Intercalation/deintercalation of oxygen: a sequential evolution of phases in Ce₂O₃/CeO₂−ZrO₂ pyrochlores

Abstract

In this paper, we report the structural and compostional variation of Ce₂Zr₂O₇ pyrochlore under a controlled oxidation/reduction process. On oxidation Ce₂Zr₂O₇ transforms to Ce₂Zr₂O₈ via an intermediate lattice at Ce₂Zr₂O_7.5. Crystal structures of the Ce₂Zr₂O₇, Ce₂Zr₂O_7.5, and Ce₂Zr₂O₈ have been determined accurately from the neutron diffraction data of polycrystalline samples. The retention of cubic pyrochlore-type arrangements is observed even up to the fully oxidized Ce₂Zr₂O₈ composition. The unit cell parameters systematically decreased from Ce₂Zr₂O7 (10.6924(3) Å) to Ce₂Zr₂O₈ (10.5443(2) Å). All three compositions retain the original pyrochlore-type cation ordering without any intermixing Ce and Zr atoms. All three structures can be explained as cubically coordinated metal atoms, which share the edge of their AO₈ polyhedra similar to that in the fluorite lattice. The transformation from Ce₂Zr₂O₇ to Ce₂Zr₂O₈ systematically lowers the symmetry from Fd3m to P2₁3. A complete oxidation of Ce³⁺ to Ce⁴⁺ is observed, in contrast to the kinetically and sterically hindered oxidation proposed earlier. The deep crystallographic insights associated with the oxygen intercalation and deintercalation process in Ce₂O₃/CeO₂−ZrO₂ will have immense importance in the development and understanding of new oxygen storage capacitors.