Conduction and disorder in Y 3 NbO 7 - Zr 2 Y 2 O 7
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1126-S06-07-PP05-07
Conduction and disorder in Y3NbO7 - Zr2Y2O7 Dario Marrocchelli1, Paul A. Madden1,2, S. T. Norberg3,4 and S. Hull3 1
School of Chemistry, University of Edinburgh, Edinburgh EH9 3JJ, UK Department of Materials, University of Oxford, Oxford OX1 3PH, UK 3 The ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom. 4 Department of Chemical and Biological Engineering, Chalmers Institute of Technology, SE-412 96 Gothenburg, Sweden 2
ABSTRACT The construction of interaction potentials for the Y0.5+0.25xNb0.25xZr0.5−0.5xO1.75 system, on a purely ab-initio basis, is described. These potentials accurately reproduce experimental data on both the structure and the dynamics of these systems; the computer simulations also reproduce the experimental trend of the conductivity, which decreases as x increases, and of the level of static disorder within the O2− sublattice, which increases with x. A detailed analysis of these phenomena shows that the static disorder in Y3NbO7 is caused by the high Nb5+ charge and that in this material the conduction is heterogeneous, i.e. some anions are completely immobile while some others are very mobile. The role of the cation sublattice is explained in detail. INTRODUCTION Materials with high values of oxide-ion conductivity are currently the subject of extensive research activity, motivated by their technological applications within Solid Oxide Fuel Cells (SOFCs), oxygen separation membranes and gas sensors. Binary compounds of stoichiometry AO2 possessing the cubic fluorite structure are of particular interest, especially when some of the host cations are replaced by species of a lower valence to produce aniondeficient phases (i.e. A4+1−yB3+yO2−y/2 and A4+ 1−yB2+yO2-2−y). The charge compensating vacancies formed on the anion sublattice become mobile at elevated temperatures, leading to the impressive ionic conductivities shown by, for example, yttria-stabilized zirconia Zr1−yYyO2−y/2 [1]. As is well-known, vacancy ordering effects mean that the conductivity of doped zirconias does not increase with increasing vacancy concentration for y >0.1. Vacancy ordering effects have been investigated extensively in zero temperature, first-principles electronic structure calculations [2] and the reasons why the degree of vacancy ordering depends on the ionic radius of the dopant species explored. Here we introduce ab initio-based simulation methods to allow such phenomena to be examined in the temperature régime of practical interest (~1000K) and to examine the link between structure and conduction mechanism. The electronic structure calculations [2] show that the vacancy ordering effects are associated with energy scales of ~0.05 eV so entropic effects should be expected to be playing an important part in determining structural effects on the dynamics at 1000 K. We illustrate these methods with simulations of the Zr2Y2O7 - Y3NbO7 series. Compounds of stoichiometry A3NbO7 (and A3TaO7) are known to adopt anion-deficient fluorite structures,
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