Study on the effects caused by defect LaK in KH2PO4 crystal

Based on first-principles calculations, we have investigated the effects of different charged states of LaK on the defect formation energies (DFE), lattice distortions, electronic structures, and optical properties of paraelectric phase (PE-KDP) and ferroelectrical phase (FE-KDP) crystals. Our calculations indicate that LaK·· is readily formed in both crystal structures, and the degree of H–O bond distortion is more pronounced in PE-KDP. This could explain the formation of small growth mounds on the surface of La-doped crystals observed in the experiments, as well as the large decrease in the hardness of La-doped crystals. Furthermore, the band gap value of La-doped crystals is smaller than that of perfect crystals. This can be attributed to the fact that the 5d orbitals of La affect the 2p orbitals of O located in the conduction band minimum (CBM), resulting in a splitting of the energy levels and the formation of a defective energy level that enters the forbidden band. The optical spectra have been obtained considering the electron-phonon coupling. Large stokes redshift and nonradiative jump will lower the crystal laser damage threshold. The La-doped PE-KDP has an absorption band at 3.03 eV (409 nm) in good agreement with the experimentally observed absorption band at 390 nm.