Phase evolution, microstructure and chemical stability of Ca1-xZr1-xGd2xTi2O7 (0.0 ≤ x ≤ 1.0) system for immobilizing nuclear waste

In order to ascertain the structural relationship of zirconolite and pyrochlore for their potential application in HLW immobilization, the Gd-doped zirconolite-pyrochlore composite ceramics (Ca1-xZr1-xGd2xTi2O7) were systematically synthesized with x = 0.0-1.0 by traditional solid-phase reaction method. The phase evolution and microstructure of the as-prepared samples have been elucidated by XRD and Rietveld refinement, Raman spectroscopy, BSE-EDS and HRTEM analysis. The results showed that zirconolite-2M, zirconolite-4M, perovskite and pyrochlore, four phases were identified in Ca1-xZr1-xGd2xTi2O7 system and could be coexisted at x = 0.4 composition. With the increase of Gd3+ substitution, the phase evolution was followed by zirconolite-2M→zirconolite-4M→pyrochlore. It is illustrated that the phase transformation from zirconolite-2M to zirconolite-4M was promoted by the preferential substitution of Gd3+ for Ca2+. And the solubility of Gd3+ in zirconolite-2M, zirconolite-4M and pyrochlore increased in sequence. The chemical stability test was also measured by the PCT leaching method. The normalized elemental release rates of Ca, Zr, Ti and Gd in Ca1-xZr1-xGd2xTi2O7 system were fairly low and in the range of 10−6−10−8 g m−2 d−1, which indicated a potential ceramics composite ensemble of CaZrTi2O7-Gd2Ti2O7 system for nuclear HLW immobilization.