F-Interstitial passivation preserves host-like optoelectronic properties in ²²⁹Th:YLF nuclear-clock platforms

In this work, we use density-functional theory to study charge compensation in ²²⁹Th:LiYF₄ and its impact on the optoelectronic response. We compare three Li-vacancy configurations (near, mid, and far from Th) with an adjacent fluorine interstitial (F_i). The near vacancy has the lowest formation energy, while F_i is only 0.11 eV higher, and the mid and far vacancies are significantly less favorable. We find a clear mechanism-level contrast between vacancy- and interstitial-based compensation: vacancy-induced changes reduce the local electrostatic (Madelung) field and shift Th d/f character toward the band edges, narrowing the gap, whereas F_i screens the Th center and preserves a more host-like gap without introducing midgap states. Local PDOS including spin-orbit coupling supports this passivating behavior, and optical ε₂(ω) shows transparency near the 8.36 eV nuclear-transition energy; importantly, only the F_i route maintains a host-like absorption edge. Overall, F_i emerges as an energetically competitive passivation route that preserves host-like optoelectronic behavior among the charge-compensation paths considered here.