Effect of S Substitution for P Point Defects in KDP Crystals: First-Principles Study

The electronic structure and geometric distribution of phosphor replaced by sulfur in potassium dihydrogen phosphate (KDP) are investigated by first-principles calculations. The point defect narrows down the energy gap to about 4.9 eV, corresponding to a two-photon absorption of 355 nm after correction. This can explain the decrease of the laser damage resistance in KDP crystals. Moreover, the defects twist the crystal structure and weaken bonds, especially the O-H bonds, so these bonds may be the first sites to crack under laser irradiation.