We present an ab initio study of the stability and defect reactions of neutral and charged H interstitial (${\mathrm{H}}_i$) and H vacancy (${\mathrm{H}}_v$) in ${\mathrm{K}\mathrm{H}}_2{\mathrm{P}\mathrm{O}}_4$ (KDP). We find that while there is no interaction between the neutral ${\mathrm{H}}_i$ and the host, the addition of an electron leads to the ejection of a H host atom and the subsequent formation of an interstitial ${\mathrm{H}}_2$ molecule and a ${\mathrm{H}}_v$. In sharp contrast, the addition of a hole results in the formation of a hydroxyl bond. Thus, ${\mathrm{H}}_i$ in both charged states severs the H-bonded network. For the ${\mathrm{H}}_v$, the addition of a hole leads to the formation of a peroxyl bridge. The neutral ${\mathrm{H}}_i$ and the positively charged ${\mathrm{H}}_v$ induce states in the gap. The results elucidate the underlying atomic mechanism for the defect reactions suggested by experiment.

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