Structure and Oxide Ion Conductivity: Local Order ... - ACS Publications

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Structure and Oxide Ion Conductivity: Local Order, Defect Interactions and Grain Boundary Effects in Acceptor-Doped Ceria Francesco Giannici,*,† Giuliano Gregori,‡ Chiara Aliotta,† Alessandro Longo,§ Joachim Maier,‡ and Antonino Martorana† †

Dipartimento di Fisica e Chimica, edificio 17, viale delle Scienze, I-90128 Palermo, Italy Max Planck Institute for Solid State Research, Heisenbergstr. 1, D-70569 Stuttgart, Germany § Istituto per lo Studio dei Materiali Nanostrutturati, Consiglio Nazionale delle Ricerche, via U. La Malfa, I-90146 Palermo, Italy ‡

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ABSTRACT: The long-range and short-range structure of nanocrystalline and microcrystalline acceptor-doped ceria is investigated by a combined approach using EXAFS, XANES, Raman, and XRD, and correlated with the oxide-ion conductivity in the bulk and in grain boundaries. Compared to Yb3+ and Er3+, the positive influence of Sm3+ is attributed to the ability to repel oxygen vacancies, and to keep a localized disorder around the dopant. The long-range structural analysis shows lattice contraction for Yband Er-doping and lattice expansion for Sm-doping. The short-range analysis around the dopants and cerium highlights that a more complex structural rearrangement has to be assumed to explain the complementary results of the different techniques. Nominally trivalent dopants are also shown to have an effect on the electronic structure of ceria, and the consequences on oxide-ion conductivity are highlighted.

1. INTRODUCTION Cerium oxide is a versatile and widespread material used in solid state oxygen transport due to the easy formation of oxygen vacancies (through reduction, or addition of acceptorsM3+), and to the rather low activation energy associated with their migration across the lattice.1−3 The fine-tuning of the properties of ceria is achieved through the control of point defects in the bulk, but also by suitable tailoring of the grain boundaries. These are particularly important in nanocrystalline ceria (with grain size