The structure of higher rare earth oxides: role of the coordination defect

A milestone in the understanding of the chemistry of the defect solid state has been achieved through new insights provided by recently published neutron diffraction studies of five rare earth oxides with compositions in the range M2O3-MO2. For more than four decades the exact way by which gross concentrations (i.e., up to 25%) of vacant sites can be accommodated in the fluorite MO2 lattice has remained largely unresolved. There is now compelling evidence that the vacancies do not exist in isolation but as octahedral 'coordination defects'. It is also known that the intricate planar patterns formed by anion vacancies in the fluorite lattice arise from the unique and structure-determining topology of these coordination defects. The decisive role played by the coordination defect in generating extended defects in these fluorite-related oxides is highlighted in this paper. 13 refs., 35 figs