Fabrication by Graphite Reduction and Structure Characterization of Ce₂Zr₂O_7.97 Phase Possessing Ordered Cation Ions

The phase composition and phase structure of CeO₂-ZrO₂ binary system were studied via graphite reduction method under different reducing temperatures and time. The various reduced products were characterized by powder X-ray diffraction and infrared spectra. Experimental results show that Ce₂Zr₂O_7.97 phase can be synthesized successfully at a relative low temperature (T_red＝1050 ℃) and short time (t＝24 h) by graphite reduction method, which is more simplified and economic than traditional reduction-reoxidation method with Ar-H₂ (5% V/V hydrogen in argon) reduction atmosphere. Ce₂Zr₂O_7.97 phases prepared in a temperature range of 1050-1300 ℃ for 24-72 h are stable and can be retained until room temperature without phase decomposition. The mechanism may be attributed to a weak reduction environment formed by graphite. Ce₂Zr₂O_7.97 phase with an ordered cation subunit cell structure is similar with pyrochlore Ce₂Zr₂O₇, but the structure symmetry of Ce₂Zr₂O_7.97 phase is lower than that of pyrochlore Ce₂Zr₂O₇ due to the oxygen vacancy (O3, O4, O11) filled partially. Graphite reduction method is expected to become a new, simple and economical preparation method comparing with the traditional preparation process for Ce₂Zr₂O_7.97. The Ce₂Zr₂O_7.97 phase retained at room temperature in a weak reducing atmosphere can provide a reference for the storage environment of An₂Zr₂O_7+x waste forms.