Comparison of hydrogen vacancies in KDP and ADP crystals: a combination of density functional theory calculations and experiment

Abstract

The hydrogen vacancy (V_H) is the most common point defect that may lead to optical damage of potassium dihydrogen phosphate (KDP) and its analog ammonium dihydrogen phosphate (ADP), further limiting their practical application in high-power laser systems. In this work, we have grown KDP and ADP crystals by using a rapid growth method, and investigated the physical origin of the different stability of V_H as well as the defect-induced electronic structure and optical absorption in KDP and ADP crystals. The inclusion of van der Waals correction to density functional theory calculations is found to have little influence on V_H energetics of KDP whereas it largely reduces the charge transition level ε(+/−) of V_H by >2 eV in ADP. It is found that hydrogen vacancies mainly contribute to the redshift of the measured absorption edges of both KDP and ADP crystals. Owing to the varied lattice environments and locations, the V_H defects exhibit different stability, and electronic and optical properties in KDP and ADP crystals. Notably, the extra optical absorption caused by the positively-charged V_H in KDP could be largely reduced by decreasing the defect concentration, whereas ADP exhibits defect-location dependence – the optical damage center of the V_H in the NH₄⁺ group could not be eliminated because of electron capture of its neighboring N atoms. The calculation results help us to better understand the origin of laser damage in KDP and ADP crystals.