Electron- or Hole-Assisted Reactions of H Defects in Hydrogen-Bonded KDP

We present an ab initio study of the stability and defect reactions of neutral and charged H interstitial (H₈) and H vacancy (Hₕ) in KH₂PO₄ (KDP). We find that while there is no interaction between the neutral H₈ and the host, the addition of an electron leads to the ejection of a H host atom and the subsequent formation of an interstitial H₂ molecule and a Hₕ. In sharp contrast, the addition of a hole results in the formation of a hydroxyl bond. Thus, H₈ in both charged states severs the H-bonded network. For the Hₕ, the addition of a hole leads to the formation of a peroxyl bridge. The neutral H₈ and the positively charged Hₕ induce states in the gap. The results elucidate the underlying atomic mechanism for the defect reactions suggested by experiment.