Influence of the dual charge compensator on solid solution of the air-sintered Ca1-xCexZrTi2-2xFexCrxO7 zirconolite

Abstract

In this study, a dual charge-compensator formulation was developed for zirconolite with the nominal composition Ca1-xCexZrTi2-2xFexCrxO7 (x = 0–0.30). The design strategy here was such that trivalent Fe and Cr were both targeted to substitute across the Ti site(s) to charge balance an inventory of CeO2 included as a structural analogue for Pu. The targeted solid solution was prepared by sintering constituent oxides at 1400 °C for 10 h under an air atmosphere. By means of powder XRD refinement and selected area electron diffraction analysis, the dominant zirconolite polytype was confirmed to be 2M across the solid solution. The obtained product density was significantly increased when compared to the previously discussed Ca1-xCexZrTi2-2xCr2xO7 system, suggesting that the partial inclusion of Fe in a 1:1 molar ratio with Cr may improve sintering behaviour. The limit of solid solution was reached at approximately x = 0.30, for which the segregation of CeO2 and Cr2O3 phases was clearly evidenced. An evaluation of obtained chemical compositions and bulk oxidation states was performed to inform the solid solution mechanism of Ce, Cr and Fe within zirconolite.