Structural Characterization and Ionic Conductivity of Metastable Gd2(Ti0.65Zr0.35)2O7 Powders Prepared by Mechanical Mil
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0972-AA09-04
Structural Characterization and Ionic Conductivity of Metastable Gd2(Ti0.65Zr0.35)2O7 Powders Prepared by Mechanical Milling Antonio F. Fuentes1, Karla J. Moreno1, Jacobo Santamaria2, Carlos Leon2, and Ulises Amador3 1 Unidad Saltillo, Cinvestav, Carretera Saltillo-Monterrey Km. 13, Ramos Arizpe, Coahuila, 25900, Mexico 2 Departamento de Fisica Aplicada III, Facultad de Fisica, Universidad Complutense, Madrid, 28040, Spain 3 Departamento de Quimica, Facultad de Farmacia, Universidad San Pablo-CEU, Boadilla del Monte, Madrid, 28668, Spain
ABSTRACT We analyze in this work the influence of ordering on the oxygen ion dynamics in the ionic conductor Gd2(Ti0.65Zr0.35)2O7, prepared by mechanical milling. As-prepared powder phase presents a metastable anion deficient fluorite-type of structure below 800°C becoming a disordered pyrochlore above this temperature. Such phase transformation implies a significant increase in the ionic conductivity of this material as a result of a systematic decrease in the activation energy for the dc conductivity, from 1.23 to 0.78 eV. Electrical conductivity relaxation is well described by the Kohlrausch-Williams-Watts stretched exponential function with the fractional exponent n decreasing with increasing sintering temperature (increasing ordering) as a result of decreasing ion-ion interactions in better ordered samples. INTRODUCTION Oxygen transport in oxide-ion conductors takes place mainly by ion hopping to adjacent vacancies in the structure in a thermally-activated long-range motion with activation energies typically of the order of 1 eV. Although the activation energy is mostly determined by the energy barrier that oxygen ions must overcome to hop to neighboring vacant sites, there some other factors affecting ion dynamics and consequently, the values found for Edc [1-3]. In this work, we analyze the influence of structural ordering (sintering temperature) in oxygen hopping dynamics and their importance in determining the effective activation energy for long-range oxygen diffusion in pyrochlore-type Gd2(Ti0.65Zr0.35)2O7 powders. The fully ordered or “ideal” pyrochlore oxide A2B2O(1)6O(2) can be described in terms of a superstructure of an ideal aniondeficient fluorite structure MO1.75 with twice the cell constant, a ≈ 10 Å, and oxygen vacancies ordered in a particular direction. However, some disordered pyrochlores such as Gd2Zr2O7, are high-temperature fast-ionic conductors [4]. Interestingly, different degrees of atomic disorder and high oxygen mobility can be obtained in A2B2O7 systems by using the appropriate substitutions on the A and B sites [5]. Thus, cation and anion substructures in Gd2(Ti1-yZry)2O7 disorder gradually as the Zr content increases leading to the appearance of oxygen vacancies at the O(1) positions (48f sites), which are known to be the main contributors to oxygen hopping and diffusion [4]. Correspondingly, a marked increase in conductivity is observed when y ≥ 0.3 which has been related to the onset of the anion disordering and partial occupation
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