First-principles study of interstitial oxygen in potassium dihydrogen phosphate crystals

The electronic structure and geometric distribution of neutral and charged states of interstitial oxygen in potassium dihydrogen phosphate (KDP) are investigated using a first-principles method. The energy gap is lowered to about 4.2 eV from the density-functional theory (DFT) value of 5.9 eV by a neutral O interstitial, which corresponds to a two-photon absorption of 364 nm (3.4 eV) after correction of 1.3 eV due to the underestimation of the band gap using the DFT. The addition of a single electron lowers the gap to 1.4 eV. The addition of two electrons leads to the formation of an isolated interstitial H₂O molecule in KDP accompanied by the breaking of two hydrogen-bonded chains, and thus no defect states appear in the energy gap. The results can be utilized to explain the recently reported decomposition of KDP during a laser-induced breakdown.