Ba/Ti ratio effect on oxygen re-equilibration kinetics of donor-doped BaTiO 3 Chung-Eun Lee, Han-Ill Yoo ⁎ Solid State Ionics Research Laboratory, School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, South Korea Received 24 July 2007; received in revised form 21 December 2007; accepted 19 February 2008 Abstract In an oxygen activity gradient, BaTiO 3 ceramic generally relaxes twofold: a fast global relaxation in the oxygen sublattice and a sluggish in- grain relaxation in the cation sublattice(s), thus allowing one to simultaneously determine the two chemical diffusivities. [Yoo et al., J. Am. Ceram. Soc., 88, 617–623 (2005)]. Upon this ground, the chemical diffusivities have been measured, by a conductivity relaxation technique, on donor (La Ba • )-doped BaTiO 3 ceramics with different Ba/Ti ratios, Ba 1 − x La x Ti 1 − x /4 O 3 , Ba 1 − 3x /2 La x TiO 3 and Ba 1 − x La x TiO 3 all with x = 0.01, against oxygen activity in the range of − 15 ≤ log(Po 2 /atm) ≤ 0 at 1200 °C. It has been found that the chemical diffusivities for the cation sublattice(s), and those for oxygen as well, are essentially the same irrespective of the Ba/Ti ratios. This fact indicates either that only either of Ba or Ti is mobile or both have about the same mobility. Circumstantial evidences support the latter. © 2008 Elsevier B.V. All rights reserved. Keywords: Donor-doped BaTio 3 ; Chemical diffusivity; Twofold relaxation; Ba/Ti ratio 1. Introduction BaTiO 3 has been serving as a most basic substance of modern electroceramics, e.g., multilayer ceramic capacitors (MLCC) and positive temperature coefficient resistors. One of the modern trends of the electroceramic technology is to get the devices thereof thinner and thinner (e.g., b 0.5 μm thickness of BaTiO 3 dielectric layers in MLCC). The thinner the material, the more satisfied is the technological greed, but the more unexpected problems start. One of those unexpected problems may be attributed to cation migration in various thermodynamic driving forces, which has been normally ignored particularly for perov- skite BaTiO 3 . Cation migration in perovskite oxides is actually attracting increasing attention now [1] because they comprise most of the component materials of solid oxide fuel cells and they are normally operated at elevated temperatures in various thermodynamic driving forces. We have recently observed [2] that upon a step-wise change of oxygen activity in the surrounding, donor(La Ba • )-doped BaTiO 3 ceramic (nominal composition Ba 0.99 La 0.01 Ti 0.9975 O 3 ) generally relaxes twofold to a new diffusive equilibrium: a fast global relaxation (with the decisive diffusion length of the overall specimen size) followed by a sluggish in-grain relaxation (with the decisive diffusion length of the grain size). The twofold relaxations are rendered visible only when the concentration changes of vacancies in the oxygen sublattice (V o •• ) and cation sublattices (say, V Ti VVVV ) are comparable, e.g., by doping donor impurities (La Ba • ). From the two different relaxation times, the two chemical diffusivities were evaluated to be on the order of 10 − 4~ − 6 and 10 − 11 ~ − 14 cm 2 /s, respec- tively, over an oxygen activity range of − 15 ≤ log a o 2 ≤ 0 at 1200 °C. The former is no doubt due to the chemical component O on the basis of our own experience with undoped or acceptor doped BaTiO 3 [3–5]. The latter, however, has been tentatively assigned to the component Ti, presuming the majority, whence the most mobile, disorders in the cation sublattices to be V Ti VVVV on the basis of the nominal composition of the specimen employed, Ba 1 − x La x Ti 1 − x /4 O 3 (x = 0.01). As regards the cationic disorders and their mobilities in BaTiO 3 , the literature reports appear to be still in conflict: Jonker and Havinga [6] first reported, by examining the phase equilibria of donor-doped BaTiO 3 at elevated temperatures, that the donors should be compensated ionically by V Ti VVVV .A simulation study by Lewis and Catlow [7] subsequently showed that V Ti VVVV formation is energetically more favorable than V Ba VV , but Available online at www.sciencedirect.com Solid State Ionics 179 (2008) 338 – 346 www.elsevier.com/locate/ssi ⁎ Corresponding author. E-mail address: hiyoo@plaza.snu.ac.kr (H.-I. Yoo). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2008.02.038