Effect of stability and diffusivity of extra-framework oxygen species on the formation of oxygen radicals in 12CaOd 7Al 2 O 3 Katsuro Hayashi a,b, * , Naoto Ueda c , Masahiro Hirano b,c , Hideo Hosono a,b,c a Frontier Collaborative Research Center, S2-13, Tokyo Institute of Technology, Yokohama 213-0012, Japan b Transparent Electroactive Materials Project, Japan Science and Technology Agency, KSP C-1232, Kawasaki 213-0012, Japan c Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan Received 30 April 2004; received in revised form 15 July 2004; accepted 28 July 2004 Abstract The thermodynamics and kinetics of oxygen radical formation process in a nanoporous crystal, 12CaOd 7Al 2 O 3 , were examined by isothermal annealing experiments in dry atmospheres. It is clarified that the formation of oxygen radicals, O 2  and O  , in the crystal occurs in the following three steps; outward diffusion of extra-framework oxide ions, its oxidation by absorbed oxygen molecules to form O  and O 2  at the surface, and subsequent inward diffusion of the oxygen radicals. The instability of the extra-framework oxide ions is essentially the driving force for the process. Further, the analyses allow to predict the total oxygen radical content for a given annealing condition. D 2004 Elsevier B.V. All rights reserved. Keywords: Cage; Active oxygen; Superoxide ion; Oxygen anion radical, C12A7 1. Introduction 12CaOd 7Al 2 O 3 (C12A7) [1–15] crystal has a lattice framework composed of nano-size cages as shown in Fig. 1. Because the lattice framework has a positive charge equivalent to +1/3 per cage (2.3210 21 cm 3 ), incorporation of compensating extra-framework anions inside the cages are inevitable. Consequently, oxide ions, O 2  , occupy 1/6 of the inner-cage sites under a stoichiometric composition [1]. These oxide ions are likely responsible for the fast oxide- ionic conduction [2]. Further, they can be partially or completely replaced by various monovalent anions such as OH  [3],F  [4], Cl  [4],O 2  [5–10],O  [6–10] and H  [11] as well as electrons [12]. Among them, oxygen radical anions, O 2  and O  , are particularly important due to their high oxidation reactivity [16], which makes C12A7 attractive as an oxidation catalyst [13] or an oxide ion conducting-solid electrolyte with catalytic activity. The oxygen radical-loaded C12A7 enables efficient electric-field extraction of the O  in a vacuum [14], which provides a novel method for generating high density O  -ion beam. In the present study, the thermodynamics and kinetics of oxygen radical formation process in dry atmospheres are examined based on isothermal annealing experiments [10] to elucidate the mechanism for the incorporation of the oxygen radicals. Furthermore, results of the thermodynamic and kinetic analyses are applied to predict the oxygen radical concentration for given process conditions. 2. Thermodynamic and kinetic analyses of oxygen radical formation 2.1. Experimental and results C12A7 ceramic samples with ~80% of the theoretical density and an average grain size of 11F4 Am were heated at 1350 8C for 2 h in a dry oxygen atmosphere and then thermally quenched to room temperature. These samples were again heated to 700, 625, or 550 8C, held for 2 min– 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.07.057 * Corresponding author. Tel: +81 45 924 5359; fax: +81 45 924 5339. E-mail address: k-hayashi@lucid.rlem.titech.ac.jp (K. Hayashi). Solid State Ionics 173 (2004) 89 – 94 www.elsevier.com/locate/ssi