Study on the optical properties and electronic structures of Ba-doped KH2PO4 crystals

Abstract

Potassium dihydrogen phosphate (KDP) is widely used in electro-optical switches and inverter crystals in inertial confinement fusion (ICF). Ba substituted for K (Ba_K) was found to be one of the common impurities in KDP crystals. Defect formation energies (DFEs), electronic structures, and optical properties of various charge states (0, +1, +2) are investigated for KDP crystalized in the paraelectric phase (P-KDP) and ferroelectric phase (F-KDP). The DFE results indicate that the most stable state of Ba_K is more likely to be positive monovalent. We found a large lattice distortion (9.26–22.33%) in the H–O bonds that can interpret the visible foggy inclusions observed by the experiment in the P-KDP. In sharp contrast, the Ba_K defect has a smaller influence on the F-KDP in the crystal structure (maximum 12.03%). The optical properties of Ba_K defects in the KDP crystals are obtained by considering electron–phonon coupling. The optical transition in the P-KDP and F-KDP is as small as 0.62 eV and 0.40 eV, respectively. The calculated results indicate that Ba_K defects in the KDP crystals easily lead to nonradiative components. The 6.07 eV (204.45 nm) absorption band caused by the optical transition from the (+2/+1) level to the conduction band minimum (CBM) agrees with the experimentally observed strong absorption and is probably related to the experimentally observed linear and nonlinear absorption at 210.5 nm, which may affect the application of P-KDP crystal in the deep ultraviolet region.