Synthesis and full characterization of the phase-pure pyrochlore Ce2Zr2O7 and the κ-Ce2Zr2O8 phases

• Synthesis of phase pure pyrochlore Ce2Zr2O7 and κ-Ce2Zr2O8 requires reduction at 1500 °C.
• The coordination shell ZrO6 in the pyrochlore Ce2Zr2O7 consists with six equivalent ZrO bonds.
• The coordination shell ZrO8 in the κ-Ce2Zr2O8 phase consists of two sets of distorted tetrahedra.

The κ-Ce2Zr2O8 phase has been synthesized starting from t-Ce0.5Zr0.5O2 solid solution which is reduced by hydrogen at high temperatures to form the pyrochlore Ce2Zr2O7 phase (pyr-Ce2Zr2O7) with high degree of ordering of the cationic sublattice. The final step in the synthesis of the κ-Ce2Zr2O8 phase includes a mild re-oxidation of pyr-Ce2Zr2O7 at around 600 °C under atmospheric conditions. Yet, the synthesis of phase-pure κ-Ce2Zr2O8 phase (phase-pure and with highest ordering) is still not settled since most of the previous studies used reduction temperatures of 1300 °C and below. We show in this contribution that 1300 °C is not sufficient to warrant the preparation of phase-pure pyr-Ce2Zr2O7 and κ-Ce2Zr2O8, but that rather temperatures as high as 1500 °C are required to assure phase purity of both phases. This conclusion is drawn from extended X-ray diffraction and X-ray absorption spectroscopy analyses together with Raman spectroscopy, providing in-depth details of the structure on the level of both, the evolution of the special periodic structure and the coordination of the metal atoms.

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