Synchrotron X-ray Spectroscopy Investigation of the Ca1-xLnxZrTi2-x(Al,Fe)xO7 Zirconolite Ceramics (Ln=La, Nd, Gd, Ho, Yb)

Abstract

When incorporating actinides into zirconolite for high-level radioactive waste immobilization, Al3+ and Fe3+ ions generally act as charge compensators. In this study, we rationally designed a series of urn:x-wiley:00027820:media:jace16832:jace16832-math-0001 (Ln = La, Nd, Gd, Ho, Yb) to unravel the dopant solubility and evolutions of the crystalline phase and local environment of cations through synchrotron X-ray methods. It was found that single zirconolite phase is difficult to obtain and the fraction of perovskite have an increase with x from 0.1 to 0.9 in urn:x-wiley:00027820:media:jace16832:jace16832-math-0002. Formation of both zirconolite-2M and zirconolite-3O phases was observed in urn:x-wiley:00027820:media:jace16832:jace16832-math-0003 and urn:x-wiley:00027820:media:jace16832:jace16832-math-0004. Phase transformation from zirconolite-2M to 3O occurs at x = 0.7 for urn:x-wiley:00027820:media:jace16832:jace16832-math-0005 while x = 0.9 for urn:x-wiley:00027820:media:jace16832:jace16832-math-0006. The solubility of urn:x-wiley:00027820:media:jace16832:jace16832-math-0007 and urn:x-wiley:00027820:media:jace16832:jace16832-math-0008 to form single zirconolite-2M can reach to 0.9 f.u. and 0.7 f.u., respectively. The evolution of lattice parameters of zirconolite in urn:x-wiley:00027820:media:jace16832:jace16832-math-0009 is greatly related to the ionic radii of cations and substitution mechanism among the cations. X-ray absorption near edge spectroscopy revealed that Fe3+ ions replace both five- and six-coordinated Ti sites and the ratio of TiO5 to TiO6 decreases when increasing dopant concentration in the urn:x-wiley:00027820:media:jace16832:jace16832-math-0010. For the local environment of Zr4+, the major form is ZrO7 with a trace of ZrO8.