Study on the Fe _P Defects in KH ₂ PO ₄ Crystals

The experimental results indicate that the impurity Fe tends to occupy P sites and enhances the 220–300 nm absorption band, thereby reducing the laser‐induced damage threshold (LIDT) in KH 2 PO 4 (KDP) crystals. This work conducts a systematic study on the defect models of Fe P in different oxidation states using the first‐principles method. The calculated defect formation energies (DFE) demonstrate that the defect mainly exists in the form of Fe P ″ and Fe P ‴. The defect transition level is near the center of the bandgap and the minimal structural differences between them confirm that they can be easily converted into each other. Through analyses, it is confirmed that Fe exists in Fe P ″ and Fe P ‴ in the form of Fe 3+ and Fe 2+ .It is concluded that both Fe 3+ and Fe 2+ exist in KDP crystals and can easily transform into each other. Density of states analysis indicates that strong spin polarization defect states appear in the bandgap when Fe substitutes P in the KDP crystal. The optical absorption spectra exhibit excellent agreement with experimental results, particularly with the characteristic peaks at 263 nm. The configuration coordinate diagram reveals substantial lattice relaxation energy associated with these transitions, suggesting their significant impact on LIDT.