First-principles study on the optical and electronic properties of YK defects in KH2PO4 crystals

The defect formation energy, electronic structures and optical properties of paraelectric phase (PE-KDP) and ferroelectric phase (FE-KDP) crystals with Y substitute K(Y-K) defects have been investigated using density function theory. The doping of Y atoms leads to changes in bond lengths around the defects for the different charged defect. The most pronounced changes occur in the O-H and O-Y bond lengths, which disrupts the geometrical symmetry of the original crystals. According to the defect formation energy, defects are likely to be most stable in the +2 charged state. The conduction band shifts down and band gap reduces caused by Y-K defect. New defect states in the bandgap mainly arise from the interaction between 2p orbitals of O and 4d orbitals of Y. The calculated absorption peaks are at 1.715(723 nm), 6.907 eV(180 nm), 3.178(390 nm), and 6.299 eV(197 nm) for PE-KDP. The 197 nm absorption band is close to the absorption of the experimental samples at 190-250 nm. The absorption bands of FE-KDP presents in the UV and visible range.