Neutron Powder Diffraction Studies of Fluorite and Pyrochlore Nd_xZr_1-xO_2-x/2 Solid Solutions with 0.25<x<0.55

Neutron powder diffraction studies on stoichiometric and non-stoichiometric pyrochlore phases of the ZrO2- Nd2O3 system show remarkable differences in the occupation factor of the 8b site of space group Fd3m to which the pyrochlore phase belongs. Stoichiometric Nd2Zr2O7 can be described as a perfect pyrochlore phase in which the 8b site is either vacant or at least has a low occupation factor. The occupation factors of the 8b sites of non-stoichiometric NdxZr1-xO2-x/2 pyrochlores reveal that these contain a large degree of disorder in the anion sublattice. Best refinement results on non-stoichiometric materials are achieved when the overall structure of the non-stoichiometric pyrochlore phase is described as a hybrid phase consisting of a major volume fraction of the stoichiometric pyrochlore phase and a minor fraction of the defect fluorite phase.