Relationship between oxide-ion conductivity and ordering of oxygen vacancy in the Ln2Zr2O7 (Ln = La, Nd, Eu) system having a pyrochlore composition

The crystal structures of the Ln2Zr2O7 (Ln = La, Nd, Eu) systems having a pyrochlore-type composition were refined by the Rietveld analyses of the XRD data combined with MEM-based pattern fitting. All the samples showed a single phase of the pyrochlore-type structure. The oxide-ion conductivities of the present samples were discussed on the basis of the ordering of oxygen vacancy in the pyrochlore-type structure. The Rietveld analyses of XRD data showed that the degree of the oxygen 8b site occupancy at R.T. was estimated to be 0.0, 0.27 and 0.48 for La2Zr2O7, Nd2Zr2O7 and Eu2Zr2O7, respectively. The increase in the 8b site occupancy means the increase of incompleteness of the pyrochlore-type structure. The oxide-ion conductivity increased with increase in the 8b site occupancy which can be recognized to be a measure of the incompleteness of the pyrochlore-type structur. The oxide-ion conduction of Eu2Zr2O7 was ascribed to diffusion of oxygen along <100> direction, while the hole conduction of La2Zr2O7 occurred along <110> direction, according to the charge density map estimated by the MEM analysis.