Study on point defects and La doping in KH2PO4 crystal from combined first-principles

We investigated the defect formation energy, lattice distortion, electronic structure, and optical properties of KH₂PO₄ (KDP) crystals with interstitial La defects of different charged states based on first principles. The results indicate that interstitial La defects with +1 and + 3 charged states are more stable in the crystals. The degree of H–O bond distortion is more pronounced. This can explain the significant decrease in crystal hardness. Furthermore, the electronic structure of KDP crystals containing La interstitial defects has changed compared to that of perfect crystals. Defect energy levels appear in bandgap for all charged defect states, potentially promoting multiphoton absorption. The obtained spectra considered electro-phonon coupling exhibit larger Stokes shifts and non-radiative jumps, which significantly lower the laser damage threshold of the crystals. Furthermore, PE-KDP containing La interstitials defects exhibits an absorption peak at 5.36 eV (231 nm), which aligns well with the experimentally observed absorption band at 250 nm due to La impurities.