A Theoretical Study on the Hydrogen Transport Mechanism in SrTiO 3 Perovskite. II. Scandium Doping at Titanium Site Taku Onishi* [a,b,c] and Trygve Helgaker [c] Hybrid Kohn–Sham calculations are performed to clarify the proton-conduction mechanism in Sc-doped SrTiO 3 perovskite, from the viewpoint of energetics and bonding. The calculated potential energy curves are discussed and a molecular-orbital analysis performed. Much hydrogen can exist around doped scandium, where it is energetically stabilized. However, scandium-doping has no energetic advantage in the activation energy for proton-conduction. V C 2012 Wiley Periodicals, Inc. DOI: 10.1002/qua.24086 Introduction As is well known, cubic SrTiO 3 perovskite shows high proton conductivity. [1] Although many experimental and theoretical works on the proton-conduction path have been performed, [2–5] the detailed energetics and bonding of the proton-conduction path are unclear. In part I, [6] we investigated proton-conduction in Sc-undoped SrTiO 3 perovskite using hybrid Kohn–Sham theory. As illustrated in Figure 1, we found that a diagonal path and a two-dimensional OAH rotation path dominate proton-conduc- tion inside the Ti 4 O 4 square, whereas a three-dimensional OAH rotation path dominates conduction between two such squares. To introduce a proton inside SrTiO 3 perovskite, a trivalent anion may be doped at an oxygen site or a trivalent cation may be doped at a titanium site. In part I, we discussed nitro- gen-doping at an oxygen site in SrTiO 3 perovskite. There are then two cases to consider. In one case, the doped nitrogen exists as part of NH 2 rather than as N 3 , without creating an oxygen vacancy. [7] This happens when hydrogen dissolves to compensate the dopant already during the synthesis of nitro- gen-doped SrTiO 3 perovskite, the covalent bonding between nitrogen and hydrogen being stronger than that between oxy- gen and hydrogen. The other case occurs when, in wet atmos- pheres, OH  and H þ ions of water dissolve into an oxygen va- cancy (created to compensate the nitrogen dopant at oxygen) and into a nitrogen site, respectively. The latter reaction can be expressed in the Kr€ oger–Vink notation [8] as: H 2 O þ V  O þ N X O ! OH  þ NH  (1) In both cases, the activation energy for proton-conduction decreases or is similar to the Sc-undoped case, and much hydrogen can exist as part of NH 2 . We concluded that nitro- gen-doping enhances the proton conductivity. In contrast, when trivalent cations such as scandium (Sc 3þ ), [9–11] iron (Fe 3þ ), [12] and aluminium (Al 3þ ) [13] are doped at a titanium site, an oxygen vacancy is created to compensate the dopant. In wet atmospheres, OH  and H þ from water dis- solve into an oxygen vacancy and an oxygen site, respectively, in the same manner as in the nitrogen-doped case: H 2 O þ V  O þ O X O ! 2OH  (2) In this study, we have investigated in detail the proton-con- duction mechanism in Sc-doped SrTiO 3 perovskite, from the viewpoint of the energetics and bonding. Computational Method The calculations presented here have been performed using the BHHLYP hybrid Kohn–Sham method, [14] which properly reproduces the electronic structure of the strongly correlated perovskite-type transition metal oxides. In BHHLYP theory, the total exchange and correlation energy is expressed by 50% HF exchange, 50% Becke exchange, and LYP correlation energies. We have used the Tatewaki–Huzinaga MINI basis [15] for tita- nium, strontium and scandium, combined with the 6-31G(d) basis for oxygen and hydrogen. All calculations were per- formed with the GAMESS program. [16] The molecular orbitals (MOs) have been plotted using MOLEKEL 4.3. [17] [a] T. Onishi Department of Chemistry for Materials, Graduate School of Engineering, Mie University, 1577 Kurimamachiya-cho, Tsu, Mie 517-8507, Japan E-mail: taku@chem.mie-u.ac.jp taku.onishi@kjemi.uio.no [b] T. Onishi The Center of Ultimate Technology on Nano-Electronics Mie University (MIE- CUTE), 1577 Kurimamachiya-cho, Tsu, Mie 517-8507, Japan [c] T. Onishi, T. Helgaker Department of Chemistry, The Centre for Theoretical and Computational Chemistry (CTCC), University of Oslo, Postbox 1033, Blindern 0315 Oslo, Norway Contract grant sponsor: The Norwegian Research Council (CoE Centre for Theoretical and Computational Chemistry); contract grant number: 179568/ V30. Contract grant sponsor: Iwatani Naoji Foundation. V C 2012 Wiley Periodicals, Inc. International Journal of Quantum Chemistry 2013, 113, 599–604 599 FULL PAPER WWW.Q-CHEM.ORG