Defect thermodynamics and electrical properties of nanocrystalline oxides: pure and doped CeO 2

Abstract

Electrical characterization of fully-dense, nanocrystalline CeO 2 (10 nm grain diameter) of undoped and Gd-doped compositions illustrates the influence of size scale on defect formation thermodynamics and transport properties. In undoped n-CeO 2, the heat of reduction is less than one-half the value for conventional polycrystals and single crystals, and the electronic conductivity is correspondingly enhanced. Preferential oxygen vacancy formation at grain boundary sites is indicated. Ionically conducting n-Ce 0.74Gd 0.26O 1.87 exhibits no conductivity enhancement, indicating that oxygen vacancy conductivity is not significantly increased along grain boundaries. Lightly-doped Ce 0.9846Gd 0.0154O 2-x, which is normally an ionic conductor, becomes electronically conducting at nanocrystalline grain size. In all compositions, reduction of grain size also results in a lower resistance per grain boundary, which is attributed to size-dependent grain boundary segregation. The results show that size reduction to the nanometer scale provides a new way to control stoichiometry and electronic conductivity in semiconducting oxides.