Ordering and grain boundary effects on ionic conductivity in ZrO2-Gd2O3 and ZrO2-Nd2O3 ceramics

Single crystal and powder X-ray refinements of pyrochlores are described with the main aim of evaluating the degree of perfectness of cation ordering in this type of system. Neutron refinement results from solid solutions indicate substantial disorder in the anion-sublattice with a more or less dilute concentration of mobile oxygen ions. Conductivity measurements of fluorite and pyrochlore solid solutions indicate that at the stoichiometric pyrochlore composition, sigma0 and ΔH are minimal. Complex admittance measurements show that the grain boundary regions have a much lower specific conductivity than the grain bulk, and that very pure materials can have a larger grain boundary resistivity than less pure materials. The variation of the electrical conductivity is explained as a function of composition and ordering in a model which has three basic elements - the strain energy contribution to the activation enthalpy for ionic motion in fluorite and pyrochlore solid solutions, the magnitude of the pre-exponential factor in terms of the degree of disorder in the oxygen sublattice and the occurrence of a hybrid phase for pyrochlore compositions which deviate strongly from stoichiometry. (Auth.)