Zn interstitial defects in KH 2 PO 4 : a first-principles study of optical and electronic properties

Abstract The structural and optical properties of KH 2 PO 4 (KDP) crystals with zinc interstitial (Zn i ) defects are investigated using first-principles calculations. The formation energies of Zn i in various charged states are evaluated for both paraelectric (PE) and ferroelectric (FE) phases, incorporating the Freysoldt-Neugebauer-Van de Walle (FNV) correction. The results indicate that the +1 charged state is difficult to stabilize. The Zn i defect is also found to be thermodynamically more favorable than an isolated Zn k defect or cluster defect Zn k + V H . Lattice distortion analysis indicates pronounced changes in H-O and K-O bond lengths, with variations ranging from −18.39% to 106.02% and −7.96% to 27.45%, respectively. These significant distortions may induce structural instability. Additionally, the Zn i defect introduces defect states in band-gap that could promote multi-photon absorption, a process known to cause substantial crystal damage. Optical calculations reveal a large lattice relaxation energy of 3.28 eV for the +2 to +1 charged state transition in the FE phase. When electron–phonon coupling is considered, an absorption peak at 3.78 eV (328 nm) is observed, consistent with experimental measurements near 328 nm. A pronounced Stokes shift further indicates efficient conversion of optical energy into lattice vibrations, thereby lowering the laser-induced damage threshold.