Oxygen ion dynamics in pyrochlore-type ionic conductors: Effects of structure and ion–ion cooperativity M.A. Frechero a,1 , O.J. Durá b , M.R. Díaz-Guillén c , K.J. Moreno d , J.A. Díaz-Guillén e , J. García-Barriocanal a , A. Rivera-Calzada a , A.F. Fuentes f , C. León a, ⁎ a GFMC, Departamento de Física Aplicada III, Facultad de Física, Universidad Complutense de Madrid, 28040 Madrid, Spain b Departamento de Física Aplicada and INEI, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain c GMyPQ, Instituto de Investigaciones Eléctricas, 62490 Cuernavaca, Morelos, Mexico d Instituto Tecnológico de Celaya, Apartado Postal 57, 38010 Celaya, Guanajuato, Mexico e División de Estudios de Posgrado e Investigación, Instituto Tecnológico de Saltillo, Saltillo 25280, Coahuila, Mexico f Cinvestav Unidad Saltillo, Apartado Postal 663, Saltillo 25000, Coahuila, Mexico abstract article info Article history: Received 26 May 2014 Received in revised form 23 July 2014 Available online 6 September 2014 Keywords: Ionic conductivity; Conductivity relaxation; Electric modulus We summarize recent studies of oxide ion dynamics in ionically conducting defect pyrochlores of general formulae Ln 2 Ti 2 - y Zr y O 7 . Unit cell volume, structural disorder, and ion–ion interactions, are all relevant in determining ion hopping probability and thus the mobility of oxide ions at high temperatures. At enough low temperatures ion hopping is scarce and oxide ion dynamics are restricted to local displacements within the cage. © 2014 Elsevier B.V. All rights reserved. 1. Oxide ion conductivity in disordered pyrochlores Fuel cells are considered one of the most promising alternatives to fossil energy sources for power generation [1–4]. Oxygen ion conductors of fluorite structure, such as yttria stabilized zirconia (YSZ), are currently used as electrolytes in commercial solid oxide fuel cells. However, due to the low values of the oxygen ion conductivity at room temperature, op- erating temperatures higher than 500 °C are required. In fact, a major challenge is to improve oxygen conductivity for fuel cells to operate at lower temperatures. This has triggered a huge research effort involving the search for materials as novel electrolytes with higher ionic conduc- tivity near room temperature. Long-range migration of oxygen ions takes place by thermally activated hopping to adjacent oxygen vacancies that yields a dc conductivity of the form σ dc =(σ ∞ /T)exp(-E dc /kT). Thus, lowering the operation temperature would require increasing the prefactor σ ∞ and/or decreasing the activation energy E dc . But increasing the number of oxygen vacancies to increase σ ∞ leads to an undesired increase in E dc and eventually to lower conductivity values [1], thus limiting the strategies to obtain high oxygen conductivity at lower temperatures. The origin of this behavior had remained not well understood for a long time [1,4–6], but recently, systematic and comprehensive studies of oxygen transport in a wide family of oxide- ion conductors of pyrochlore structure have provided experimental evidence of the important role that cooperative effects in oxygen dynamics have on the observed increase of the activation energy E dc [7–13]. In this work we present a perspective of these experimental results and on how they can be rationalized within the framework of Ngai's Coupling Model. Oxide-ion conductors of pyrochlore structure A 2 B 2 O(1) 6 O(2) have been proposed as alternative electrolytes in solid oxide fuel cell devices [14–17]. The pyrochlore cubic crystal structure (S.G. 227) might be de- rived from that of an anion deficient fluorite by doubling the unit cell, removing one out of every eight anions and placing cations and anions in four crystallographically non-equivalent sites. Thus, A (R A ≈ 1 Å) and B (R B ≈ 0.6 Å) cations are respectively found at the 16d (8-coordinated) and 16c (6-coordinated) sites (origin choice 2 of space group 227) whereas anions are distributed between two tetrahedrally coordinated positions, 48f [O(1)] and 8b [O(2)] [18,19]. In addition, there is another tetrahedral site available for anions in the unit cell, 8a, which is system- atically vacant in fully ordered pyrochlores that makes them poor oxygen ion conductors. However, defect pyrochlores such as Gd 2 Zr 2 O 7 which are intrinsically disordered and exhibit the three aforementioned anion positions partially occupied, are good oxygen ion conductors at high temperatures. Different theoretical calculations have shown that the most stable intrinsic defect in these compounds is an oxygen Frenkel pair consisting of a vacant 48f position and an interstitial ion located at the 8a site [17,20–22]. Thus, oxygen conductivity in pyrochlores depends Journal of Non-Crystalline Solids 407 (2015) 349–354 ⁎ Corresponding author. E-mail address: carlos.leon@fis.ucm.es (C. León). 1 Permanent address: Departamento de Química—INQUISUR, Universidad Nacional del Sur, Av. Alem 1253, 8000 Bahía Blanca, Argentina. http://dx.doi.org/10.1016/j.jnoncrysol.2014.08.046 0022-3093/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol