Cluster defect Zn_K + Si_P in KH₂PO₄ crystals: effects on electronic structure and optical properties

First-principles calculations were implemented to explore the performance evolution of paraelectric (PE) and ferroelectric (FE) KH₂PO₄ (KDP) crystals induced by the cluster defect Zn_K + Si_P. The Freysoldt-Neugebauer-Vande Walle (FNV) finite-size correction was employed to calculate accurate defect formation energies (DFEs). This study systematically investigates the lattice configurations, electronic features, as well as optical absorption and luminescence spectra. Our calculations directly connect atomic-scale defect physics to experimentally measurable macroscopic properties. We find that the negatively charged defect (Zn_K + Si_P)' causes the most drastic local lattice deformation (34.59%) and orbital reconstruction, which quantitatively explains the experimentally observed sub-bandgap absorption and provides a microscopic mechanism for laser-induced damage. Specifically, the in-gap localized defect states promote multi-photon absorption, a key precursor to irreversible lattice damage under long-term laser irradiation. Considerable electron-phonon coupling leads to a high Huang-Rhys factor in PE-KDP, indicating phonon-dominated non-radiative energy dissipation. This non-radiative pathway quantitatively accounts for the thermal load that ultimately reduces the laser-induced damage threshold (LIDT). The simulated absorption peak at 3.08 eV (403 nm) matches the experimental absorption signal at 395 nm. Remarkable Stokes redshifts confirm that most photoexcitation energy transforms into lattice heat, which directly correlates with the reduced operational lifetime of KDP optical components in high-power laser systems, particularly for inertial confinement fusion (ICF) applications.