Electronic structure calculations of intrinsic and extrinsic hydrogen point defects in KH₂PO₄

We report first-principles total-energy density-functional theory electronic structure calculations for the neutral and charge states of H intrinsic (Frenkel pair) and extrinsic (H vacancy or interstitial) point defects in KH₂PO₄. 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 O—O 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 O—O 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 H₂ 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.