The electronic and optical properties of Al interstitial defects in KH2PO4 crystal: First principles study

Abstract

KH2PO4 (KDP) crystals are widely used as frequency multipliers in inertial confinement controlled fusion ignition devices. The electronic and optical properties of KDP crystals with Al interstitial defects based on first principles are investigated. The results show that neutral and +3 charged Al interstitial can exist stably in the crystal. The +3 charged defect produces a larger lattice distortion. Bond length varies from −18.78 % to 43.71 % in the paraelectric phase(P-KDP) and from −25.57 % to 66.09 % in the ferroelectric phase (F-KDP). At the same time, +3 charged Al interstitial may also cause HO bond to break, resulting in the generation of H interstitials, which may compensate for the H vacancies in as-grown KDP. This may make the laser damage threshold slightly increase when KDP crystal is doped with +3 charged Al ion at low concentration. The optical spectra of Al interstitial are calculated, and the results show that in the paraelectric and ferroelectric phases, the optical transition of Al ions generally produces a large lattice relaxation energy, such as the lattice relaxation energy of up to 4.00 eV in the +2 to +3 charged defect transition in the paraelectric phase. This shows that the optical transition of Al interstitial will cause serious damage to the crystal. In the paraelectric phase, the absorption peaks of optical transitions found in the ultraviolet and visible regions are consistent with experiments. The absorption peak of the +1 to +2 charged defect transition is at 260 nm, which is very close to the absorption peak at 270 nm measured by experiments. The analysis of the Huang-Rys factors and transition displacement further confirms the conclusion that the optical transition of the defect charged state will cause serious crystal damage.