Study on the Properties of Al Substitute for K Defects in KH₂PO₄ Crystals: First-Principles Study

The defect formation energies, electronic structures, and optical properties of potassium dihydrogen phosphate (KDP) crystals with Al substitute for K(AlK) defects are calculated based on first principles. Neutral and +2 charged AlK defects are thought to exist stably in crystals, with +2 charged defects dominating. The lattice distortion caused by +2 charged defects is symmetric, while neutral defects are not. At the same time, neutral defects can change the bonding mode of H ions, which may cause H ions to form H interstitials more easily. Neutral and +1 charged defects create new states in the band gap, which may facilitate multiphoton transitions of “1 + 2” or “1 + 1+1”. In P-KDP crystals, the absorption peaks of electron transitions of neutral defect to CBM, +1 charged defect to CBM, and +2 charged defect to CBM are located at 274, 280, and 253 nm, respectively, which is consistent with the experimental results, and which can prove the existence of AlK defects in P-KDP crystals. These defects may indirectly cause the degradation of optical properties of KDP crystals by participating in multiphoton transition and causing lattice distortion.