Fe-vacancy complexes in KDP: contrasting force partitioning reveals the Fe 3d-O 2p hybridization bottleneck

A critical missing step in understanding how Fe impurities lower the laser damage threshold of potassium dihydrogen phosphate (KH₂PO₄, KDP) crystals is the transduction of sub-bandgap photon absorption into localized structural instability, which provides the initial structural driving force for material failure. Using first-principles constrained-occupation Δ_SCF calculations, we investigate two charge-compensated Fe_K-vacancy complexes and find that the nature of the compensating vacancies partitions the photoexcited driving force in fundamentally distinct ways in the 64-atom supercell studied here: K-vacancy compensation disperses the force throughout the PO₄ framework, whereas H-vacancy compensation concentrates it within the Fe-O coordination shell. Fluorine substitution, introduced as an extreme stress test, disrupts the Fe 3d-O 2p hybridization and collapses the defect-assisted optical absorption and photoexcited forces, regardless of the targeted oxygen site. The spatial pattern of this suppression is compensation-dependent: the framework-mediated pathway is most fragile at its PO₄ nodes, while the shell-localized pathway is most fragile at its Fe-O core. These findings identify the Fe 3d-O 2p hybridization architecture, shaped by the vacancy environment, as the bottleneck that governs the coupling of optical excitation to structural forces, and point to this hybridization as a pivotal control point for mitigating photoexcited structural responses in KDP.