We present an ab initio study of the stability and defect reactions of neutral and charged H interstitial (H i ) and H vacancy (H v ) in KH 2 PO 4 (KDP). We find that while there is no interaction between the neutral H i and the host, the addition of an electron leads to the ejection of a H host atom and the subsequent formation of an interstitial H 2 molecule and a H v . In sharp contrast, the addition of a hole results in the formation of a hydroxyl bond. Thus, H i in both charged states severs the H-bonded network. For the H v , the addition of a hole leads to the formation of a peroxyl bridge. The neutral H i and the positively charged H v induce states in the gap. The results elucidate the underlying atomic mechanism for the defect reactions suggested by experiment.
We report first-principles total-energy density-functional theory electronic structure calculations for the neutral and charge states of H intrinsic ͑Frenkel pair͒ and extrinsic ͑H vacancy or interstitial͒ point defects in KH 2 PO 4 . The relaxed atomic structures, the formation energy, the ionization energy, and electron and hole affinities for the various defects have been calculated. For the Frenkel pair, the additional hole leads to a decrease of the OuO bond length between the two O atoms next to the H vacancy, while the effect of the additional electron is small. For the H vacancy, the added hole is trapped and shared by the two O atoms adjacent to the vacancy, reducing dramatically the OuO bond length, thus forming a molecular-type polaron. We find that the positively charged H vacancy introduces states in the gap, in contrast with its neutral state, confirming the experimental suggestion that it is a relevant absorbing center. The negatively charged H vacancy leads to an increase of the two O atoms close to the H vacancy, and does not induce states in the gap. The H interstitial does not interact with the host atoms in the neutral state. However, the addition of an electron leads to the ejection of a H host atom and the subsequent formation of a H 2 interstitial molecule and a H vacancy, in agreement with experimental suggestions. In the positively charged state the H interstitial binds to its nearest-neighbor O atom forming a hydroxyl bond. The H interstitial in both positive and negative charge states induces no defect states in the band gap, in contrast with its neutral state. The calculations provide insights into the role of the charged and neutral defects on the transient optical absorption under irradiation by high-intensity laser beam.
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