Diversity of hydrogen configuration and its roles in SrTiO3−δ

As a source of carrier electron, various configurations of hydrogen in SrTiO3 are searched by using first-principles calculations. The most stable form of hydrogen is found to be H−, where doubly charged oxygen vacancy $V_{\rm O}²⁺$VO2+ changes into singly charged ${\rm H}_{\rm O}^+$HO+. Most importantly, an additional H− is found to be weakly trapped by ${\rm H}_{\rm O}^+$HO+, which completely neutralizes carrier electrons by forming ${\rm (2H)}_{\rm O}^0$(2H)O0. These unexpected behaviors of hydrogen, which can explain reported experimental results, expand the role of the hydrogen in carrier-control technology in transition-metal oxides.