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 (H-i) and H vacancy (H-v) in KH2PO4 (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.