Unraveling the Hf4+ Site Occupation Transition in Dy: LiNbO3: A Combined Experimental and Theoretical Study on the Concentration Threshold Mechanism

Precise control over defect structures is essential for tuning the functional properties of lithium niobate (LiNbO3) crystals. Although the threshold effect of Hf4+ doping is well recognized, its underlying atomic-scale mechanism, especially in systems co-doped with luminescent rare earth ions, remains unclear. In this study, we combine experimental and theoretical approaches to elucidate the Hf4+ concentration-driven threshold behavior in Dy: LiNbO3 crystals. A series of crystals with Hf4+ concentrations of 2, 4, 6, and 8 mol% were grown using the Czochralski method. Characterization through XRD and IR spectroscopy identified a threshold near 4 mol%, evidenced by an inflection in lattice constants and a pronounced blue shift of the OH− absorption peak. UV–Vis–NIR absorption spectra revealed a systematic enhancement of Dy3+f–f transition intensities, linking the global defect structure to the local crystal field of the optical activator. First-principles calculations showed that Hf4+ ions preferentially occupy Li sites, repairing antisite Nb defects (NbLi4+) below the threshold, and incorporate into Nb sites beyond it, inducing structural reorganization. Electron Localization Function analysis visualized strengthened Hf-O covalent bonding in the post-threshold regime. This work establishes a complete atomic-scale picture connecting dopant site preference, chemical bonding, and macroscopic properties, providing a foundational framework for the rational design of advanced LiNbO3-based materials.