Solid State Ionics 127 (2000) 109–123 www.elsevier.com / locate / ssi Local electronic structures around hydrogen and acceptor ions in perovskite-type oxide, SrZrO 3 * M. Yoshino , K. Nakatsuka, H. Yukawa, M. Morinaga Department of Materials Science and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi, 464-8603, Japan Received 19 September 1998; accepted 13 September 1999 Abstract Local electronic structures around hydrogen and acceptor ions in SrZrO are simulated by the DV-Xa molecular orbital 3 method. In pure SrZrO , there is a band gap of about 6 eV between the O–2p valence band and the Zr–4d conduction band, 3 in agreement with experiments. When Y or Sc is doped into SrZrO , an acceptor level appears just above the valence band. 3 When hydrogen is introduced into SrZrO , a donor level appears below the conduction band. Also, an oxygen ion vacancy 3 makes the defect level below the conduction band. It is shown that charge compensation takes place among these acceptor, donor and defect levels. In addition, local electronic structure around hydrogen is found to change largely with the acceptor dopants, Sc and Y. For example, the metal–oxygen bond strength changes in the order, Y–O , Zr–O , Sc–O, which probably modifies the thermal vibrational amplitude of oxygen ions near hydrogen, and affects protonic conductivity in the doped SrZrO .  2000 Elsevier Science B.V. All rights reserved. 3 Keywords: Perovskite-type oxide; Protonic conductor; SrZrO ; SrTiO ; Electronic structures; Charge compensation 3 3 ¨ 1. Introduction the charge balance. Following the Kroger-Vink nota- tion, it is expressed as, It is known that many perovskite-type oxides (e.g., 1 1 1 3 3 ?? SrTiO , SrCeO ) show protonic conduction at high 3 3 ] ] ] 9 Y 1 Zr 1 O → Y 1 V 1 Zr 1 O, (1) Zr O Zr O 2 2 2 4 temperatures when acceptor ions are doped into them 31 41 ?? [1]. These oxides have a large potential for use in 9 where Y is a Y ion at the Zr site and V is an Zr O fuel cells, steam electrolysis and hydrogen gas oxygen ion vacancy. In a wet atmosphere, some of sensors [2–4]. this oxygen ion vacancy is readily filled with an 31 ? When trivalent cations such as Y are substituted oxygen ion, and interstitial protons H , are intro- i 41 for tetravalent ions such as Zr in SrZrO , oxygen 3 duced into the crystal. This is expressed as, ion vacancies are introduced into the crystal to keep ?? 3 ? HO 1 V 5 O 1 2H , (2) 2 O O i 3 where O is an oxygen ion at the oxygen site. As a O *Corresponding author. Fax: 181-52-789-3233. result, protonic conductivity through the interstitial E-mail address: yoshino@silky.numse.nagoya-u.ac.jp (M. ? Yoshino) H is induced in such oxides. i 0167-2738 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738(99)00274-X