A Study on V _H ‐Fe _K ‐V _H Defect Clusters and Their Aggregation Effects in KDP Crystals

ABSTRACT It has been revealed that defect aggregation serves as a primary factor responsible for the reduction in the laser‐induced damage threshold (LIDT) of KDP crystals. In accordance with the electrical neutrality principle and defect recombination rules, intrinsic point defects and extrinsic impurity species inside KDP lattices tend to assemble into defect clusters. On this basis, taking the experimentally verified Fe‐doped K‐site substitutional defect associated with hydrogen vacancy complexes as a representative model, we systematically investigate the aggregation behavior of defect clusters in KDP crystals via first‐principles calculations. Based on the calculation of defect formation energy (DFE), the ‐1 charged defect clusters are considered to exist stably. This conclusion is further verified by our investigations on lattice distortion and density of states (DOS). The calculated absorption peaks corresponding to the electron transitions from the ‐1 charged defect state to the conduction band minimum (CBM) and from the neutral defect state to the CBM are 230 nm and 271 nm, respectively, which are in good agreement with the experimental values (225 nm and 275 nm). In addition, analysis of the configuration coordinate diagrams reveals that electron transition from this defect state to the conduction band minimum (CBM) induces a relatively large lattice relaxation energy. Studies on cluster aggregation indicate that the DFE reaches its minimum value of 7.344 eV when the two clusters are in the next‐nearest neighbor configuration with an intercluster distance of 6.39 Å. This result validates the rationality of the defect cluster aggregation model. Furthermore, analysis of lattice distortion and Bader charge distribution reveals that differences in the interaction strength between defect clusters and their surrounding lattice atoms constitute the dominant driving force underlying cluster aggregation. By calculating the binding energies corresponding to the aggregation of neutral and charged defect clusters, we confirm that charged defect clusters can readily aggregate and maintain thermodynamically stable configurations. Thus, a multiphoton transition and cluster aggregation model involving defect clusters V H ‐Fe K ‐V H under laser irradiation is proposed.