In this work, we investigated whether this affinity gap is also present within the solid phase and how it depends on the surface configuration. We modeled adsorptions onto β-YF<sub>3</sub> (waimirite) and isostructural β-HoF<sub>3</sub> by periodic density functional theory. To draw conclusions on the affinity toward fluoride and chloride vs. water, adsorbates of HF, HCl, or H<sub>2</sub>O onto any of the four highly abundant surfaces of (010), (100), (011), and (101) were studied. Among others, the conformational landscape was explored by 200 ps of ab initio molecular dynamics. For stoichiometric surfaces of both MF<sub>3</sub>, we indeed found stronger adsorptions for HF than HCl. All <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>(</mo><mi>h</mi><mi>k</mi><mi>l</mi><mo>)</mo><mo>·</mo></mrow></semantics></math></inline-formula>H<sub>2</sub>O showed slightly stronger adsorption energies for HoF<sub>3</sub>, while for HF and HCl, the metal preferences varied by the surface. While (100) showed the strongest preference for HoF<sub>3</sub>, (101) preferred YF<sub>3</sub> by the same magnitude.
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