F-Interstitial Passivation Preserves Host-Like Optoelectronic Properties in ²²⁹Th:YLF Nuclear-Clock Platforms

Abstract

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 (Fᵢ). The near vacancy has the lowest formation energy, while Fᵢ 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 Fᵢ 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 Fᵢ route maintains a host-like absorption edge. Overall, Fᵢ emerges as an energetically competitive passivation route that preserves host-like optoelectronic behavior among the charge-compensation paths considered here.