Crystal structures of CeO2–ZrO2–Ta2O5 ternary system studied by Rietveld method

Solid solutions of the pseudo-ternary system Ce0.6/(1+x)Zr(Hf)(0.4−x)/(1+x)Ta2x/(1+x) (0.00 ≤ x ≤ 0.06), i.e. having molecular composition 1, 2, 3, 4, 5 and 6% in Ta2O5, have been prepared by traditional ceramic methods (1500°C for 3h). SEM observations and analysis of the X-ray diffraction powders allowed to establish that the solubility of Ta2O5 in the above mentioned system is 4%mol. The Rietveld analysis of the powder X-ray diffraction data has shown that all the samples of the solid solution crystallize in tetragonal space group P42/nmc (No. 137) but are nearly cubic, space group Fm3¯m (No. 225). The freshly prepared specimens (from 1 to 4%mol of Ta2O5) show additional peaks due to a tetragonal supercell and also to a modulated structure. However, the modulated peaks disappear after 8 months. Compositions containing 5 and 6% of Ta2O5 show also additional peaks due to a small quantity of two crystalline phases having approximate composition: ZrO2–2CeO2–Ta2O5 and 2CeO2–Ta2O5 (later on identified as CeTaO4).