Damage mechanisms of Pb substitution at K sites in KH 2 PO 4 crystals: a hybrid density functional theory study

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Abstract This study investigates the effects of lead substitution for potassium defects in KH 2 PO 4 (KDP) crystals in both paraelectric (PE) and ferroelectric (FE) phases, focusing on various charge states (−1, 0, +1, +2), using first-principles calculations. We systematically investigate the relative stability, electronic structures, lattice distortion, and the effects of charged-state transitions on the optical properties of the material. Simulation results reveal that the Pb K defect is most stable in the +1 charged state. This defect induces significant lattice distortions, with changes in H–O bond lengths ranging from −10.50% to 24.60%. Moreover, Pb K defects introduce defect states in band-gap, which may enhance multi-photon absorption and consequently cause severe damage to the crystal. Taking electron-phonon coupling into account, the classical barrier in PE-KDP and FE-KDP are as small as 4.25 meV and 66.10 meV, making non-radiative transitions highly probable. An absorption peak observed at 5.70 eV (218 nm) in the PE phase likely corresponds to the experimentally detected strong absorption near 210 nm and may contribute to electron ionization. The observed Stokes redshift further indicates that a considerable fraction of the optical energy dissipates as heat within the lattice, which consequently lowers the laser-induced damage threshold.