Unusual lattice evolution and magnetochemistry of Nb doped CeO2

We report on the effect of Nb substitution on the lattice evolution and magnetic properties of the Ce1-xNbxO2. Based on the microstructure analysis the solubility limit of Nb was revised to x = 0.03. The lattice response to the Nb dopant shows unexpected non-monotonic dependence which cannot be described by an ideal solution of non-interacting point defects. By slow cooling, quenching, and sealed sample anneals, the ideal solution model, including oxygen vacancies, Nb dopant, and oxygen interstitials as key defects does not adequately describe the observed concentration of Ce3+. Magnetic measurements reveal that for ceramics prepared in air at ambient pressure the concentration of Ce3+ ions linearly increases with Nb content at approximately 1:2 ratio up to the Nb solubility limit. The effect of the crystal field on the energy splitting between the Ce3+ Gamma 8 and Gamma 7 multiplets shows strong dependence on the Nb concentration especially at low doping levels. This serves as an evidence of the possible point defect association in the title compound even at a very diluted Nb concentration. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.