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 (Hi) and H vacancy (Hv) in KH2PO4 (KDP). We find that while there is no interaction between the neutral Hi 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 Hv. In sharp contrast, the addition of a hole results in the formation of a hydroxyl bond. Thus, Hi in both charged states severs the H-bonded network. For the Hv, the addition of a hole leads to the formation of a peroxyl bridge. The neutral Hi and the positively charged Hv induce states in the gap. The results elucidate the underlying atomic mechanism for the defect reactions suggested by experiment.